HEWLETT-PACKARD JOURNAL

JANUARY 1968 © Copr. 1949-1998 Hewlett-Packard Co.

Three and One-Half Decades in One Clean Sweep N e w h i g h - a c c u r a c y s w e e p g e n e r a t o r c o v e r s 1 0 k H z t o 3 2 M H z i n o n e r a n g e w i t h l o w r e s i d u a l F M

By Robert B. Bump and Myles A. Judd

OSCILLOSCOPE DISPLAYS OF SWEPT FREQUENCY TRACES should be compatible in accuracy with pre

cise point-by-point plots on graph paper. However, a sweep oscillator with superior accuracy specifications must also be easy to operate. Measurement speed gained with a swept technique could be negated if measurement time were increased due to complicated controls and operating procedures.

Sweepers generally cannot produce an accurate scope display, and it is often necessary to spot-check points of interest to achieve the desired accuracy. Besides the lack of accuracy, some of these instruments are complicated to operate. Ideally, the sweeper should be able to produce a display as accurate as a plotted curve and be as easy to operate as a modern signal generator.

Many innovations have been combined in a new 10

Fig. 1. Covering 10 kHz to 32 MHz in a single sweep, the new HP Model 675A Sweeping Signal Generator makes swept measurements with an accuracy approaching that n i r i r r r i \ i n n p n i n t - h \ - - p i i i n t p i n t * .

kHz to 32 MHz sweep oscillator, Fig. 1, to achieve point- by-point accuracy of measurement without sacrificing speed. The new sweeper has a linearity of ±0.5% of sweep width, a dial accuracy of ±0.5% and an overall system flatness (sweeper plus its internal detector) of bet ter than 0.25 dB. Sweep and CW output signals are avail able over the three and one-half decade frequency range in one band on an easy-to-read 3-place digital dial.

Non-interacting, calibrated controls set the two end- points in either start-stop sweep or the delta sweep mode. Delta sweep, or f,,/Af, is used where very narrow, cali brated sweeps are desired, especially where the start-stop sweep dials cannot be read that accurately. It is possible to set the center frequency to within 0.5% of full scale and have calibrated sweep around that frequency. Sweep width can be read at a glance without the need for sub

tracting any numbers. Sweep time is continuously

variable from 0.01 to 100 sec onds. Manual and single sweep is provided. Residual (line related) FM is less than 70 Hz peak, which prevents degradation of the sweep at narrow widths. Both RF and vertical (dc) blanking are provided.

Theory of Operat ion

Mixing of a variable 100 to 132 MHz signal with a fixed, crystal-controlled 100 MHz sig

nal. Fig. 2, results in the 0 to 32 MHz output frequency. Dial accuracy, frequency stability and sweep linearity are all dependent upon the variable 100 to 132 MHz signal. These requirements, alonp with the neetl for electronic

© Copr. 1949-1998 Hewlett-Packard Co.

Fig. entire variable-frequency maintain frequency stability with temperature, the entire variable-frequency oscillator is placed in a proportionally-controlled oven.

tuning, indicated a preference for a voltage-variable capacitor and field effect transistors in the VFO, Fig. 3. The VFO output frequency is 50 to 66 MHz and is doubled and mixed with the 100 MHz crystal frequency.

The voltage-variable capacitor is the dominant capaci tance in the VFO tank circuit, and a control voltage de termines the VFO output frequency. To reduce unwanted FM, a high Q tank circuit is needed, which requires high output impedance of the driver stage and high input im pedance of the following stage. These requirements are well met at these frequencies by field effect transistors. Diode limiting stabilizes the oscillator output level, and an emitter follower provides a low output impedance to drive a coaxial cable, followed by an amplifier which drives the frequency doubler.

A proportionally-controlled oven maintains the entire VFO at a constant temperature.

O n t h e C o v e r : W i d e b a n d c a p a b i l i t i e s o f t h e M o d e l 6 7 5 A S w e e p G e n e r a t o r a r e d r a m a t i z e d b y p h o t o g r a p h e r G e o r g e W a r d i n t h i s ' f i s h - eye ' l ens pho to . He re , t he Mode l 675A , i n t he f 0 / A f m o d e , i s s w e e p i n g a 1 0 . 7 M H z I F a m p l i f i e r , a n d 1 M H z c o m b m a r k e r s c a n b e s e e n on the d i sp lay .

A l s o i n t h i s I s s u e : A d v a n c e s i n S p e c t r u m A n a l y s i s ; p a g e 7 . H o w a Y I G F i l t e r W o r k s ; p a g e 9 .

Sweep Control Circui try

Since the capacitance of a voltage variable capacitor varies approximately as the inverse square root of applied voltage, and the frequency of an LC oscillator also varies as the inverse square root of capacitance, the frequency of the VFO is roughly proportional to the fourth root of the applied voltage. To achieve a precisely linear sweep it is necessary to compensate for this non-linear voltage- to-frequency characteristic. A 16-line segment diode- shaping network modifies the control voltage so that the output frequency is directly proportional to the control voltage within ±0.5%.

The control voltage is generated by summing ramps, adjustable dc voltages, or externally applied signals de pending on the function desired. In Start-Stop sweep, where the end points are set, a ramp is applied to a ten- turn potentiometer which sets the frequency at which the sweep starts, Fig. 4. Another ramp, the inverse of the first, is fed to a similar pot which sets the frequency at which the sweep stops. The outputs of these pots are summed to produce a control voltage ramp representing the difference between the pot settings. The output fre quency starts at one setting and stops at the other, allow ing completely independent sweep end points and up-or- down-going sweep frequency.

In delta sweep, the F,, pot sets a dc voltage which de termines the center frequency. A ramp with no dc com ponent is added to the dc, and the amplitude of the ramp determines the delta-sweep width. Because of this, the

© Copr. 1949-1998 Hewlett-Packard Co.

Fig. 3. Equivalent circuit of the variable- frequency oscil lator. Field effect transistors were used to provide the high Q in the tank circuit to reduce unwanted FM.

controls setting center frequency and sweep width are independent. In CW operation, only the dc from the pot determines the output frequency.

In External Frequency Control, an externally applied

voltage is summed with the dc from the CW control. If frequency modulation is desired, the CW control can be used to set the carrier and an ac-coupled external voltage will determine the modulation. To use the Model 675A for programming or voltage-to-frequency conversions, the CW control can be set to zero, and a de-coupled external voltage will determine the output frequency.

Reference power supplies, from which the control volt ages are derived, have to be very clean and stable for low residual FM and long-term frequency stability. The built-in reference supplies are regulated once to provide hum and ripple reduction and line regulation, and a sec ond time to obtain long term stability and low noise. A temperature compensated zener diode is located in the VFO oven to provide an invariant reference voltage for these supplies. These reference supplies have noise levels of less than 4 /¿V; the control circuitry has less than 2 ¡iV*

Markers

If accuracy greater than that offered by a CRT grati cule is needed, or if test limits are to be indicated for repeated operations, a system of by-pass markers are available. Crystal-derived harmonic comb markers at

* Techn iques used in measur ing these low no ise leve ls a re desc r ibed in the a r t i c le by R o b e r t P r e B u m p e n t i t l e d ' I n c r e a s i n g I n s t r u m e n t S e n s i t i v i t y w i t h a L o w N o i s e P r e amp l i f i e r , 1 'Hew le t t -Packa rd Jou rna l , ' Vo l . 17 , No . 11 , Ju l y 1966 , page 14 .

Fig. VFO. fre ramp circuitry provides the control signal fed to the VFO. So that swept fre quency integrator linear with time, the constant current source charges a Miller integrator capacitor which Start-Stop a highly-linear ramp. Two additional ramps required by the Start-Stop func

tion are obtained hv thr pair of inverl-affxpt opprntinnnl amplifiers shntuij.

© Copr. 1949-1998 Hewlett-Packard Co.

100 kHz or 1 MHz intervals are available as an option, and single crystal markers can be obtained for any fre quency from 100 kHz to 32 MHz. These markers are created by mixing marker frequencies with the sweep output frequency and using an active low-pass filter to obtain a beat note. When the output frequency is equal to the marker frequency, a signal passes through the filter and appears as a marker on the swept display, giving precise frequency identification.

The device under test is in this way bypassed by the marker frequencies and is only affected by the sweep frequency. Bandwidth and amplitude characteristics of the active filter are adjustable from the front panel, allow ing the width and size of the markers to give accurate frequency identification without obliterating the trace. Marker controls and a jack for external marker frequen cies are included on the standard instrument. Marker widths are variable and markers may be tilted by 90° for better frequency identification on steep skirts.

Manua l Sweep

For maximum precision of frequency readout at any point on a response curve, the sweeper can be switched to manual sweep. The frequency at the point of interest can be read out on a counter from a rear-panel unatten- uated jack. Manual sweep also saves time in setting up an X-Y recorder. Sweep speeds down to 100 seconds are provided on the Model 675A. These are compatible with X-Y recorders.

Blanking

Two types of blanking are provided — RF and verti cal. Turning the RF on and off through the device has been the standard method ot blanking the retrace. How ever, switching the RF on and off through a device may not always be desirable. In a high-Q tank circuit, for ex ample, turning the RF on and off abruptly may result in a transient, which might show on the display. Where a detector has a relatively slow decay, the decay manifests itself as a curved baseline when the RF is turned off. Therefore vertical blanking, in which the vertical output is grounded during retrace, is provided. In other words, the RF is always on, but the detected signal is turned off.

Applicat ions

Because the HP Model 675 A covers 3l/2 decades in a single sweep, it is possible to see the entire frequency re sponse curve of many wideband devices, Fig. 5. Some telephone repeaters, for example, are spec'd from 60 kHz to 20 MHz. In checking these repeaters, wide bandwidth

Fig. 5. Bandpass filler response front 11 to 21 MHz show ing 1 MHz comb markers positioned exactly on each verti cal line of the graticule.

Fig. 6. Very low residual F M enables presentation of nar row sweep widths such as this crystal filter. This is a 20 kHz sweepwidth and the bandwidth of the filter is 1.5 kHz.

viewing simplifies adjustment of controls, since any inter action between adjustments can be seen on the display.

In checking wideband amplifiers, the wide sweep width reduces the possibility of missing anomalies in the response curve. For example, there may be a dip due to power supply resonance, and the effects of power supply bypassing may be seen immediately.

Narrow band sweeps are practical due to low residual FM of less than 70 Hz peak. Calibrated delta sweep from 1 kHz to 1 0 MHz makes narrow-band measurement con venient and fast. Bandwidth of the crystal filter, Fig. 6, is 1.5 kHz, and the effect on the display by residual FM is negligible. With an accurate sweep, it is possible to ex-

5 © Copr. 1949-1998 Hewlett-Packard Co.

S P E C I F I C A T I O N S

H P M o d e l 6 7 5 A S w e e p i n g S i g n a l G e n e r a t o r

F R E Q U E N C Y R A N G E : 1 0 k H z t o 3 2 M H z i n o n e r a n g e . O U T P U T :

M a x i m u m + 1 3 d B m ( 1 V r m s i n t o 5 0 ' . Ã , 2 V r m s o p e n c i r cu i t ) .

C o n t i n u o u s l y a d j u s t a b l e . Impedance : 50 ' . . '

R F F L A T N E S S I N T E R N A L L Y L E V E L E D :

50 kHz to 200 kHz: ±1 dB; 2 0 0 k H z t o 3 2 M H z : - 0 . 1 5 d B .

S Y S T E M F L A T N E S S U S I N G I N T E R N A L R F D E T E C T O R , I N T E R N A L L Y L E V E L E D :

1 0 k H z t o 5 0 k H z : M d B ; 50 kHz to 200 kHz: ±0.4 dB; 200 kHz to 32 MHz: :±0.25 dB.

I N T E R N A L D E T E C T O R O U T P U T ( V E R T I C A L ) : A t l e a s t 1 . 2 V d c f o r 1 V r m s .

A T T E N U A T O R R A N G E : 9 9 d B i n 1 0 a n d 1 d B s t e p s . A C C U R A C Y : Â ± 0 . 3 d B , + 1 0 d B t o - 1 2 d B .

 ± 0 . 4 d B , - 1 3 d B t o - 8 9 d B + 6 M V c o n s t a n t e r ro r .

O U T P U T M O N I T O R R A N G E : - 3 t o + 3 d B ( 0 . 5 V t o 1 V ) . A C C U R A C Y : Â ± 0 . 3 d B , 2 0 0 k H z - 3 2 M H z .

C O U N T E R O U T P U T : > 3 0 0 m V r m s . D I S T O R T I O N

H A R M O N I C : > 3 0 d B d o w n f r o m f u n d a m e n t a l . S P U R I O U S : > 5 0 d B d o w n f r o m f u n d a m e n t a l .

R E S I D U A L ( l i n e r e l a t e d ) F M : < 7 0 H z p e a k . S P U R I O U S F M : < 6 0 H z r m s . F R E Q U E N C Y D R I F T : < 1 k H z / h r , < 3 k H z / Â ° C . A U X I L I A R Y O U T P U T ( r e a r p a n e l ) : 1 0 0 M H z t o 1 3 2 M H z u n -

leve led. S W E E P F U N C T I O N S

L I N E A R I T Y :  ± 0 . 5 % o f s w e e p w i d t h  ± 1 0 0 H z . S T A R T - S T O P : S w e e p s u p o r d o w n f r o m s t a r t t o s t o p f r e

q u e n c y s e t t i n g s . R A N G E : 1 0 k H z t o 3 2 M H z . E N D P O I N T A C C U R A C Y :

±1% of ful l scale, 20°C to 30°C. ±2% of full scale, 0°C to 20°C, 30°C to 50°C.

Fo/AF: Sweeps ¿F centered on Fo set t ing . Fo RANGE: 10 kHz to 32 MHz. F o A C C U R A C Y :  ± 0 . 5 % o f f u l l s c a l e . _ \ F W I D T H : C o n t i n u o u s l y a d j u s t a b l e f r o m 2 0 0 H z t o 1 0

M H z , o r c a l i b r a t e d s t e p s i n 1 - 2 - 5 s e q u e n c e f r o m 1 k H z t o 1 0 M H z .

^ F W I D T H A C C U R A C Y : = 5 % o f c a l i b r a t e d s t e p s  ± 1 0 0 Hz.

S W E E P M O O E S A U T O : R e p e t i t i v e s w e e p s . S I N G L E : B e g i n n i n g - t o - e n d s w e e p . R E T R A C E : E n d - t o - b e g i n n i n g r e t r a c e . M A N U A L : M a n u a l c o n t r o l o f s w e e p a n d r e t r a c e .

S W E E P T I M E : 0 . 0 1 s t o 1 0 0 s  ± 2 0 % ; 4 d e c a d e s t e p s w i t h vern ier .

R E T R A C E T I M E : 0 . 0 1 s f o r 0 . 0 1 s t o 1 s s w e e p t i m e s . 1 s f o r 1 s t o 1 0 0 s s w e e p t i m e s .

H O R I Z O N T A L O U T P U T : 0 t o + 5 V d C . B L A N K I N G

RF : RF ou tpu t o f f du r i ng r e t r ace . V E R T I C A L : V e r t i c a l o u t p u t s h o r t e d t o g r o u n d d u r i n g r e

t race

n a l s h o r t e d d u r i n g s w e e p , o p e n d u r i n g r e - P E N L I F T : T e n t race.

C R Y S T A L M A R K E R S ( o p t i o n a l ) : 1 0 0 k H z . 1 M H z h a r m o n i c c o m b a n d / o r u p t o 5 f i x e d f r e q u e n c i e s f r o m 1 0 0 k H z t o 32 MHz. A C C U R A C Y : Â ± 0 . 0 0 5 % o f f r e q u e n c y . WIDTH: Ad jus tab le 5 s teps , 4 kHz t o 100 kHz .

* HP 11300A single frequency marker (frequency must be specified). E X T E R N A L M A R K E R : F r o n t - p a n e l B N C i n p u t ( 5 0 1 . ' i m p e d

a n c e ) , 5 0 m V t o 5 0 0 m V r m s . S I G N A L G E N E R A T O R F U N C T I O N S

C W D I A L A C C U R A C Y : -0 .5% of fu l l sca le , 20°C to 40°C. ± 1 % of full scale 0°C to 20°C. 40°C to 50°C.

C W S E T T A B I L I T Y : 1 k H z . C W R E S O L U T I O N : 2 0 k H z . I N T E R N A L A M : 0 % t o 5 0 % s i n u s o i d a l 9 8 5 H z t o 1 0 1 5 H z

c o n t i n u o u s l y a d j u s t a b l e . E X T E R N A L A M :

0 % t o 5 0 % , d c t o k H z l e v e l e d . 0% to 50%. 50 Hz t o 600 kHz un leve l ed . S E N S I T I V I T Y : A t l e a s t 5 0 % f o r 2 . 5 V r m s i n p u t .

E X T E R N A L F R E Q U E N C Y C O N T R O L A N D E X T E R N A L F M S E N S I T I V I T Y : 1 M H z / V . INPUT IMPEDANCE: 1 M IÃ . R A T E : D C t o 4 k H z . A b o v e 4 k H z t h e r a n g e a n d s e n s i

t i v i t y d e c r e a s e 2 0 d B / d e c a d e . GENERAL

TEMPERATURE RANGE: 0°C to 50°C. P O W E R : 1 1 5 V o r 2 3 0 V  ± 1 0 % , 5 0 H z t o 1 0 0 0 H z , < 8 0 W . W E I G H T : N e t 4 6 I b . ( 2 0 . 8 k g ) , s h i p p i n g 5 1 I b . ( 2 3 , 2 k g ) . PRICE: HP 675A, $2250 .00 .

M A N U F A C T U R I N G D I V I S I O N : L O V E L A N D D I V I S I O N P.O. Box 301 815 Four teen th S t ree t S .W. Love land , Co lo rado 80537

periment with filters without the need for a great deal of calculation.

Frequently an application requires a sweep just beyond the 32 MHz range of the Model 675A. A typical case would be a response check of a 30 MHz IF amplifier where the response from 23 to 37 MHz is desired. Here, a doubler can be used with the sweeper to extend its range. Combined system flatness is better than 0.25 dB from 4 to 64 MHz with the greatest deviation at 64 MHz. Marker width is not affected.

A c k n o w l e d g m e n t s The authors would like to recognize the contributions

to the Model 675A by the other members of the design team. Paul Thomas was responsible for the design of the marker circuitry. Gerald Nelson designed the power sup plies, oven and oven controls and also contributed to the design of the variable frequency oscillator. Arthur Minich designed the attenuators. Kay Danielson did the product design, and Brian Smith gave valuable assistance in test ing and breadboarding. *

Robert B. Bump Bob Bump rece i ved h i s BSEE i n June , 1962 , f r om Ca l i f o rn ia I ns t i t u te o f Techno logy , and j o i ned the HP Love land D iv i s ion as a d e v e l o p m e n t e n g i n e e r i m m e d i a t e l y

s a f t e r g r a d u a t i n g . H e d i d t h e ' e l e c t r o n i c d e s i g n o f t h e H P

Mode l 208A Tes t Osc i l l a t o r and the HP Mode l 465A Genera l Pu rpose Amp l i f i e r , and had p ro j ec t respons ib i l i t y fo r the 675A S w e e p i n g S i g n a l G e n e r a t o r .

B o b r e c e i v e d h i s M S E E i n D e c e m b e r , 1 9 6 6 f r o m C o l o r a d o S ta te Un i ve r s i t y on t he HP Hono rs Coope ra t i ve P rog ram.

I n S e p t e m b e r , 1 9 6 7 h e t r a n s f e r r e d t o t h e A v o n d a l e , P e n n s y l v a n i a D i v i s i o n , w h e r e h e i s c u r r e n t l y p r o j e c t l e a d e r o f a n e l e c t r o m e t e r i n v e s t i g a t i o n t e a m .

1

Myles A. Judd My les rece i ved h i s Bache lo r o f E n g i n e e r i n g S c i e n c e d e g r e e f r o m Br igham Young Un ive rs i t y i n 1962 .

He j o ined the Love land D iv i s i on r e s e a r c h a n d d e v e l o p m e n t l a b o r a t o r y a s a d e v e l o p m e n t eng ineer in tha t year . S ince j o i n i ng HP , My les has wo rked on seve ra l t es t osc i l l a to r s i nc lud ing t he Mode l s 651A and 652A . He a lso worked on the 236A T e l e p h o n e T e s t O s c i l l a t o r a n d t h e Mode l 313A T rack ing Osc i l l a t o r .

He i s p resen t l y a g roup leader and i s con t i nu ing wo rk re la ted to th is area.

© Copr. 1949-1998 Hewlett-Packard Co.

Advances in Spectrum Analysis A new p rese lec to r , va r i ab le pe rs i s tence w i th s to rage , be t te r sens i t i v i t y , and f l a t t e r f r equency r esponse make spec t r um ana l ys i s cons ide rab l y eas ie r and more power fu l .

By John J. Dupre, Richard C. Keiter, and John R. Page, Jr.

A 'though spectrum analysis has been an important signal-analysis tool for twenty-five years, it has

made its greatest strides in the three years since 1964, when the first accurately calibrated microwave spectrum analyzer was developed. It now seems probable that the spectrum analyzer will one day be almost as common and as necessary as the osci l loscope. Tending to strengthen this conviction are several recent develop ments, chiefly new capabilities for dealing with certain types of signals that have traditionally been difficult to analyze.

Aiding in the analysis of strong signals, multiple sig nals, and broadband signals is a versatile new preselector for the HP spectrum analyzer. This instrument is a voltage-tunable YIG filter which automatically tracks a selected harmonic of the analyzer's local oscillator, so that only signals in a single frequency range are dis played. It can also be used separately as a voltage-tuned or manually tuned filter.

A new variable-persistence display unit for the spec trum analyzer significantly enhances the analyzer's ability to measure intermittent, low-level, and low-repetition- rate signals. It also makes it convenient to use slower sweeps, which are necessary for maximum resolution.

Greater accuracy in measuring widely spaced and low- level signals comes from a new broadband input mixer, which extends the upper limit of the analyzer's coaxial range to 12.4 GHz. The mixer has greater sensitivity and flatter frequency response than its predecessor.

Preselector The broadband harmonic input mixer and wide-

sweeping first local oscillator in the HP spectrum analyzer allow simultaneous observation of all input signals be tween 10 MHz and 12.4 GHz. The analyzer acts like a wide-open superheterodyne receiver. It accepts, in coax, any input signals between 10 MHz and 12.4 GHz. and mixes them with the fundamental and the harmonics of a local oscillator which sweeps from 2 GHz to 4 GHz. (The harmonics are generated in the mixer.) The result

ing IF signals, centered at 2 GHz, are converted down to 20 MHz, amplified and detected, and displayed on the CRT.

This wide-open, untuned input system is an advantage when widely spaced signals must be observed simultane ously. Tuning a parametric amplifier is one such occa sion; signal, pump, and idler frequencies can be seen on a single display. Another task which calls for a wide spectrum display is observing the output of a varactor multiplier chain.

However, there are situations when the untuned input, combined with the action of the harmonic input mixer, can make the display difficult to interpret. For example, 1 GHz on the display also corresponds to input frequen cies of 4, 5, 7, 8, and 1 1 GHz; thus signals separated widely in frequency can appear close together. Also, a single input signal can produce responses at more than one point on the display. Hence the display can become ambiguous and complicated, especially if there are sev eral input signals.

Display ambiguities can be eliminated by using frequency-selective circuitry (preselection) ahead of the input mixer to confine the displayed signals to a definite frequency range. The HP Model 8441 A Preselector, designed primarily for use with the HP spectrum ana lyzer, employs an Yttrium-Iron-Garnet (YIG) bandpass filter whose center frequency is electrically tunable from 1.8 to 12.4 GHz. The preselector obtains from the spectrum analyzer a tuning voltage proportional to the analyzer's local-oscillator frequency. Its center frequency then automatically tracks a selected harmonic of the spectrum analyzer's local oscillator, with a frequency offset equal to the intermediate frequency. Thus only signals in a selected frequency range can reach the mixer and be displayed on the CRT. Nominal bandwidth of the preselector is 40 MHz. wide enough to avoid inter ference with desired signals (maximum IF bandwidth of the analyzer is 1 MHz), but narrow enough to provide good rejection of most unwanted signals.

© Copr. 1949-1998 Hewlett-Packard Co.

Fig. 1. Coaxial input mixer of HP spectrum analyzer con verts input signals to 2-GHz intermediate frequency. An IF response is produced whenever f , = nf , .n ± 2 GHz, w h e r e n = 1 , 2 , 3 , 4 , . . .

Responses of the Harmonic Mixer To understand multiple responses and how the pre

selector rejects them, consider the tuning curve for the spectrum analyzer, Fig. 1 . This is a graphical presenta tion of the harmonic mixing responses of the spectrum analyzer. A signal of frequency fs produces a response whenever

f s = n f L0 ± fit- where fj.o = first-local-oscillator frequency, 2 to 4 GHz.

f,K = first intermediate frequency, 2 GHz (A 200 MHz IF can also be selected, for observing signals in the vicinity of 2 GHz),

n = l , 2 , 3 , . . . Responses produced by a particular harmonic with a plus or minus sign in the above equation are spoken of as having been generated by the n^ or n mixing modes, respectively.

Harmonic Responses Without preselection, the analyzer will respond to

several different input frequencies for a particular local- oscillator setting. For example, if the local-oscillator fre quency is 3 GHz. the analyzer will respond to signals at 1, 4, 5, 7, 8, and 1 1 GHz. Corresponding mixing modes are 1~, 2~, 1% 3 . 2', and 3 .

Harmonic responses like these can be prevented by the preselector. The preselector is connected to the ana lyzer as shown in Fig. 2, and a front-panel switch on the preselector is turned to the desired mixing mode. The preselector's center frequency then automatically tracks the proper harmonic of the analyzer's local oscillator —

offset above or below it by an amount equal to the first intermediate frequency — so that only responses pro duced by the desired mixing mode are displayed. Only where the tuning curves for two mixing modes come within approximately 40 MHz of each other is there a possibility that an unwanted harmonic response can occur. Such areas are indicated in Fig. 1; they can be avoided by selecting another mixing mode.

"

Fig. 2. New HP Model 8441 A Preselector, connected to spectrum analyzer as shown, confines displayed signals to a \clected frequency range. The preselector is a narrow band Y IG filter which automatically tracks a selected har- .w.'.-rc mixing mode of the analyzer's LO anil mixer.

© Copr. 1949-1998 Hewlett-Packard Co.

How a YIG Filter Works

T h e e l e c t r i c a l l y t u n a b l e f i l t e r u s e d i n t h e P r e s e l e c t o r d e s c r i b e d i n t h e a c c o m p a n y i n g a r t i c l e i s o f t h e r e c e n t l y deve loped Y t t r i um- I r on -Ga rne t (Y IG ) t ype . H igh l y po l i shed s p h e r e s o f s i n g l e - c r y s t a l Y I G , a f e r r i t e m a t e r i a l , w h e n p l a c e d i n a n R F s t r u c t u r e u n d e r t h e i n f l u e n c e o f a d c m a g n e t i c f i e l d , e x h i b i t a h i g h - Q r e s o n a n c e a t a f r e q u e n c y p r o p o r t i o n a l t o t h e d c m a g n e t i c f i e l d .

T o u n d e r s t a n d t h e p h e n o m e n o n o f f e r r i m a g n e t i c r e s o n a n c e , c o n s i d e r d i a g r a m s ( a ) t h r o u g h ( e ) . I n t h e f e r r i t e w i t h n o d c m a g n e t i c f i e l d a p p l i e d , t h e r e i s a h i g h d e n s i t y o f r a n d o m l y o r i e n t e d m a g n e t i c d i p o l e s , e a c h c o n s i s t i n g o f a m i n u t e c u r r e n t l o o p f o r m e d b y a s p i n n i n g e l e c t r o n . V i e w e d m a c r o s c o p i c a l l y , t h e r e i s n o n e t e f f e c t b e c a u s e o f t h e r a n d o m o r i e n t a t i o n s . W h e n a d c m a g n e t i c f i e l d , H , , o f s u f f i c i e n t m a g n i t u d e i s a p p l i e d , t h e d i p o l e s a l i g n p a r a l l e l t o t h e a p p l i e d f i e l d , p r o d u c i n g a s t r o n g n e t m a g n e t i z a t i o n , M j , i n t h e d i r e c t i o n o f H . . I f a n R F m a g n e t i c f i e ld i s app l ied a t r i gh t ang les to H- . , the ne t magne t i za t ion vec to r w i l l p recess , a t t he f r equency o f t he RF f i e l d , abou t a n a x i s c o i n c i d e n t w i t h H . T h e p r o c e s s i n g m a g n e t i z a t i o n v e c t o r m a y b e r e p r e s e n t e d a s t h e s u m o f M 0 a n d t w o c i r c u l a r l y p o l a r i z e d R F m a g n e t i z a t i o n c o m p o n e n t s m , a n d m y . T h e a n g l e o f p r e c e s s i o n 0 , a n d t h e r e f o r e t h e m a g n i t u d e s o f m , a n d m , , w i l l b e s m a l l e x c e p t a t t h e n a t u r a l p r e c e s s i o n f r e q u e n c y . T h i s f r e q u e n c y , k n o w n a s t h e f e r r i m a g n e t i c r e s o n a n t f r e q u e n c y , i s a l i n e a r f u n c t i o n o f t h e dc f i e ld H . .

D i a g r a m ( f ) s h o w s t h e b a s i c e l e m e n t s o f a Y I G b a n d p a s s f i l t e r . 1 T h e f i l t e r c o n s i s t s o f a Y I G s p h e r e a t t h e

cen te r o f two l oops , whose axes a re pe rpend i cu la r t o each o t h e r a n d t o t h e d c f i e l d H . O n e l o o p c a r r i e s t h e R F i n p u t cu r ren t , and t he o the r l oop i s connec ted t o t he l oad . When H , i s z e r o t h e r e i s l a r g e i n p u t - t o - o u t p u t i s o l a t i o n , s i n c e t h e t w o l o o p s a r e p e r p e n d i c u l a r . W i t h H , a p p l i e d , t h e r e i s a n e t m a g n e t i z a t i o n v e c t o r i n t h e d i r e c t i o n o f H . T h e m a g n e t i c f i e l d h , p r o d u c e d b y t h e R F d r i v i n g c u r r e n t i n t h e i n p u t l o o p c a u s e s t h e n e t m a g n e t i z a t i o n v e c t o r t o p r e c e s s a b o u t t h e z - a x i s . T h e r e s u l t i n g R F m a g n e t i z a t i o n c o m p o n e n t , m , i n d u c e s a v o l t a g e i n t o t h e o u t p u t l o o p . A t f r e q u e n c i e s a w a y f r o m t h e f e r r i m a g n e t i c r e s o n a n t f r e q u e n c y , m a n d t h e v o l t a g e i t i n d u c e s a r e s m a l l , s o i n p u t - t o - o u t p u t i s o l a t i o n i s h i g h . W h e n t h e i n p u t c u r r e n t i s a t t h e f e r r i m a g n e t i c r e s o n a n t f r e q u e n c y , 0 a n d m v a r e m a x i m u m . T h e r e i s a l a r g e t r a n s f e r o f p o w e r f r o m i n p u t t o o u t p u t , a n d i n s e r t i o n l o s s i s l o w . T h u s t h e f i l t e r c e n t e r f r equency i s t he f e r r imagne t i c r esonan t f r equency and can be tuned by varying (- !„. Commonly, the YIG sphere and RF s t r u c t u r e a r e l o c a t e d b e t w e e n t h e p o l e s o f a n e l e c t r o m a g n e t , a n d t u n i n g i s a c c o m p l i s h e d b y f u r n i s h i n g a c o n t r o l cu r ren t t o the magne t co i l s .

T o a c h i e v e i m p r o v e d s e l e c t i v i t y a n d o f f b a n d i s o l a t i o n , t h e Y I G f i l t e r u s e d i n t h e p r e s e l e c t o r e m p l o y s t w o Y I G f i l t e r s t ages i n se r i es and i n t he same magne t i c f i e l d . Th i s f i l t e r w a s d e v e l o p e d a n d i s m a n u f a c t u r e d b y t h e W a t k i n s - J o h n s o n C o m p a n y . 1 For 'Magnetically detailed treatment see, for instance, P. S. Carter, Jr., 'Magnetically

T u n a b l e M i c r o w a v e F i l t e r s U s i n g S i n g l e - c r y s t a l Y t t r i u m - l r o n - G a r n e t R e s o n a t o r s , ' I R E T r a n s a c t i o n s o n M i c r o w a v e T h e o r y a n d T e c h n i q u e s V o l M T T - 9 No. 3, May 1961.

( a ) R a n d o m l y o r i e n t e d m a g n e t i c d i p o l e s i n t h e u n m a g n e t i z e d f e r r i f e .

( b ) M a g n e t i c d i p o l e s a l i g n e d u n d e r t h e i n f l u e n c e o à - a m a g n e t i c H e l d .

( c ) A n e q u i v a l e n t r e p r e s e n t a t i o n o f ( b ) s h o w i n g t h e c o m b i n e d e l f e c t o f t h e a l i g n e d d i p o l e s .

â € ¢ â € ¢ " ^ ^ ^ ^ ^ ^ â € ¢ ^ ^ ^ ^ ^ â € ¢ ^ â € ¢ â € ¢ â € ¢ â € ¢ ^ ^ ^ ^ ^ ^ ^ â € ¢ â € ¢ ^ ^ ^ ^ ^ ^ â € ¢ ^ B

( d ) P r e c e s s i o n o f t h e n e t m a g n e t i z a t i o n v e c t o r d u e t o R F m a g n e t i c e x c i t a t i o n .

( e ) E q u i v a l e n t r e p r e s e n t a t i o n o f p r e c e s - s i n g m a g n e t i z a t i o n v e c t o r .

( I ) T u n e d b a n d p a s s f i l t e r c o n s i s t s o f Y I G s p h e r e a t c e n t e r o f t w o m u t u a l l y o r t h o g o n a l l o o p s .

© Copr. 1949-1998 Hewlett-Packard Co.

( b )

Fig. 3. (a) Spectrum-analyzer display fur 5-GHz input signal, without preselection. Input signal produces re sponses by mixing with one or another LO harmonic when LO is tuned to 2.33, 3.00, and 3.50 GHz. (b) Model 844IA Preselector eliminates multiple responses by re moving input when LO is tuned to 2.33 and 3.50 GHz.

Mult ip le Responses

A single input signal above 4 GHz will produce re sponses for two or more different local-oscillator frequen cies. A 5-GHz input, for example, will produce responses by mixing with one or another LO harmonic when the local oscillator is tuned to 2.33, 3.00, and 3.50 GHz. Like harmonic responses, these multiple responses can be eliminated by the preselector.

Fig. 3(a) shows the spectrum analyzer display result ing from a 5-GHz input signal when the local oscillator is sweeping over its entire range from 2 to 4 GHz, without preselection. The three multiple responses from left to right are the 3 , T, and 2 mixing responses.

With preselection, any selected one of these responses is tracked by the YIG filter and the others rejected. Fig. 3(b) shows the resulting display when the 1 response is

preselected. The YIG filter sweeps from 4 to 6 GHz in synchronism with the local oscillator. When the local oscillator is at the frequencies required to produce the 3~ and 2 responses from the 5-GHz signal, the filter is tuned away from 5 GHz, so these responses are rejected. Again, the amount of rejection is large except at the crossings of two response curves (indicated in Fig. 1).

Possible harmonic and multiple responses which can occur, even with preselection, when the tuning curves for two mixing modes intersect, can be avoided by switching to another mixing mode. By proper selection of analysis bands, multiple responses can be reduced by more than 38 dB throughout the range of 1.8 to 12.4 GHz.

Spurious Responses Spurious responses are harmonic and intermodulation

distortion products generated by the nonlinear behavior of the analyzer's input mixer when it is driven by signals greater than —30 dBm. Intermodulation distortion prod ucts are caused by the interaction of two or more strong input signals in the mixer.

The new preselector is effective in reducing spurious responses. Fig. 4 shows the analyzer displays both with and without preselection when two high-power signals are applied to the input mixer. Without preselection, harmonic and intermodulation distortion products make it difficult to find the desired signals. Preselection greatly simplifies the display.

With preselection, harmonics of either input signal are actually still generated in the mixer, but only while the preselector is tuned to the signal fundamental. At this time the local oscillator is tuned so that only the signal fundamental produces a 2-GHz IF signal; thus no response to the harmonics is produced. When the local oscillator is at such frequencies as to produce responses from signal harmonics, the preselector is tuned off the fundamental, so no harmonics are produced in the mixer. The only responses that appear on the display represent the true harmonic content of the signal.

Intermodulation distortion is also reduced by the pre selector, if the strong signals causing it are separated by more than the YIG filter bandwidth (40 MHz). The pre selector passes only one signal at a time to the mixer, and attenuates the other signal by an amount given by the selectivity characteristic, Fig. 5. For example, if the pre selector is tuned to an 8-GHz signal, another signal 200 MHz lower in frequency will be attenuated 3 1 dB.

Using the preselector to reduce spurious responses increases the spectrum analyzer's spurious-free dynamic range — and. therefore, the analyzer's distortion-measur-

10 © Copr. 1949-1998 Hewlett-Packard Co.

ing capability — by at least 30 dB. Normally, spurious responses are more than 50 dB down for —30 dBm input signals, but increase drastically for higher input levels. The preselector allows input signals up to one milliwatt to be applied without the multitude of unwanted responses that occur without preselection (Fig. 4). The resulting wide dynamic range may be used to advantage, for example, in observing the harmonic output of micro wave signal sources. For a fundamental input level up to one milliwatt, harmonics may be observed at as low a level as spectrum-analyzer sensitivity will permit. Har monics as much as 100 dB below the fundamental may be observed with assurance that they are not being gen erated in the mixer.

Observing Impulses In analyzing the spectra of high voltage, narrow width

pulses, such as those from impulse generators used for receiver calibration, preselection is a definite require ment. These pulses often have amplitudes of 100 V or more. If such a pulse were connected directly to the ana lyzer, its amplitude would be limited and its shape would be distorted by mixer saturation. This would change the spectrum being measured. The preselector, on the other hand, passes only the spectral lines within its bandpass, so the total power applied to the mixer at any time is extremely small, and no mixer saturation occurs.

Using the Preselector as a Tunable Filter Although it is designed primarily as a preselector for

the HP spectrum analyzer, the preselector can also be used independently as a tunable bandpass filter. The center frequency can be set anywhere between 1.8 GHz and 12.4 GHz. This can be done manually, using the calibrated front-panel control, or remotely, by means of a control voltage. There is also an internal sweep gen erator which can sweep the filter over its entire frequency range or any portion of it. In this sweeping mode of operation the preselector, in conjunction with a broad band detector and a sensitive oscilloscope, becomes a low-sensitivity but remarkably wideband spectrum ana lyzer (see Fig. 6).

Variable-Persistence Display Unit Variable persistence, already in use on some HP

oscilloscopes,1 is now available in a new display unit for the HP spectrum analyzer (top instrument in photo graph. Fig. 2). Designated Model 852A, the new display unit is similar to the standard display unit- except for the variable-persistence CRT and associated circuitry. 1 R . H . Ko la r , 'Va r i ab le pe rs i s tence i nc reases osc i l l oscope ' s ve rsa t i l i t y , ' E lec t ron i cs ,

November 29, 1965. ' H . L . H a l v e r s o n , ' A N e w M i c r o w a v e S p e c t r u m A n a l y z e r , ' H e w l e t t - P a c k a r d J o u r n a l ,

Vol . 15, No. 12, August 1964.

The persistence of the CRT in the new display unit can be varied between approximately 0.2 second and more than one minute. There is also a storage mode, in which a trace can be stored for more than an hour. With the instrument turned off, the trace can be stored for a day or more.

Unlike an oscilloscope, which would probably be oper ated with short persistence most of the time and switched to variable-persistence only for special applications, a spectrum analyzer display is most readable when it is operated in a variable-persistence mode. For all but the fastest spectrum-analyzer sweeps, the trace flickers no ticeably with normal-persistence phosphors. Yet many routine applications call for slow sweep rates. For ex ample, it is necessary to use a narrow IF bandwidth for better resolution or sensitivity, and this requires a slow sweep rate, since narrow-band filters respond slowly to

Fig. 4. (a) Without preselection, spectrum-analyzer dis play for strong (about 0 dBm) input signals at 4.9 and S.I GH- is complicated by ¡ntennodulation products and mul tiple responses, (b) Model 8441A Preselector reduces multiple responses and distortion products.

1 1 © Copr. 1949-1998 Hewlett-Packard Co.

4 0 0 6 0 0 8 0 0 1 0 0 0 2 0 0 0

FREQUENCY

Fig. 5. Selectivity characteristic of Model 8441 A Preselector.

suddenly applied input signals; if a signal sweeps past too rapidly, the IF filter's response will be incomplete, and a small signal may be missed entirely.1 Flicker can be reduced by confining the sweep to a narrow frequency range, but this is also undesirable in many applications. Variable' persistence makes slow sweep speeds usable without flicker. Reduction of the spectrum width can be avoided, and there is no need to resort to photographic techniques.

Dif f icul t Measurements Made Easy Variable persistence makes it possible to do things

with a spectrum analyzer that would be difficult or im possible with normal persistence.

With normal persistence, at the faster sweep rates, a pulsed RF signal which has a repetition rate of 500 Hz or less appears on the analyzer's display as random lines.

' R u l e o f t h u m b :  ¡ f B W i s t h e 3 - d B b a n d w i d t h o f t h e I F f i l t e r s , t h e n t h e s w e e p r a t e should be less than (BW)¡ Hz/second.

This type of pulse is commonly found in radar spectrum measurements. The reason for the apparent randomness is that a line will appear on the display only when an RF pulse occurs. The maximum number of lines visible at any time will be the pulse repetition rate times the per sistence of the CRT phosphor. With a pulse repetition rate of 500 Hz and a persistence of 0.1 second, 50 lines will be visible. At 50 Hz and 0.1 second, only five lines will be visible. Since the analyzer's sweep is not synchron ized with the pulses, the entire spectrum will eventually be displayed, but only a few lines of it will be visible at any time.

With variable persistence, individual lines can be stored long enough for a complete pattern to form (Fig. 7). Complete patterns can be seen either for fast or for slow sweeps, whereas with normal persistence, a com plete pulse spectrum can be seen only with a slow sweep, and then flicker becomes a problem.

Q D E T E C T O R - o n t a l S w e e p J

Fig. 6. low- HP Model 8441 A Preselector, with detector and oscilloscope, forms a low- sensitivity but very wideband spectrum analyzer, (b) Spectrum of high-power frequency

comb obtained with spectrum analyzer setup (a). Sweep limits are 2 and 12 GHz.

1 2 © Copr. 1949-1998 Hewlett-Packard Co.

Similar improvements in spectrum presentations can be obtained in noise measurements. The peak noise envelope becomes well defined after several sweeps have been stored (Fig. 8). This can be useful in measuring impulse noise, such as that encountered in RFI measurements. Impulse noise appears as faint spikes on a short-persistence CRT, but becomes a bright steady display when variable persistence is used. What's more, signals very near the noise level per easier to detect against the sharp envelope provided by variable per sistence. Thus the effective sensitivity of the analyzer is increased.

Many easily or spectrum-surveillance measurements are made more easily with variable persistence. For example, intermittent signals can be stored over periods of a minute or longer, permitting their frequencies and amplitudes to be measured.

Antenna pattern measurements can be made using storage, simply by re ducing CRT analyzer's local-oscillator sweep to zero and scanning the CRT horizontally with an external signal proportional to antenna rotation. The stored display will then be a pattern of antenna gain or radiation versus rotation angle.

Coaxial Input Mixer Advantages of the new coaxial input mixer include the following. • Extended coaxial range, 0.01 to 12.4 GHz. The upper limit was 10 GHz. • Five to 15 dB more sensitivity over most of the coaxial range. This means

lower-level signals can be measured. • Flatter frequency response. Typical variations are now ±0.5 dB for funda

mental mixing and ±1.5 dB for harmonic mixing, over any 2 GHz spectrum width. These variations include those caused by components, cables, and con nectors preceding the mixer. Flatter frequency response means greater ac curacy in comparing the levels of signals at different frequencies.

• Easily replaceable mixer diode. The mixer diode is located on the front panel and is readily changed by unscrewing a cap and unplugging the diode. The diode is a standard type (D-5282-C), not a specially selected one. The design of the mixer is somewhat different from that of most microwave

mixers. Not only is the mixer's frequency range very broad (0.01 to 12.4 GHz), but the magni frequency is 2 GHz, which is about two orders of magni tude higher than typical microwave-receiver IP's. The local-oscillator port is broadband from 2 to 4 GHz.

Another unusual feature of the mixer is that, to allow the mixer diode to be located on the front panel, the diode has to be separated from the mixer structure by a length of transmission line, instead of being integral to it. To get the IF special out of the diode and into the IF amplifier efficiently, a special two-cavity arrangement was developed. Fig. 9 is a schematic diagram of the mixer, and Fig. 10 is a photograph of its exterior.

Mixer Operat ion To cover the required input bandwidth of 0.01 to 12.4 GHz, the mixer takes

. s i g n a l p o w e r + n o i s e p o w e r â € ¢ S e n s i t i v i t y = s i g n a l p o w e r f o r w h i c h - i s e o w e r â € ” ~ = 2 ; 1 0 k H z I F b a n d w i d t h .

Fig . 7 . Spec t rum o f a CW s igna l pu l se -modu la t ed a l 50 Hz. Envelope o f spectrum would be poorly de f ined with normal persistence, but gradually becomes well de f ined with variable persistence of new HP Model 852A

Spectrum Analyzer Display Unit.

13 © Copr. 1949-1998 Hewlett-Packard Co.

Fig. persistence becomes noise envelope, difficult to define with normal persistence (a), becomes well defined with longer persistence (I)).

advantage of harmonic mixing. It converts input signals to 2 GHz according to the equation

fs = nf,x, ± 2 GHz where fs is the input-signal frequency, fLO is 2 to 4 GHz, and n is 1, 2, or 3.

Operation of the mixer in the fundamental mixing

mode (n = 1) can be shown best by an example. Assume that the local oscillator is tuned to 3 GHz. The directional coupler in Fig. 9 couples some of the 3-GHz power into the transmission line which leads to the diode. This signal causes the diode to switch on and off at the local- oscillator frequency.

P A R T I A L S P E C I F I C A T I O N S

H P M o d e l 8 4 4 1 A P r e s e l e c t o r

C H A R A C T E R I S T I C S A S A P R E S E L E C T O R F O R T H E 8 5 1 B / 8 S 5 1 B ( o r 8 5 2 A / 8 5 5 1 B ) S P E C T R U M A N A L Y Z E R

( M a y a l s o b e u s e d w i t h t h e 8 S 1 A / 8 S S 1 A S p e c t r u m A n a l y z e r : a s l i g h t m o d i f i c a t i o n t o t h e 8 5 S 1 A i s r e q u i r e d . )

F R E Q U E N C Y R A N G E : 1 8 t o 1 2 . 4 G H z . I n p u t c o n n e c t o r . T y p e N f e m a l e .

B A N D W I D T H ( 3 d B ) : 2 0 t o 7 0 M H z .

I N S E R T I O N L O S S : I n s e r t i o n l o s s i n t h e p a s s b a n d i s l e s s t h a n 5 d B ; m i n i m u m V S W R i n t h e p a s s b a n d i s l e s s t h a n 2 1 . T h e t i l l e r r e f l e c t s a p p l i e d s i g n a l s a t f r e q u e n c i e s o t h e r t h a n t h e p a s s b a n d , s o t h e V S W R i s v e r y h i g h o u t s i d e t h e p a s s b a n d .

L I M I T I N G L E V E L : ( M a x i m u m i n p u t l e v e l f o r â € ¢ 2 d B s i g n a l c o m p r e s s i o n ) - 2 0 d B m , 1 . 8 G H z t o 2 G H z . + 1 0 d B m , 2 G H z t o 1 2 . 4 G H z .

A B S O L U T E M A X I M U M I N P U T L E V E L : + 3 0 d B m ; 8 5 5 1 I n p u t A t t e n u a t o r m u s t b e s e t t o k e e p p o w e r t o a n a l y z e r i n p u t m i x e r b e l o w 0 d B m t o p r e v e n t d a m a g e t o t h e m i x e r .

U N D E S I R E D R E S P O N S E R E D U C T I O N : ( R e d u c t i o n o f r e s p o n s e s o f t h e 8 5 5 1 t o h a r m o n i c m i x i n g m o d e s o t h e r t h a n t h e o n e p r e s e l e c t e d . ) A t l e a s t 3 5 d B .

C O N T R I B U T I O N T O 8 5 5 1 B F R E Q U E N C Y R E S P O N S E :

P r e s e l e c t o r H a r m o n i c M i x i n g M o d e

A d d i t i o n t o 8 S 5 1 B V a r i a t i o n

1 t ; 2 0 0 M H z I F 1 - ; 2 G H z I F 2 - : 2 G H z I F

 ± 2 . 5 d B o v e r 2 G H z r a n g e  ± 2 5 d B o v e r 2 G H z r a n g e  ± 3 . 5 d B o v e r 2 G H z r a n g e

4 5 d B o v e r 2 G H z r a n g e

â € ¢ 8 5 5 1 i n p u t A t t e n u a t o r s e t t i n g 1 0 d B ; 8 4 4 1 A P e a k i n g c o n t r o l a d j u s t e d a t c e n t e r o f 2 G H z s w e p t r a n g e .

â € ¢ A p p l i e s t o 8 S 5 1 B ' s w i t h s e r i a l n u m b e r 5 5 0 - 0 0 2 8 3 a n d a b o v e ; s l i g h t l y h i g h e r o n e a r l i e r m o d e l s .

C O N N E C T I O N S T O 8 5 1 B / 8 5 5 1 B : 8 5 1 B H o r i z o n t a l O u t p u t . B N C f e m a l e . 8 5 5 1 B P r e s e l e c t o r D r i v e O u t p u t , B N C f e m a l e . 8 5 5 1 B R F I n p u t . T y p e N f e m a l e .

C H A R A C T E R I S T I C S A S A V O L T A G E - T U N A B L E B A N D P A S S F I L T E R

I N T E R N A L S W E E P : C e n t e r f r e q u e n c y c o n t i n u o u s l y v a r i a b l e f r o m 1 . 8 G H z t o 1 2 . 4 G H z . W I D T H : C o n t i n u o u s l y v a r i a b l e f r o m 0 t o 1 0 G H z a b o u t c e n

t e r f r e q u e n c y . ( F r e q u e n c y e x t r e m e s c a n n o t e x c e e d 1 . 8 G H z t o 1 2 . 4 G H z . )

R E P E T I T I O N R A T E : 6 0 H z .

EXTERNAL SWEEP: C E N T E R F R E Q U E N C Y ( f o r e x t e r n a l s w e e p i n p u t l e v e l o f

z e r o v o l t s ) : 1 . 8 G H z t o 1 2 . 4 G H z c o n t i n u o u s l y v a r i a b l e . F i l t e r c a n b e m a n u a l l y t u n e d f r o m 1 . 8 G H z t o 1 2 . 4 G H z .

P R I C E : M o d e l 8 4 4 1 A . $ 2 . 9 5 0 . 0 0

H P M o d e l 8 5 2 A S p e c t r u m A n a l y z e r D i s p l a y S e c t i o n

( W h e n c o n n e c t e d t o B 5 5 1 B R F S e c t i o n )

D I S P L A Y C H A R A C T E R I S T I C S V E R T I C A L D I S P L A Y ( 7 D i v f u l l s c a l e d e f l e c t i o n ) :

S c a l e F a c t o r A c c u r a c y * *

L I N E A R R e l a t i v e V o l t a g e / D i v  ± 3 % f u l l s c a l e S Q U A R E R e l a t i v e P o w e r / D i v - _ 5 % f u l l s c a l e * L O G A R I T H M I C 1 0 d B / D i v c a l i b r a t e d <  ± 0 . 2 d B / d B b u t

o v e r 0 t o 6 0 d B o n n o t m o r e t h a n - _ 2 d B C R T D i s p l a y o v e r f u l l c a l i b r a t e d

6 0 d B C R T d i s p l a y r a n g e

* E x c e p t p u l s e s p e c t r a o n 1 M H z I F b a n d w i d t h . " 0 t o 4 0 " C .

C A T H O D E - R A Y T U B E : T Y P E : P o s t - a c c e l e r a t o r s t o r a g e t u b e , 7 3 0 0 - v o l t a c c e l e r a t

i n g p o t e n t i a l : a l u m i n i z e d P 3 1 p h o s p h o r ; e t c h e d s a f e t y g l a s s l a c e p l a t e r e d u c e s g l a r e .

G R A T I C U L E : 7 X 1 0 d i v i s i o n s ( a p p r o x i m a t e l y 5 . 9 X 8 . 5 c m ) p a r a l l a x - f r e e i n t e r n a l g r a t i c u l e ; 5 s u b d i v i s i o n s p e r m a j o r d i v i s i o n o n m a j o r h o r i z o n t a l a n d v e r t i c a l a x e s .

W A R R A N T Y : C R T s p e c i f i c a t i o n s ( p e r s i s t e n c e , b r i g h t n e s s . s t o r a g e t i m e ) w a r r a n t e d f o r o n e y e a r .

P E R S I S T E N C E : S H O R T : L e s s t h a n 0 . 2 s . V A R I A B L E :

N o r m a l w r i t i n g r a t e m o d e : c o n t i n u o u s l y v a r i a b l e f r o m l e s s t h a n 0 . 2 s t o m o r e t h a n 1 m i n ( t y p i c a l l y t o 2 o r 3 m i n ) .

M a x . w r i t i n g r a t e m o d e : t y p i c a l l y v a r i a b l e f r o m 0 2 t o 1 5 s .

E R A S E : m a n u a l ; e r a s u r e t a k e s a p p r o x i m a t e l y 0 . 5 s ; C R T r e a d y t o r e c o r d i m m e d i a t e l y a f t e r e r a s u r e .

B R I G H T N E S S : G r e a t e r t h a n 1 0 0 f o o t - l a m b e r t s i n N O R M A L o r V I E W ; t y p i c a l l y 5 f o o t - l a m b e r t s i n S T O R E .

S T O R A G E T I M E :

N O R M A L W r i t i n g R a t e M o d e

M A X . W r i t i n g R a t e M o d e

S T O R E M o d e ( d i m d i s p l a y ) V I E W M o d e ( b r i g h t d i s p l a y )

1 m i n u t e t y p i c a l

1 0 s e c o n d s t y p i c a l

C R T B A S E L I N E C L I P P E R : F r o n t p a n e l c o n t r o l p e r m i t s b l a n k i n g o f C R T t r a c e b a s e l i n e t o a l l o w m o r e d e t a i l e d a n a l y s i s o f l o w r e p e t i t i o n r a t e s i g n a l s .

P R I C E : M o d e l 8 5 2 A , $ 3 , 4 0 0 . 0 0

H P M o d e l 8 5 5 1 A S p e c t r u m A n a l y z e r R F S e c t i o n

( W h e n c o n n e c t e d t o 8 5 1 B o r 8 5 2 A D i s p l a y S e c t i o n )

C O A X I A L I N P U T C H A R A C T E R I S T I C S F R E Q U E N C Y R A N G E : 1 0 . 1 M H z t o 1 2 . 4 G H z . I n p u t c o n

n e c t o r . t y p e N f e m a l e

S E N S I T I V I T Y / s i g n a l p o w e r + n o i s e p o w e r

n o i s e p o w e r

I F b a n d w i d

= 2 ; 1 0 k H z

w i d t h ) :

1 0 . 1 M H z t o 1 . 8 G H z - 1 0 0 d B m , f u n d a m e n t a l m i x i n g

1 , 8 t o 4 . 2 G H z - 1 0 0 d B m , f u n d a m e n t a l m i x i n g ( u s i n g 2 0 0 M H z 1 s t I F )

- 9 2 d B m , s e c o n d h a r m o n i c m i x i n g

- 1 0 0 d B m , f u n d a m e n t a l m i x i n g

à - â € ” 9 0 d B m , s e c o n d h a r m o n i c m i x i n g j â € ” 8 6 d B m , t h i r d h a r m o n i c m i x i n g

- 8 7 d B m , s e c o n d h a r m o n i c m i x i n g

1 0 t o 1 2 . 4 G H z - 8 0 d B m , t h i r d h a r m o n i c m i x i n g

W i t h s o u r c e s t a b i l i t y b e t t e r t h a n 1 k H z , g r e a t e r s e n s i t i v i t y c a n b e a c h i e v e d u s i n g n a r r o w e r I F b a n d w i d t h .

F R E Q U E N C Y R E S P O N S E : ( I n c l u d e s m i x e r a n d R F a t t e n u a t o r r e s p o n s e w i t h a t t e n u a t o r s e t t i n g > 1 0 d B ) .

F r e q u e n c y M i x i n g M o d e R e l . G a i n F l a t n e s s F l a t n e s s R a n g e n * I F ( a p p r o  » . } " f u l l r a n g e 1 0 0 M H z

2 . 4 t o 4 G H z

4 t o 6 G H z

6 t o B G H Z

8 t o 1 0 G H z

1 0 . 1 M H z t o 1 . 8 G H z 1 . 8 t o 4 . 2 G H z 2 . 4 t o 4 G H z 4 t o 6 G H z

8 G H z 8 t o

1 0 G H z 1 0 t o 1 2 4 G H z

1 2 G H z 0 d B

1 â € ¢ 2 0 0 M H z 0 d B

2 2 G H z - 6 d B

 ± 1 d B  ± 1 d B

 ± 3 . 5 d B  ± 2 . 0 d B

 ± 2 d B  ± 1 . 5 d B

2 G H z 0 d B Â ± 1 . 5 d B Â ± 1 d B 2 G H z - 1 0 d B Â ± 2 d B Â ± 1 . 5 d B 2 G H z - 1 4 d B Â ± 2 . 5 d B Â ± 1 . 5 d B

2 G H z - 1 3 d B Â ± 3 . 5 d B Â ± 1 . 5 d B

2 G H z - 1 0 d B Â ± 5 d B Â ± 3 d B

* n = L O h a r m o n i c . " T h e s i g n a l s d i s p l a y e d a m p l i t u d e s o f e q u a l - a m p l i t u d e i n p u t s i g n a l s

f o r v a r i o u s h a r m o n i c m i x i n g m o d e s .

I M A G E S E P A R A T I O N : 4 G H z ( 2 G H z F i r s t I F ; 4 0 0 M H z s e p a r a t i o n w h e n u s i n g 2 0 0 M H z I F ) .

R E S I D U A L R E S P O N S E S ( n o i n p u t s i g n a l ) : L e s s t h a n - 9 0 d B m r e f e r r e d t o S i g n a l I n p u t o n f u n d a m e n t a l m i x i n g ( - 8 5 d B m w h e n L O i s w i t h i n 6 0 M H z o f 2 o r 4 G H z ) .

M A X I M U M I N P U T P O W E R ( f o r 1 d B s i g n a l c o m p r e s s i o n ) :

I n p u t A t t e n . S e t t i n g T y p i c a l M a x . I n p u t ( P e a k o r A v e r a g e )

0 d B 1 0 d B 2 0 d B 3 0 d B 4 0 - 6 0 d B

- 5 d B m + 5 d B m

+ 1 5 d B m + 2 5 d B m + 3 0 d B m

M I X E R D I O D E : S t a n d a r d D - 5 2 8 2 - C r e p l a c e a b l e f r o m t h e f r o n t p a n e l .

P R I C E : M o d e l 8 5 5 1 B , 5 7 , 5 5 0 . 0 0 .

M A N U F A C T U R I N G D I V I S I O N : H P M I C R O W A V E D I V I S I O N 1 5 0 1 P a g e M i l l R o a d P a l o A l t o , C a l i f o r n i a 9 4 3 0 4

14 © Copr. 1949-1998 Hewlett-Packard Co.

Now assume a smaller 1-GHz signal is applied to the input port. This signal propagates to the diode uninter rupted by the cavities because they are tuned to 2 GHz.

At the diode, the 1-GHz signal sees a large reflection coefficient switching from one phase to another as the diode is switched on and off by the local oscillator. In the ON state the diode looks like a small inductive reactance in series with some spreading resistance. In the OFF state it looks like a large capacitive reactance. The non linear process which results from the 1-GHz signal meet ing the phase-switching reflection coefficient produces currents at 3 — 1 = 2 GHz and 3 + 1=4 GHz. Waves at these frequencies travel back up the transmission line, to the left in Fig. 9.

The two cavities perform the task of taking the 2-GHz power off the main line and routing it to the IF amplifier. Cavity #2 is loaded by the 50 i> input resistance of the IF amplifier via a coupling loop in the cavity. To 2-GHz signals, cavity #2 presents a resistance in series with the main line.

Cavity #1 is unloaded, so it is a virtual open circuit at 2 GHz. Cavity #1 is a quarter wavelength ("at 2 GHz) from cavity #2. so the high impedance of cavity #1 is transformed into a short circuit just to the left of cavity #2. Looking left into cavity #2, therefore, 2-GHz IF signals see just the 50 f! input resistance of the IF ampli fier across the main line. Thus all of the IF power goes to the IF amplifier. This scheme routes the 2-GHz power traveling left from the diode to the IF amplifier with a loss of only 0.3 dB, and does it without perturbing the

main line at frequencies other than 2 GHz. Cavity #1 also helps prevent 2-GHz input signals from

going directly to the IF amplifier and causing the entire baseline of the display to lift. This cavity is a reflective trap at 2 GHz.

Diode on Front Panel

The length of transmission line between the diode and the rest of the structure is inconsequential. Thus the diode can be placed on the front panel. Should too much power be applied at the input port, thereby destroying the diode, a new diode can be installed immediately and the instrument returned to service.

The local-oscillator signal in the HP spectrum analyzer is generated by a backward-wave oscillator that has large variations in its output power as it is swept from 2 to 4 GHz. These variations ordinarily would cause the bias on the mixer diode to vary, thereby causing the diode's conduction angle to vary. (The conduction angle is the portion of one LO cycle that the diode is on.) However, for the mixer to have a flat frequency response, the diode's conduction angle must be held constant.

For first- and third-harmonic mixing the desired con duction angle is 180°. To achieve this conduction angle it is necessary to keep the dc voltage across the diode very close to zero volts, regardless of diode current. What is needed is a dc return which acts as a near short circuit at low frequencies, and a near open circuit at frequencies between .01 and 12.4 GHz. The classical inductive dc return couldn't be used because it is virtually impossible

Fig. diode Two-cavity design of new coaxial input mixer allows mixer diode to be located on front panel, instead of being integral to mixer structure.

15 © Copr. 1949-1998 Hewlett-Packard Co.

Fig. 10. Exterior view / new mixer. Diode is inside c at left.

to build inductive dc returns which have three decades of bandwidth. The problem was solved by bridging the line with a low-capacitance 1000 Ã-Ã- resistor, and by removing the resistor at the low frequencies at which the diode bias tends to vary (sweep frequencies and harmonics of sweep frequencies). The resistor is removed by means of a very simple three-transistor negative-resistance generator. When the current through the diode changes because of changing local-oscillator power, the voltage across the 1000 resistor changes too. But the voltage across the negative resistance generator changes an equal and oppo site amount, so the dc voltage across the diode remains close to zero.

When second-harmonic mixing is used, the —1000 generator is shorted out so that the dc return has a resist ance of f 1000 tj. This allows the diode to bias itself to a conduction angle of approximately 90°.

The mixer has a conversion loss of about 9 dB on fundamental mixing, 15 dB on second harmonic, and 22 dB on third harmonic.

Acknowledgments

The H P Model S44 1 A Preselector was designed under the direction of Harley L. Halverson and Roderick Carl son. Mechanical design was by Edgar C. Hurd.

The initial design of the Model 852A Display Unit was done by Robert E. Jacobsen. Final design was done by Charles W. Cook.

Russell B. Riley provided many valuable suggestions during the design of the new mixer. John E. Nidecker did the product design of the mixer. S

f

John J . Dupre Jack Dupre came to HP in 1964 , a f te r g radua t i ng f r om Ca l i f o rn i a S ta te Po l y t echn i c Co l l ege w i t h a BS deg ree i n e l ec t r on i c eng inee r i ng . He des igned the 8441A Prese lec to r , then tu rned to h i s p resen t ac t i v i t y , deve lop i ng m i c rowave t r ans i s t o r osc i l la tors . In June, 1967 Jack rece ived h is MS deg ree i n e l ec t r i ca l eng inee r i ng f r om S t a n f o r d U n i v e r s i t y . H e h a d a t t e n d e d

S tan fo rd on t he Hono rs Coope ra t i ve p rog ram w h i l e w o r k i n g a t H P . Jack i s a member o f IEEE. He i s mar r ied and has two ch i l d ren .

Richard C. Kei ter : R i t K e i t e r e a r n e d h i s B S d e g r e e i n

e l e c t r i c a l e n g i n e e r i n g a t t h e Un i ve rs i t y o f Co lo rado , g radua t i ng i n 1965. He came to HP the same year .

I A t HP , R i t wo rked on t he 431 C Power Me te r and the 8402B Power Me te r Ca l i b ra to r , and des igned t he new

à coaxial input mixer for the HP spec t r um ana l yze r . Now a p ro j ec t eng inee r i n t he M ic rowave Labora to ry , he i s deve lop ing

m ic rowave m ixe rs and so l i d - s ta te sou rces . R i t i s a member o f S igma Tau and E ta Kappa Nu . He has been a t tend ing S tan fo rd Un ive rs i t y on the Honors Coopera t i ve p rog ram, and w i l l r ece i ve h i s MS degree nex t June . He i s mar r ied , and he and h is w i fe share an in te res t in f l y ing .

John R. Page, Jr. John Page rece i ved h i s BS deg ree i n e l e c t r i c a l e n g i n e e r i n g f r o m S t a n f o r d Un ivers i ty in 1959. He then en tered the U.S. Navy and served f rom 1960 to 1961 as an o f f i cer on the a i rc ra f t ca r r ie r USS Cora l Sea . Then i t was

| b a c k t o S t a n f o r d , w h e r e h e r e c e i v e d f ; h i s MS deg ree i n e l ec t r i ca l

eng ineer ing in 1962. In 1963 John jo ined the HP Mic rowave D i v i s i o n , h a v i n g p r e v i o u s l y w o r k e d

two summers the re . He he lped des ign the 312A Wave Ana l yze r and the 8708A Synch ron i ze r , and in 1965 assumed p ro jec t respons ib i l i t y fo r the 312A. He was a lso the p ro jec t leader fo r the 852A D isp lay Un i t . A t p resen t , John i s work ing on spec t rum ana lyze r deve lopment , aga in as p ro jec t l eader . John i s mar r ied , has two ch i l d ren , and en joys o u t d o o r s p o r t s a n d p h o t o g r a p h y .

HEWLETT-PACKARD JOURNAL ® JANUARY wea volume 19 -Number 5 T E C H N I C A L C A L I F O R N I A F R O M T H E L A B O R A T O R I E S O F T H E H E W L E T T - P A C K A R D C O M P A N Y P U B L I S H E D A T 1 5 0 1 P A G E M I L L R O A D P A L O A L T O C A L I F O R N I A 9 4 3 0 4

© Copr. 1949-1998 Hewlett-Packard Co.

Three and One-Half Decades in One Clean SweepAdvances in Spectrum AnalysisHow a YIG Filter works