Agilent PN 8753-2

RF Component Measurements—Mixer Measurements Using the 8753B Network AnalyzerProduct Note

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This note describes several procedures and hardware setups for measuring theperformance of a mixer or frequency translator using the Agilent Technologies8753B vector network analyzer.

The measurements described in this note are conversion loss, conversion com-pression, amplitude and phase tracking, two-tone third order intermodulationdistortion, isolation (feedthrough), and SWR.

Vector network analyzers have typically been used for measuring the transmis-sion and reflection characteristics of linear components and networks. The8753B simplifies and speeds the testing of non-linear devices such as mixers(the focus of this note) and amplifiers (see Product Note 8753-1, AmplifierMeasurements Using the Agilent 8753, lit. no. 5956-4361).

Traditionally, vector network analyzers working at a single stimulus andresponse frequency were unable to test the transmission characteristics of amixer. The 8753B has the ability to offset or decouple its receiver from its owninternal synthesized source. This enables you to stimulate a device over onefrequency range and view its response over another. This, along with its vectornetwork analyzer capabilities, makes the 8753B a significant enhancement toany environment where comprehensive mixer testing is required.

Because a mixer is a 3-port non-linear device it is impossible to take advantageof traditional 2-port vector accuracy enhancement. However, there are neces-sary considerations when making any mixer measurement: IF port filtering,reducing the number of unwanted signals that enter the receiver, attenuationat all mixer ports, reducing refections, and frequency selection (frequency listmode). These techniques will be discussed in greater detail in the “Measure-ment Considerations” section of this note.

Throughout the procedures described in this note, front panel keys appear inbold type, e.g. [MENU]. Softkeys such as POWER appear in bold italics, e.g.[POWER].

Typical equipment list used to make the measurements in this noteNetwork analyzer 8753BTransmission/Reflection test set 85044AExternal synthesized signal generator 8656A/B

(8657A/8642A/B could also be used.)Power meter GPIB 436A/7B/8A Mixer Under Test Mini-Circuits ZFM-15 or equivalentVarious cables, filters, amplifiers, connectors, and attenuators

Introduction

Table of contents

2 Introduction 3 Mixer term definitions4 Measurement considerations 5 Conversion loss 8 Conversion compression 9 Amplitude and phase tracking

10 Two-tone third order intermodulation Distortion

12 Isolation 13 SWR/Return loss 14 Appendix. Spur analysis and prediction

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Mixer term definitions

Figure 1a. Spectrum of RF, LO, and IF sig-nals present in mixer measurements

Figure 1b. Plot of conversion loss and IFoutput power as a function of RF inputpower level, note that the IF output powerincreases linearly with the increasing RFsignal, until mixer compression begins andthe mixer saturates.

Figure 1c. Diagram showing the signalflow in a mixer, note that RF and LOfeedthrough signals may appear at themixer IF output, together with the desiredIF signal.

Conversion lossConversion loss is the measure ofefficiency of a mixer. It is the ratio of sideband IF power to RF signalpower, and is usually expressed indB. The mixer translates the incomingsignal, RF, to a replica, IF, displacedin frequency by the local oscillator,LO. This frequency translation exactsa penalty that is characterized by aloss in signal amplitude and the gen-eration of additional sidebands. For a given translation, two equal outputsignals are expected, a lower sidebandand an upper sideband (Figure 1a).

Conversion compressionConversion compression is a measureof the maximum RF input signal levelfor which the mixer will provide lin-ear operation. The conversion loss isthe ratio of the IF output level to theRF input level, and this value remainsconstant over a specified input powerrange. When the input power levelexceeds a certain maximum, the con-stant ratio between IF and RF powerlevels will begin to change. The pointat which the ratio has decreased 1 dBis called the 1-dB compression point.See Figure 1b.

Amplitude and phase trackingThe match between mixers is definedas the absolute difference in ampli-tude and/or phase response over a

specified frequency range. The track-ing between mixers is essentially howwell the devices are matched over aspecified interval. This interval maybe a frequency interval or a tempera-ture interval, or a combination ofboth.

Third order intermodulation distortionThis term describes the distortion ofthe mixer’s conversion loss, causedby two or more sinusoidal signalsinteracting at the mixer’s RF port.The two-tone third order distortionterm is the amount of the signal atthe IF port of the mixer due to thirdorder frequency terms. This is usuallyexpressed relative to the desired IFmixing product, (dBc). These thirdorder terms are given by (2RF1 – RF2)±LO, and (2RF2 – RF1) ±LO.

RF1 = Fixed RF frequency # 1RF2 = Fixed RF frequency # 2LO = Local oscillator frequency

IsolationIsolation is the measure of signalleakage in a mixer. Feedthrough isspecifically the forward signal leak-age to the IF port. High isolationmeans that the amount of leakage orfeedthrough between the mixer’sports is very small. Figure 1c dia-grams the signal flow in a mixer.

The LO to RF isolation and the LOfeedthrough are typically measuredwith the third port terminated in 50 ohms. Measurement of the RF feed-through is made as the LO signal isbeing applied to the mixer.

SWR/Return lossReflection coefficient (G) is definedas the ratio between the reflectedvoltage (Vr) and incident voltage (Vi).Standing wave ratio (SWR) is definedas the ratio of maximum standingwave voltage to the minimum stand-ing wave voltage, and can be derivedfrom the reflection coefficient (G)using the equation shown below.

G = Vr/Vi

1 +|G|SWR =1 –|G|

Return loss is equal to –20 log |G|.

Because mixers are three-port devices,making an SWR measurement on oneis more complicated than making anSWR measurement on a two-portdevice. The operating conditions the mixer will encounter during useshould be the test levels at which theSWR measurements are made. Forexample, to make RF port SWR meas-urements, the LO must be connectedand set at the desired frequency andpower level, and the IF port must beterminated in 50 ohms.

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In mixer transmission measurementsyou have RF and LO inputs and an IFoutput. Also emanating from the IFport are several other mixing prod-ucts of the RF and LO signals. Inmixer reflection measurements, leak-age signals propagate from one mixerport and appear at the other twomixer ports. These unwanted mixingproducts or leakage signals can causedistortion by mixing with a harmonicof the Agilent 8753B’s first down con-version stage. To ensure that meas-urement accuracy is not degraded,certain frequencies must be filteredor avoided by frequency selection.Attenuators placed at all mixer portscan be used to reduce mismatchuncertainties.

For frequency offset measurementsmade on the 8753B, it is necessary tochoose the 8753B’s source to supplythe highest frequency in the measure-ment, whether it is being used todrive the RF or LO of the mixer. It isalso necessary to configure the meas-urement so that the lowest frequencyis incident upon the 8753B’s receiver;this simply means measuring thelower of the two IF mixing products.

FilteringProper filtering between the mixer’sIF port and the receiver’s input portcan eliminate unwanted mixing andleakage signals from entering the

analyzer’s receiver. Figure 2a shows aplot of mixer conversion loss whenproper IF filtering was neglected. Fig-ure 2b shows the same mixer’s con-version loss with the addition of alow pass filter at the mixer’s IF port.Filtering is required in both fixed andbroadband measurements, but will bemore easily implemented in the fixedsituation. Therefore, when configur-ing broadband (swept) measurementsyou may need to trade some measure-ment bandwidth for the ability tomore selectively filter signals enteringthe 8753B’s receiver.

Attenuation at mixer portsWhen characterizing linear devices,(single test frequency) vector accu-racy enhancement can be used tomathematically remove all systematicerrors, including source and load mis-matches, from the measurement. Thisis not possible when the device youare characterizing is a mixer operat-ing over multiple frequency ranges.Therefore, source and load mis-matches are not corrected for andwill add to overall measurementuncertainty.

As in a scalar measurement system,to reduce the measurement errorsassociated with the interactionbetween mixer port matches and sys-tem port matches, it is advisable to

place attenuators at all of the mixer’sports. Figure 2c shows a plot of sweptconversion loss where no attenuationat mixer ports was used. The rippleversus frequency is due to source andload mismatches. In contrast, Figure2b made use of appropriate attenua-tion at all mixer ports. Extra careshould be given to the selection of theattenuator located at the mixer’s IFport to avoid overdriving the receiver.For best results, the value of thisattenuator should be chosen so thatthe power incident on the 8753B’sreceiver ports is less than -10 dBm.

Frequency selectionChoosing test frequencies (frequencylist mode) can reduce the effect ofspurious responses on measurementsby avoiding frequencies that produceIF signal path distortion.

The first step in avoiding or eliminat-ing spurs is determining at what fre-quencies they will occur. To aid youin predicting where these frequencieswill occur, a spur prediction programis included in Appendix 1 of thisnote. Although this spur predictionprogram is specialized for the meas-urement of the swept IF/fixed LOresponse of a mixer, it can easily bemodified to accommodate othermeasurement configurations.

Measurement considerations

Figure 2a. Plot of a mixer’s conversion lossvs. IF frequency without the use of appro-priate IF signal path filtering, resulting inunusable data

Figure 2b. Plot of a mixer’s conversion lossvs. IF frequency with proper IF signal pathfiltering, and attenuation at all mixer ports

Figure 2c. Plot of a mixer’s conversion lossvs. IF frequency neglecting attenuation atmixer ports. The frequency ripple seen isdue to mixer and system port mismatches.

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Fixed IF conversion lossThe simplest of all conversion lossmeasurements is the fixed IF/fixedLO measurement where all three fre-quencies are held constant. Figure 3shows the block diagram for a fixedIF/fixed LO conversion loss measure-ment.

The frequencies to be used in thismeasurement are: RF = 1400 MHz LO = 800 MHz IF = 600 MHz

In all conversion loss measurements,the IF and LO frequencies are entereddirectly as parameters, while the RFfrequency is entered by adding the IFand LO frequencies using frequencyoffset mode.

Frequency offset modeThis mode of operation allows you tooffset the 8753B’s source, by a fixedvalue, above the 8753B’s receiver.This allows you to stimulate a deviceunder test at one frequency and viewits response at another frequency.This mode of operation has an RFsource frequency limit of 3 GHz.

Measurement procedure1. Connect the instruments as shownin Figure 3.

2. Press [PRESET] on the front panelsof the 8753B and the local oscillator(LO) source.

3. From the front panel of the 8753B,set the desired IF frequency and RFsource output power to be used.

[MENU] [CW FREQ] [600] [M/µ] [POWER] [6] [x1] [MEAS] [R]

4. On the external signal generator,select the desired LO frequency andpower level to be used in this meas-urement.

[CW] [800] [MHz][POWER] [13] [dBm]

5. Turn frequency offset on to set upa constant offset between the IF andRF signals. This sets the RF sourcefrequency.

[SYSTEM][INSTRUMENT MODE][OFFSET VALUE][800] [M/µ][FREQ OFFS ON]

6. Figure 4 shows the attenuated out-put power of the mixer’s IF at thereceiver. The conversion loss of themixer is found by subtracting theattenuation from the total lossbetween the RF source and IFreceiver.

Source power = 6 dBmOutput power = –17.5 dBTotal loss = 23.5 dBTotal attenuation = 16 dBConversion loss = 7.5 dB

Swept IF measurementsOne of the primary contributions ofthe 8753B to mixer testing is its abil-ity to make a swept IF conversionloss measurement. Frequency transla-tors can also be measured using thetechniques described in this note, forexample the downconverter modulein Figure 5.

Conversion loss

Figure 3. Block diagram for a fixedIF/fixed L0/fixed RF conversion lossmeasurement

Figure 4. Plot of a mixer’s fixed IF/fixedLO/fixed RF output power

Figure 5. Block diagram of a typical down-converter module which can be character-ized using the techniques described in thisnote.

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Figure 6. Block diagram for a B/R responsecalibration and device normalization

Figure 8. Plot of swept IF/fixed LO conver-sion loss of a mixer

Figure 7. Block diagram for a sweptIF/fixed LO conversion loss measurement

Measurement procedureThe following procedure describes thesteps necessary to perform a swept IFconversion loss measurement usingthe setups in Figures 6 and 7. Thefirst five steps in this procedure areused to measure the response of theIF signal path, so that its responsecan be mathematically removed fromthe conversion loss measurement thatfollows.

1. Connect the hardware in Figure 6with a thru connection between thepower splitter and the receiver’s Bport.

2. Press [PRESET] on the front panelsof the 8753B and the local oscillator(LO) source.

3. From the front panel of the Agilent8753B, set the desired IF frequencyrange and RF output power to beused in this measurement.

[START] [100] [M/µ][STOP] [1.0] [G/n][MENU][POWER] [6] [x1]

4. Using the keystrokes shown below,perform a frequency response cali-bration.

[MEAS][B/R][CAL][CALIBRATE MENU][RESPONSE][THRU][DONE: RESPONSE]

5. Leaving the thru cable in placeconnect the IF attenuator, filter, andcable between the power splitter andthe receiver’s B port. The IF low passfilter was chosen not only to elimi-nate unwanted mixing products fromentering the analyzer’s receiver, butalso to pass the largest of the IF fre-quencies. Store the response intomemory.

[DISPLAY][DATA → MEMORY]

This step measures the frequencyresponse of the components in the IFsignal path (attenuator, filter, andcable) and stores it into memory. Thismemory trace will be used to removethe frequency response of these com-ponents from the conversion lossmeasurement to be made with thesetup in Figure 7.

6. View the absolute input power tothe R channel.

[MEAS][R]

7. Connect the hardware as shown inFigure 7, terminating the open port ofthe power splitter with a 50-ohmload.

8. Set the external local oscillator(LO) source to the desired fixed fre-quency and power level.

[CW] [1.5] [GHz][POWER] [13] [dBm]

9. Remove the frequency response ofthe IF attenuator, filter, and cablefrom the measurement by viewing[DATA/MEM].

[DISPLAY][DATA/MEM]

10.Turn frequency offset on to set aconstant offset between the IF and RFsignals. This sets the RF source’s fre-quency range. In this example, the RFfrequency range is 1.6 GHz to 2.5 GHz.

[SYSTEM][INSTRUMENT MODE][OFFSET VALUE][1500] [M/µ][FREQ OFFS ON]

11. Since the mixer’s RF input powerwas chosen to be 0 dBm and the lossdue to the IF components wasremoved, the resulting display showsthe swept IF conversion loss of themixer versus IF frequency. A plot ofthis is shown in Figure 8.

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Optional procedureTo enhance measurement accuracy,replace the 50-ohm load at the powersplitter with the power meter andpower meter sensor, as shown in Fig-ure 7. Perform a one sweep powermeter calibration. This will level thepower splitter’s output power to auser specified value over the fre-quency range of the measurement.

Zero the power meter. Prepare the8753B to interface with the powermeter.

[LOCAL][SYSTEM CONTROLLER][SET ADDRESSES][ADDRESS: P MTR/HPIB][#] [x1]

Press the power meter softkey (shownbelow) until the desired power meterhas been chosen.

[POWER MTR: [438/437]]

Perform the power meter calibration.[CAL][PWR METER][ONE SWEEP][CAL POWER] [0] [x1][TAKE CAL SWEEP]

Once this power calibration is com-plete the power at the mixer’s RFport is leveled to 0 dBm.

Disconnect the power meter and terminate this open port with the 50-ohm load.

NOTE: For more information on powermeter calibration see the Agilent 8753BSystem Operating and ProgrammingManual (08753-90119). For moreinformation on power meter meas-urements and accuracy see the appro-priate power meter manual.

Conversion loss greater than 35 dBSince the dynamic range of thereceiver’s R channel is limited, amodification of this procedure isrequired for conversion loss measure-ments where the R channel input isless than –35 dBm. The measurementprocedure is the same as for the con-version loss measurements previously

discussed. Figure 9 shows the hard-ware configuration for this measure-ment. Note that the mixer under testis being measured on the B channelwhich has greater than 35 dBdynamic range, while the referencesignal on channel R (used for phaselock) has been increased by an exter-nal amplifier.

Fixed IF stepped LO & RFAn extension of the previous meas-urements is the case of fixed IF/stepped LO and RF frequencies. Figure 10 shows the hardware config-uration for this fixed IF measurement.The simplest way to make this type ofmeasurement without the use of anexternal controller is through the useof the 8753B’s test sequence function.An excerpt from a sequence writtento control two external synthesizersin tuned receiver mode appears below.

Tuned receiver modeIn situations when the analysis of aspecific signal or mixing product isnecessary, the 8753B’s tuned receivermode allows you to tune the 8753B’sreceiver to an arbitrary frequencyand analyze a signal without phaselocking to it. This is only possible ifthe signal we wish to analyze is at anexact known frequency. Therefore,the RF and LO must be synthesizedand synchronized with the 8753B’stime base.

Figure 11 shows a plot of the steppedLO and RF fixed IF conversion loss ofa mixer.

Figure 9. Block diagram for a conversionloss measurement greater than 35 dB

Figure 10. Block diagram for a Fixed IFstepped RF and LO measurement

Figure 11. Plot of fixed IF/stepped LO andRF conversion loss of a mixer

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The following example uses a ratio ofmixer output to input power to locatethe mixer’s 1-dB compression point.Included is an optional accuracyenhancement step using power metercalibration.

Measurement procedure1. Connect the Agilent 8753B’s sourcethrough the 20-dB IF attenuator andlow-pass filter to the receiver’s Rport.

2. Press [PRESET] on the front panelsof the 8753B and the LO source.

3. From the front panel of the 8753Bset the desired fixed frequency, powerrange, and measurement specifica-tions.

[MENU][SWEEP TYPE MENU][POWER SWEEP][RETURN][CW FREQ][700] [M/µ][START] [0] [31][STOP] [15] [31][MEAS][R]

4. Store a trace of receiver power vs.source power into memory and view[DATA/ MEM]. This removes the lossbetween the IF port of a mixer andthe receiver, and will provide a linearpower sweep for use in subsequentmeasurements.

[DISPLAY][DATA → MEMORY][DATA/MEM]

5. Connect the instruments as shownin Figure 12.

6. Set the LO source to the desiredfixed frequency and power level.

[CW] [800] [MHz][POWER] [13] [dBm]

7. Turn on a frequency offset equal tothe LO’s fixed frequency. This speci-fies the RF source, frequency.

[SYSTEM][INSTRUMENT MODE][OFFSET VALUE][800] [M/µ][FREQ OFFS ON]

8. The resulting display shows themixer’s output power as a function ofits input power.

9. Set up an active marker to searchfor the 1-dB compression point of themixer.

[SCALE REF][AUTO SCALE][MKR]

Move the marker to a point of zeroslope on the trace (zero slope indi-cates the mixer’s linear region ofoperation).

[MKR ZERO][MKR FCTN][MKR SEARCH ON][TARGET][-1] [31][MKR][∆ MODE MENU][∆ MODE OFF]

The following display (Figure 13)shows the mixer’s 1-dB compressionpoint. By changing the target value,you can easily locate other compres-sion points (e.g., 0.5 dB, 3 dB).

Optional procedureTo enhance measurement accuracy,insert a power splitter between theRF source and the mixer’s RF port.Connect the power meter (shown inFigure 12), to the power splitter’sopen port. Perform a one sweeppower meter calibration.

For the necessary procedure, see theswept IF conversion loss measure-ment on page 6.

Once this procedure is complete, themixer’s RF input power is referencedto a power meter standard.

Conversion compression

Figure 12. Block diagram for a conversioncompression measurement

Figure 13. Plot of a mixer’s conversioncompression, including a marker locatingthe 1 dB compression point

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The 8753B can be used to measureswept IF amplitude and phase track-ing between mixers over a specifiedfrequency interval. A block diagramof the hardware configuration neces-sary for this measurement is shownin Figure 14.

In this measurement, we comparemixers having the same stimulus andresponse signal paths, so that any dif-ference seen in response is due to themixers and not the measurement sys-tem. Mixer B is replaced with themixer that you wish to compare to it,while mixer A remains in place, usedby all mixers as a reference.

Measurement procedure1. Connect the hardware, includingmixers A and 13, as shown in Figure 14.

2. Press [PRESET] on all instruments.

3. Set the RF source output powerand frequency range of the IFreceiver.

[MENU] [POWER] [6] [31][START] [100] [M/µ] [STOP] [1.0] [G/n]

4. Set the fixed frequency and outputpower of your LO source.

[CW] [1.5] [GHz] [POWER] [16] [dBm]

5. Set the RF source frequency rangeusing frequency offset mode. In thismeasurement the RF range covers 1.6 GHz to 2.5 GHz.

[SYSTEM][INSTRUMENT MODE][OFFSET VALUE] [1.51] [G/n] [FREQ OFFS ON]

6. Display the magnitude and phaseof the IF output of mixer B divided bythat of mixer A. Store this data intomemory an display future data rela-tive to it. This display shows two flatlines.

[CH 1] [MEAS] [B/R][FORMAT] [LOG MAG][DISPLAY] [DUAL CHAN ON][DATA → MEMORY] [DATA/MEM][CH 2] [MEAS] [B/R] [FORMAT] [PHASE] [DISPLAY] [DATA → MEMORY] [DATA/MEM]

7. Remove mixer B from the testsetup and replace it with the mixeryou wish to compare it to (in thiscase mixer C). The resulting display is the amplitude and phase matchbetween the third mixer and the orig-inal mixer that it replaced (mixer C/mixer B), see Figure 15.

When comparing several mixers, it isgood measurement procedure to peri-odically reinsert the original mixer,(mixer B) and observe the display.This display should look as it did instep 6 of the above procedure. Thisprocedure will verify that your meas-urement system is time invariant.

Amplitude and phase tracking

Figure 14. Block diagram for an amplitudeand phase tracking measurement

Figure 15. Plot of amplitude and phasetracking between two mixers

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When two signals are applied to theinput of a device, they interact to pro-duce third order intermodulation dis-tortion. In this measurement proceduretwo closely spaced fixed frequenciesof equal amplitude are input at themixer’s RF port, while a single fre-quency is used to drive the mixer’sLO port. The size of the third orderintermodulation distortion productsrelative to the desired IF frequencieswill be measured (dBc). The Agilent8753B is used to perform this meas-urement typically made with a spec-trum analyzer. This measurementmakes use of the 8753B’s tunedreceiver mode, previously discussedin the fixed IF/stepped RF and LOmeasurement (page 7).

RF1 = Fixed RF frequency #1 RF2 = Fixed RF frequency #2 LO = Local oscillator frequency IF1 = RF1 – LOIF2 = RF2 – LO

3rd order intermodulation products 3rd1 = 2RF1 – RF2 – LO 3rd2 = 2RF2 – RF1 – LO

Figure 16 shows a block diagram for athird order INID measurement madewith the 8753B. The filters, attenua-tors, and amplifiers provide isolationand remove unwanted signals fromthe system. These are essential toaccurate measurements, because signal levels as low as 60 dBc candegrade third order IMD measurements.

Measurement procedure1. Connect the hardware in Figure 6(Page 6) with a thru connectionbetween the power splitter and thereceiver’s B port.

2. Press [PRESET] on all instruments.

3. Select the fixed frequencies and out-put power levels for all three externalsignal sources. To ensure accuratemeasurement independent of sourcedistinction, power levels should bechosen given third order IMD side-bands between –40 and –50 dBc. Thepower levels below were chosen for a+30 dB amplifier gain.

RF1: [CW FREQ] [1] [GHz] [POWER] [–24] [dBm]

RF2: [CW FREQ] [1.00003] [GHz][POWERI [–17] [dBm]

LO: [CW FREQ] [.9] [GHz][POWER] [–10] [dBm]

Two-tone third order intermodulation distortion

Figure 16. Block diagram for a two-tonethird order intermodulation distortion(IMD) measurement

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4. Set up a frequency list (frequencylist mode) of points where the 8753B’sreceiver is to take data. This includesthe four points of interest IF1, IF2,3rd1, and 3rd2, and the two endpointsthat are used to center the plot. Inthis example RF1 is 1 GHz, RF2 is1.00003 GHz, the LO is 0.9 GHz, andthe two third order products are.09997, and .10006 GHz.

Frequency list data points: .09992 GHz .09997 GHz .10000 GHz .10003 GHz .10006 GHz.10011 GHz [MENU] [SWEEP TYPE MENU][EDIT LIST]

Enter frequency points. The key-strokes found directly below will berepeated for each point in the list.

[ADD][CENTER] [#] [G/n][SPAN] [2] [k/m] [NUMBER of POINTS][5] [31] [DONE]

Next point: [DONE][LIST FREQ]

5. Reduce the IF bandwidth to lowerthe noise floor of the trace, andresolve the measurement data.

[AVG][IF BW [3000 Hz][100] [31]

6. With the configuration in Figure 6still connected, repeat steps 4 and 5of the swept IF conversion loss meas-urement (Page 6).

7. Connect the instruments as shownin Figure 16, tying all time basestogether (EXT REF). To minimize theeffect of system distortion, it is sug-gested that attenuation at the mixer’sIF port be chosen to give receiverinput levels of -10 dBm or less.

8. View the absolute power present atport B relative to the trace of the IFattenuator, filter, and cable stored inmemory.

[MEAS][B][DATA/MEM]

9. Select tuned receiver mode. Thismode of operation allows the 8753Bto receive external signals withoutthe need to phase lock.

[SYSTEM][INSTRUMENT MODE][TUNED RECEIVER]

10. Figure 17 shows a comparison oftwo-tone third order IMD measuredon a spectrum analyzer and on the8753B.

If the displayed third order IMD prod-ucts are of unequal magnitude, orappear to be unstable, change the fre-quency spacing between the RF inputsignals until the display stabilizes.

Third order intercept point (TOI)Third order intercept point can becalculated using the equation below.TOI = DR/2 + (P in), where (P in) isthe RF input signal level and DR isthe difference between mixer IF out-put power and third order productmixer output power (dBc).

Figure 17. Comparison of third order IMDmeasurements made on a spectrum ana-lyzer and the Agilent 8753B

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The equipment configuration necessaryto measure isolation (feedthrough)between mixer ports is identical tothat used in a transmission (B/R)measurement, shown in Figure 18.

Measurement procedureLO to RF isolation1. Connect the hardware as shown inFigure 18.

2. Preset the Agilent 8753B by press-ing [PRESET].

3. Using the 8753B’s source as yourlocal oscillator, select the LO fre-quency range and source outputpower.

[START] [10] [M/µ][STOP] [3] [G/n][MENU][POWER] [16] [31]

4. Perform a frequency response cali-bration.

[MEAS][B/R][CAL][CALIBRATE MENU][RESPONSE][THRU][DONE RESPONSE]

5. Terminate the mixer’s IF port witha 50-ohm load.

6. Insert the mixer to be testedbetween the power splitter and atten-uator leading to the receiver’s B port.The incident signal should be enter-ing the mixer’s LO port and exitingthe mixer’s RF port.

7. Adjust scale.[SCALE REF][AUTO SCALE]

The resulting display shows themixer’s LO to RF isolation (Figure 19).

Measuring the IF to LO or LO feed-through would follow a similar proce-dure.

RF feedthroughThe procedure and equipment config-uration necessary for this measurementare very similar to those above, withthe addition of an external source todrive the mixer’s LO port as we meas-ure the mixer’s RF feedthrough.

1. Connect the hardware as shown inFigure 18.

2. Preset the instruments by pressing[PRESET] on the instrument front panels.

3. Using the 8753B as the RF source,select the frequency range and sourcepower.

[START] [10] [M/µ][STOP] [3] [G/n][MENU][POWER] [0] [31]

4. Perform a frequency response cali-bration.

[MEAS][B/R][CAL][CALIBRATE MENU][RESPONSE][THRU][DONE: RESPONSE]

5. Insert the mixer to be testedbetween the power splitter and atten-uator leading to the receiver’s B port.The incident signal should be enter-ing the mixer’s RF port and exitingthe mixer’s IF port, with the externalsource connected to the mixer’s LOport.

6. Select a fixed LO frequency andsource power from the front panel ofthe external source. Isolation isdependent on LO power level and fre-quency. To ensure good test results,these parameters should be chosen asclose to actual operating conditionsas possible.

[CW] [300] [MHz][POWER] [10] [dBm]

7. The resulting display shows themixer’s RF feedthrough.

Measuring IF to RF isolation is donein a similar manner using the 8753B’ssource as the IF signal, driving the LOport with an external source, andviewing the leakage signal at the RFport.

Isolation

Figure 18. Block diagram for an isolationmeasurement

Figure 19. Plot of LO to RF isolation of amixer

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RF Port SWRMixer reflection measurements canbe made simply and quickly using thesetup shown in Figure 20.

Measurement procedure1. Connect the Agilent 85044A Trans-mission/ Reflection test set to the8753B.

2. Preset the 8753B by pressing [PRESET].

3. Select the frequency and sourceoutput power for the 8753B.

[START] [10] [M/µ] [STOP] [3] [G/n] [MENU] [POWER] [13] [xl][FORMAT][SWR]

4. Perform an S11 1-port calibrationat the point to be connected to the RFport of the mixer. This removes sys-tematic errors from the reflectionmeasurement.

[CAL][CALIBRATE MENU][S11 1-PORT]

5. Select a fixed LO frequency andoutput power level. SWR is dependenton both LO power level and fre-quency. To ensure accurate testresults select these parameters tosimulate the conditions that themixer will encounter during normaloperation.

[CW] [300] [MHz][POWER] [13] [dBm]

6. Connect the mixer’s RF port to the85044A test set.

7. Connect the mixer’s LO port to theoutput of the local oscillator.

8. Terminate the mixer’s IF port witha 50-ohm load.

9. Adjust scale.[SCALE REF][AUTO SCALE]

10. The resulting display (Figure 21)is the SWR of the mixer’s RF portbetween 10 MHz and 3 GHz.

IF Port SWRBy connecting the IF port of themixer to the front of the 85044A, andterminating the mixer’s RF port witha 50-ohm load, you can measure theIF port SWR by using the procedureabove.

LO Port SWRBy adding 10 dB attenuators betweenthe test set and the receiver’s R andA ports, LO port SWR can be meas-ured using a procedure similar to theone listed above, using the 8753B asthe local oscillator.

Measurement procedure1. Connect the 85044A Transmission/Reflection test set to the 8753B.

2. [PRESET] the 8753B.

3. Select the frequency and outputpower the local oscillator will be setat under the mixer’s normal operat-ing conditions. If more power isneeded than is available at the testport, you may insert an amplifier (<1 Watt) between source and test set, or replace the test set with a dual directional coupler.

4. Perform an S11 1-port calibrationat the point to be connected to themixer’s LO port.

5. Terminate the mixer’s RF and IFports with 50-ohm loads.

6. Connect the mixer’s LO port to theLO signal generator.

The resulting display of LO SWRshows signal reflection under typicaloperating conditions, and thereforemore accurately predicts the reflec-tions that will be present duringactual operation.

SWR/Return loss

Figure 20. Block diagram for anSWR/return loss measurement Figure 21. Plot of a mixer’s RF port SWR

14

As described in the “MeasurementConsiderations” section of this note,the Agilent 8753B is susceptible to spurious responses caused byunwanted mixing products of thedevice entering and mixing with theanalyzer’s sampler-based receiver.The easiest way to eliminate thesespurs is to stop the unwanted signalsfrom entering the 8753B. For fixed IFmixer measurements, this is easilyaccomplished by the use of a band-pass filter (BPF) centered around themixer’s IF signal. For swept measure-ments, filtering alone may not removeall unwanted signals. If this is the case,both filtering and frequency selection(frequency list mode) will be necessary.The spur prediction program found atthe end of this appendix will help youselect the frequencies to avoid whenmeasuring the mixer’s response.

Spur analysisShown in Figure A1 is a mixer undertest having RF and LO inputs and anIF output. Also emanating from themixer’s IF port are several other mix-ing products of the RF and LO sig-nals. These unwanted mixing prod-ucts can cause spurious responses inthe 8753B’s IF signal path that willdegrade measurement accuracy. Forthis reason, spurious responses mustbe avoided or reduced.

The method used in the 8753B to down-convert incoming IF signals to 1 MHzfor internal processing is called sam-pling. The sampling method presentsall of the frequency harmonics of thereceiver’s voltage-tuned oscillator(VTO) to the incoming IF signal. TheVTO is retuned and phase-locked sothat one of its harmonics mixes withthe incoming IF signal to give exactly1 MHz. An internal 1-MHz BPF stopsall other mixing products of the RF,LO, and VTO which are not at 1 MHzfrom continuing on inside the 8753B.However, if the incoming IF signal iscomposed of many different frequencycomponents, it is possible that someother component of the IF signal willcombine with a different harmonic ofthe VTO and also produce a signal at1 MHz (see Figure A2). This unwantedsignal will then proceed through theinternal 1-MHz BPF, along with thedesired signal, and cause a spuriousmeasurement response.

If you are concerned about spuriousmeasurement responses it is suggestedthat you reduce the instrument’s IFbandwidth, and avoid frequencies

and frequency spacings of 1 MHz.Reducing the IF bandwidth will moreselectively filter signals in the instru-ment’s IF signal path. 1 MHz is the8753B’s first internal IF frequency,therefore 1 MHz and multiplesthereof should be avoided whenchoosing frequencies and frequencyspacings for mixer measurementsmade with the 8753B.

The spur prediction programThe first step in avoiding or eliminat-ing spurs is to determine at what fre-quencies they may occur. This pro-gram predicts the frequencies thatmay cause spurious responses whenthey enter the 8753B’s receiver, sothat you may avoid them using fre-quency list mode. This spur predic-tion program is written in BASIC 5.0,and although it is specialized forfixed LO/swept IF mixer measure-ments, it can easily be modified forother measurement configurations.This program only predicts the possi-ble occurrence of a spur, it does notpredict its power level. Also, this pro-gram does not consider RF and LOsubharmonics.

Appendix. Spur analysis and prediction

Figure A1. Block diagram of the 8753Breceiver and a mixer under test

Figure A2. Diagram of mixer IF output and sampler VTO harmonics vs. frequency

15

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