rf signal generator

Elektor Electronics 11/98

A rock-solid RF signal with an accu-rately known frequency and level is amust for anyone seriously involved inrepairing radio receivers and othercommunications equipment like filtersand even antennas. In particular,

receiver RF input and IF (intermediatefrequency) sections can not be testedwith any degree of certainty if a trust-worthy RF signal generator is not tohand. Unfortunately, professional-grade RF signal generators (like themighty Hewlett Packard 8640B in ourdesign lab) cost an arm and a leg, evenin the surplus trade. None the less, youwill see at least one RF signal generator,home-made, thrown together fromother bits and pieces, or ex-MOD, inthe shack of the more advanced radioamateur, simply because this piece oftest gear is as indispensable as the plainold multimeter.

The stability of the RF signal gen-erator described in this article is suchthat it will meet the (moderate)demands of many amateurs. Offeringa frequency range of 0.5 through30 MHz and an output level down to–80 dBm, it is perfect for testing andaligning many receivers and their sub-circuits like RF/IF amplifiers, mixers

An RF signal genera-tor is used for repair-ing radio/TV circuits,

checking filters, align-ing receivers, and forcomparative sensitiv-

ity tests on all kinds ofreceivers, whether

home-made, restoredsurplus or off-the-

shelf. The generatordescribed here has an

output frequencyrange of 0.5 to

30 MHz, making itsuitable for many

applications.

10

Design by Guido Brunner

RF signal generator

RADIO, TELEVISION & VIDEO

Main specificationsà Frequency range: 0.5 MHz to 30 MHzà Output level: 0 dBm down to –79 dBm in 1-dB stepsà Max. output level: 0.63Vpp into 50 Ωà Output impedance: 50 Ωà AM inputà FM inputà LCD readoutà Microprocessor controlledà Optional serial interface

part 1: circuit descriptions

and demodulators.What requirements can be men-

tioned in relation to an RF signal gen-erator? The answer is very simpleindeed: you need to be sure of (1) thefrequency and (2) the level of the signalyou feed into the circuit (receiver)under test. If either of these is unreli-able, all testing and comparing ofreceiver specs becomes meaningless.In the present design, frequency sta-bility is assured by a PLL (phase-locked loop), while the output level isdetermined by a switched pi (pi) atten-uator, all under the control of a micro-processor.

B L O C K D I A G R A MBecause the actual circuit diagrams ofthe four modules that make up the sig-nal generator are a fairly complex lot

when presented together, it wasdecided to draw and dis-

cuss them as separateblocks. The

basic interac-tion ofthese blocksis illustratedin Figure 1.The blockd i a g r a mshows thatthe heart ofthe circuit isa PLL syn-thesizer mod-ule keeping aVCO (volt-

age-controlledoscillator) in check. The

VCO output signal is amplified andfed to the generator output as well tothe synthesizer input and the input ofthe attenuator. The PLL obtains digitalinformation on the target VCO fre-quency from a microprocessor module.The micro also takes care of the front-panel mounted user interface, whichconsists of 3 switches, a rotary encoderand an LCD (liquid-crystal display). Italso controls the amount of attenuationat the generator output, across a rangeof –1 dB through –79 dB. An optionalserial interface is available to enable theRF Signal Generator to be linked to aPC using an RS232 cable. Functionally,the instrument is completed by aninternal power supply.

P L L B O A R DThe circuit diagram of this first moduleto be discussed in detail is shown inFigure 2. It comprises three sub-cir-cuits: VCO, synthesizer and outputbuffer. The VCO and the synthesizertogether from the PLL.

VCO and buffersThe active element in the oscillator is adifferential amplifier built around tran-sistors T1, T2 and T3, whose gain

depends on the currentpassed by T3. The actualresonating element inthe oscillator is an L-Cparallel tuned circuitconnected to the inputof the difference amplifier. The LC net-work consists of inductors L1-L5 incombination with variable-capacitancediodes (varicaps) D9 and D10. Theother input of the oscillator isgrounded for RF by capacitor C10.Depending on the desired frequencyrange, one or more inductors areswitched into the oscillator. This isdone by pulling the non-commonedterminals to RF ground using +5Vcontrol voltages on PIN diodes D2, D4,D6 and D8. In the highest frequencyrange, all inductors are effectively con-nected in parallel. This is necessary tomake sure that the non-selected induc-tors and their parasitic capacitance cannot form a series tuned circuit thatwould prevent the oscillator fromoperating at the desired frequency. Allinductors are off-the-shelf miniaturechokes. The frequency range switchingtakes place at 1.024 MHz, 2.304 MHz,5.376 MHz and 13.056 MHz.

Capacitor C8 provides the neces-sary amount of positive feedback in theoscillator. An AF signal may be appliedto the emitter of T4 to effect amplitudemodulation (AM). Frequency modula-tion (FM) is also possible by superim-posing an AF signal onto the varicaptuning voltage. Although FM willcause the PLL to drop out of lock, theaverage frequency remains constantbecause the time constant of the con-

trol loop is not capableof tracking the ‘insta-bility’ caused by themodulation signal.

To make sure it isnot too heavily loaded,

the oscillator signal is first buffered by aFET (field-effect transistor), T4. Nextcomes the real amplifier, IC1, a typeNE592 which some of you may knowfrom baseband-video amplifiers insatellite-TV receivers. The amplifier isbiased at half the supply voltage byopamp IC3b, and its gain is defined byseries network R26-L8. Because of theinductor action, the gain decreases athigher frequencies. Because the VCOstrives to maintain a stable outputlevel, less gain on the NE592 automat-ically more gain in the differential oscil-lator. This purposely-created effect isessential for reliable starting of theoscillator at higher frequencies.

The NE592 being a differentialamplifier, it has two inputs, but alsooutputs. Both are used here. The signalat the first output (pin 7) is applied toemitter follower T5 which supplies theactual generator output signal at animpedance of 50 Ω (the standard in RFtest equipment). The other output sig-nal supplied by the NE592 is used todrive two sub-circuits. One branchgoes to the PLL chip via C23 and R33,the other is used to drive a voltage rec-tifier/doubler, D11-D12 which in turndrives amplitude-control opamp IC3a.The desired highest output amplitudemay be set using preset P1. The authorused a setting where 0 dBm (decibel-

11Elektor Electronics 11/98

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VCO

PLL

PSU

Interface

PC

30V

12V

5V

mains

keysF

Attenuator0 ... - 79 dB

LCD

980053 - 11

0.5 ... 30 MHz

Figure 1. Block dia-gram of the RF SignalGenerator. All intelli-gence is vested in amicrocontroller.

1

B2

R1

D1

1N4148 D2

BA243

C2

33n

L2

100µH

B3

R3

D3

1N4148 D4

BA243

C3

33n

L3

22µH

B4

R5

D5

1N4148 D6

BA243

C4

33n

L4

3µH9

B5

R7

D7

1N4148 D8

BA243

C5

33n

L5

0µH56

C8

330p

C9

330p

C13

100n

C10

33n

C15

27p

R13 R14 R15 R16 R19

R20 R21

R25

R24

R27

R29

R28

R31

R11

R17

R34

T1

BF494

T2

R41

T3

BF494

C40

100n

C12

10µ63V

C14

180p

T4

BF256B

C16

330p

L6

39µH

L7

39µH

R22

10k

R23

10k

C17

33n

L8

3µH3

R26

C214n7

R30

T5

2N5179

C22

33n

R32

47Ω

C11

68p

C19

100n

C20

47µ16V

C18

10µ16V

OUT

D10

BB130D9

R2

390Ω

R4

390Ω

R6

390Ω

R8

390Ω

2x

R10

330kR9

C6

2n2

FM

L1

330µH

C1

33n

R12

10k

C7

220n

AM

R18

100k

R37

10k

R36

10k

R35

R33

2

3

1IC3a 2k2

P1

C28

2µ216V

C26

2µ2 16V

C25

2n2

C24

330p

D12AA13

D11

AA113

6

5

7IC3b

C23

330p

R40

1kC

B

A

NE592

IC1

G2B G2A

G1B G1A

14

10

11

12

I2

I1

Q2

Q1

1

8

7

3

54

C30

2n2

C31

10n

C29

10n

C37

330n

C39

100nC36

1µ

C35

100nR39

18k

R38

R42

C34

100µ 10V

C38

10µ63V

LOCK

C33

40p

X1

4MHz

2x

A A

B B

C

D

E

F

G

H

I

J

J

K

(N14)

L

L

M

N

+30V

30V

+12V

12V

+5V

5VDLEN

SDA

SCL

U TUNING = 0 ... 30V

LOCK

PLL

8

4

IC3

IC3 = LM358P

12V U+

U+

5V

C32

47µ16V

A 11V9

B

C

D

E

F

10V2

0V

9V5

0V02

0V86

G

H

I

J

K

L

M

N3V5

4V2

0V35

5V9

5V1

K

9V

2V5

1V95980053 - 12

30V

C27

100n

SAA1057IC2

XTAL

TEST

AMIN

DLENFMIN

OUTSDA

SCL

DCSDCA

TCA TCB

IN

15

17

18

11

12

14

13

10

TR

16

CC

5

6

41 2 3

1

9

2 3

7

V

8

2

D1

1N4148

C1

100n

R3

6Ω81

R4

39Ω2

RE1

R1 R2 R5 R6

D2

1N4148

C2

100n

R9

368Ω

R10

12Ω1

RE2

R7 R8 R11 R12

D3

1N4148

C3

100n

R15

3k65

R16

24Ω3

RE3

R13 R14 R17 R18

D4

1N4148

C4

100n

R21

909Ω

R22

56Ω2

RE4

R19 R20 R23 R24

D5

1N4148

C5

100n

R27

3k92

R28

162Ω

RE5

R25 R26 R29 R30

D6

1N4148

C6

100n

R33

3k92

R34

162Ω

RE6

R31 R32 R35 R36

D7

1N4148

C7

100n

R39

3k92

R40

162Ω

RE7

R37 R38 R41 R42

D8

1N4148

C8

100n

R45

3k92

R46

162Ω

RE8

R43 R44 R47 R48

A1– 1dB

A2 A3 A4– 2dB – 4dB – 8dB

A5 A6 A7 A8– 16dB – 16dB – 16dB – 16dB

980053 - 13RE1 ... RE8 = V23042-A1001-B101

3

milliwatt) into 50 equals 0.63 Vp p a tthe generator output.

S y n t h e s i z e rThe circuit of the synthesizer largelyfollows the Application Note for theS AA1057 as supplied by Philips Semi-conductors. Some component values

in the control loop had to be modified alittle to optimise the behaviour of thePLL. The ‘LOCK’ output is only pro-vided for test purposes. The SAA 1 0 5 7receives its control information in I2Cf o rmat via its SDA, SCL and DLENinputs. These lines are connected to am i c r o c o n t r o l l e r. Basically, the SAA 1 0 5 7compares the frequency of the VCOwith that of a reference signal derivedfrom the ex t e rnal 4-MHz quartz cry s-tal. For this purpose the VCO signal isi n t e rnally divided by a factor deter-mined by the microprocessor. The fre-quency difference produces an err o rsignal which is converted into a corr e-

sponding varicap control voltage. Thiscontrol voltage is integrated by R40-C37 and has a range of 0-30 V. Remark-a b l y, the SAA1057 does not require anex t e rnal level converter for the varicapcontrol voltage — a special amplifier isincluded on the chip for this purpose,as well as a direct connection for +30 V( p i n7 ) .

Trimmer C33 allows the generatoroutput frequency to be calibratedagainst a frequency standard.

The VCO/PLL board requires threesupply voltages: +5 V for the synthe-s i z e r, +12 V for the VCO, and +30 Vfor the varicap voltage.

Figure 2. Circuit diagram ofthe VCO/PLL board. The heartof the PLL is an I2C-controlledsynthesizer chip typeSAA1057.

C1

1µ 16V

R1

K1P1

10k

R5 8x 10k 1

23456789

C5

100n

R2 R3 R4

R28

1k

PSEN

ALE

R29

3k3

T12

R26

1k

R27

3k3

T11

R24

1k

R25

3k3

T10

R22

1k

R23

3k3

T9

R12

1k

R13

3k3

T4

R10

1k

R11

3k3

T3

R8

1k

R9

3k3

T2

R6

1k

R7

3k3

T1

R20

1k

R21

3k3

T8

R18

1k

R19

3k3

T7

R16

1k

R17

3k3

T6

R14

1k

R15

3k3

T5

X1

11.059 MHz

C2

33p

C3

33p

C4

100n

S1S2S3

T0

MAX232

R1OUT

R2OUT

RS1OUT

RS2OUT

IC2

T1IN

T2IN

RS1IN

RS2IN

C1–

C1+

C2+

C2–

11

12

10

13

14

15

16V+

V-

7

8 9

3

1

4

5

2

6

C9

C10

C6

C7

C8

K2

1

2

3

4

5

6

7

8

9

S4

ENCODER

A1

A2

A3

A4

A5

A6

A7

A8

C

B

A

P2.4

EA/VP

ALE/P

RESET

89C51

P0.0

P0.1

P0.2

P0.3

P0.4

P0.5

P0.6

P0.7

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

P2.0

P2.1

P2.2

P2.3

P2.4

P2.5

P2.6

P2.7

PSEN

INT0

INT1

IC1

TXD

RXD

39

38

37

36

35

34

33

32

21

22

23

24

25

26

27

28

31

19

X1

18

X2

20

40

17 RD16 WR

29

30

11

1012

13

14T015T1

1

2

3

4

5

6

7

8

9

B5

B4

B3

B2

DGCLK

DGDIR

C12

100n

C11

220µ16V

5V

T1 ... T8 = BC557B

C6 ... C10 = 10µ / 63V

OUT

IN

RS232

RES

5V 5V 5V

LOCK

980053 - 14

+5V

RS

R/W

EN

DB4

DB5

DB6

DB7

5V

5V

5V

T9 ... T12 = BC557B

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

5V

TxD

RxD

SDA

SCL

DLEN

Figure 4. Circuit diagram ofthe controller board. An 89C51sits between a number ofinput and output devices.

4

Figure 3. Circuit diagram ofthe digitally controlled attenu-ator. Range is –1 dB to –79 dBin 1-dB steps.

A T T E N U A T O R B O A R DFigure 3 shows the circuit diagram of adigitally controlled 8-section pi RFattenuator with a range of –1 dB to–79 dB in 1-dB steps. The resistor com-binations we need to realize each ofthe 79 discrete attenuation levels areconnected into the circuit by means ofrelay contacts. The associated relays areactuated and de-actuated by micro-processor drive signals that form 8-bitcombinations at the control inputsmarked A1-A8.

The theoretical values of the resis-tors in the attenuator are realized bymeans of parallel combinations of 1%resistors from the E96 series.

Each relay coil is shunted by a back-emf suppressor diode and a decou-pling capacitor.

M I C R O C O N T R O L L E RB O A R DAll the intelligence we need to imple-ment a man/machine interface, i.e.,establish communication between theuser on the one hand, and the PLL andthe attenuator on the other, is packedin a microcontroller type 89C51. Thiscontroller executes a program written

by the author and burned into theinternal program memory by the Pub-lishers. The 89C51 is available ready-programmed from the Publishers orcertain kit suppliers advertising in thismagazine.

The 89C51 accepts information andsupplies information. Microcontrollerfreaks call this ‘I/O’ for input/output.Well, the input devices are a rotaryshaft encoder, S4, which is used for thefrequency setting, a small keyboard,S1-S2-S3, the SDA line of the I2C busand (optionally) the RxD line of theMAX232 serial interface. The outputdevices to control are the LCD con-nected to port P0, the attenuator onport P1), the VCO inductors on portline P2.0 through P2.3 and, of course,the synthesizer chip, by way of theDDA and SCL lines (P2.6 and P2.7).Actually, the I2C bus is modified into aso-called CBUS by the addition of P2.5(DLEN) and its pull-up resistor, R2.

The 89C51 is clocked at11.0592 MHz by an external quartzcrystal, X1. This frequency was chosenbecause it allows standard baud ratesto be used on the serial interface.

A classic power-on reset network,R1-C1, completes the microcontrollercircuit.

This board requires only +5 V to

operate, the MAX232 having on-chipstep-up converters for +10 V and–10 V.

P O W E R S U P P L Y B O A R DAs you can see from the circuit dia-gram in Figure 5, the power supply forthe RF signal generator is entirely con-ventional.

The 30-V varicap supply is based ona simple combination of a zener diodeand a series transistor. Current drainon the 30-V rail will be very small, soextensive regulation is not necessary.None the less, a fair number of decou-pling capacitors is used to keep the var-icap voltage as clean as possible. Afterall, all hum, noise etc. on this rail willcause frequency modulation on theoutput signal. The input voltage for the30-V regulator is supplied by a voltagedoubler, C10-D5-D5.

The 5-V and 12-V supplies are basedon two old faithfuls, the 7805 and theLM317 respectively. These ICs andtheir usual ‘satellite’ components havebeen used so many times in our pub-lished circuits that no further descrip-tion will be necessary.

A single mains transformer rated at15 V, 8VA, supplies all the necessaryalternating voltages. The mains voltageat the primary side is applied via adouble-pole switch and a fuse, bothbuilt into a Euro-style appliance socket.

N E X T M O N T HIn next month’s second and conclud-ing instalment we will be discussingthe construction of the instrument onfour printed circuit boards. The articlewill be concluded with notes on theoperation of the RF Signal Generator,miscellaneous matters and optionalextras.

(980053-1)


D3

D1

D2

D4

15V

TR1

8VA

C3

C4

C2

C1

7812

IC1

IC2

LM317T

C5

1000µ 35V

C7

2µ2 16V

C9

2µ216V

C13

10µ 63V

C12

1µ63V

C11

220µ63V

C10

470µ63V

K1K2

C6

220n

D5

C8

220n

R1

22

Ω

5W

R2

27

0Ω

R3

82

0Ω

D6

1N4001

R4

1k

D7

33V

400mW

R5

10

k

D8T1

BC141+30V

+12V

+5V

30V

12V

5V

2x

63mA T

980053 - 15

D1 ... D4 = 4x 1N4001C1 ... C4 = 4x 47n

Figure 5. Circuit dia-gram of the powersupply. Three voltagesfrom one transformer!

5

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Elektor Electronics 12/98

The RF signal generator is a quite com-plex instrument, and we should reallyadvise beginners not to attempt tobuild this project without the help orguidance of someone with consider-able experience in building RF andmicrocontroller circuits.

There are no fewer than four boardsto build up, and each of these boardscontains a fair number of components.Add to that the mounting of the fourboards in a case and the inter-board

Although the main subject of this month’ssecond and final instalment is ‘all matters

constructional’, there’s also information onadjusting the instrument and, of course, on

how to use it!

20

Design by G. Brunner

RF signal generatorpart 2 (final): construction,operation and adjustment

RADIO, TELEVISION & VIDEO

Figure 6. Copper tracklayout and componentoverlay of the powersupply board.

(C) ELEKTOR980053-4

C1

C2

C3

C4

C5

C6 C7

C8C9

C10 C11

C12

C13

D1

D2

D3

D4

D5

D6

D7

D8

IC1

IC2

K1

R1

R2

R3

R4

R5

T1

TR1

0 +30V +5V+12V

980053-4

~

~

(C) ELEKTOR980053-4

wiring, and you are looking at a projectwhich should take even advanced hob-byists several hours, winter evenings orrainy Sunday afternoons to complete.

The four boards are built up one byone in the order indicated by the textto follow. As usual, great care should betaken to fit each and every part in theright position on the board. The com-ponent overlays and associated partslist should guide you through theprocess of assembling the boards. Par-ticularly with the 1% resistors in theattenuator section, you should (1)ascertain the value and (2) look up theposition on the board, before (3) fittingany resistor.

P O W E R S U P P L Y B O A R DThis board is the simplest to build. Pop-

ulating it should be straightforward,using the relevant Components Listand the component overlay shown inFigure 6. Resistor R1 may run fairly hotand should not touch the circuit board.The LM317T voltage regulator may bemounted directly on to the heatsink —an insulating washer is not required.The ‘power on’ LED is not fitteddirectly on the board — instead, it isconnected up via a pair of thin wireswith an length of about 20 cm.

This board is simple to test by pro-visionally connecting it to the mainsand using a voltmeter to check theindicated output voltages: +5 V, +30 Vand +12V. The finished PSU board isshown in Figure 7. Check your workagainst this photograph!

C O N T R O L L E R B O A R DThe controller boardshown in Figure 8 is far

more densely populated than the PSUboard. Hence, great care and precisionis required when it comes to solderingthe parts in place.

Start with the two wire links on theboard — you’ll find them near presetP1. Next, fit the components, the bestorder is probably from low-profileparts (resistors, IC sockets) to uprightmounted parts (crystal, transistors,radial electrolytic capacitors).

The three push-buttons, S1, S2 andS3, are not mounted directly on to theboard. Their pins are inserted in socketstrips or stacked IC sockets so that theirheight can be adjusted a little. Alterna-tively, their pins are ‘lengthened’ usingpieces of stiff wire. This is necessary toenable the cap tops to protrude a littlethrough the front panel. The samemounting method is used for LCD. As

with the push-buttons,the height of the LCD


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980053-3(C) ELEKTOR

C1

C2

C3

C4C5

C6

C7

C8

C9

C10

C11

C12

H1

H3

H4

H5

H6

H7

H8

IC1 IC2

K1

P1

R1

R2

R3R4

R5

R6R7R8R9

R10R11

R12R13R14R15R16R17R18R19R20R21

R22

R23

R24

R25

R26

R27

R28

R29

S1 S2 S3

S4

T1

T2

T3

T4

T5

T6

T7

T8

T9 T10 T11 T12

X1

0

+5V

RES

C BA

T0

P2.4

ALE

PSenLock

T

out in

980053-3

A8A7A6A5A4A3A2A1

B2 B3 B4 B5

RS232

Figure 7. Finished PSUboard (prototype).

Figure 8. Controllerboard artwork.

7

COMPONENTS LIST

POWER SUPPLY BOARD

Resistors:R1 =22Ω 5WR2 =270ΩR3 =820ΩR4 =1kΩR5 =10kΩ

Capacitors:C1-C4 =47nFC5 =1000µF 35V radial

C6,C8 =220nF MKTC7,C9 =2µF2 16V radialC10 = 470µF 63V radialC11 = 220µF 63V radialC12 = 1µF 63V radialC13 = 10µF 63V radial

Semiconductors:D1-D6 = 1N4001D7 = 33V 400mW zener diodeD8 = LED, red, high efficiencyT1 = BC141IC1 = 7812

IC2 = LM317T

Miscellaneous:TR1 = mains transformer, 15V 8VA,

Monacor/Monarch type VTR8115K1 = PCB terminal block, 2-way, raster

7.5mmK2 = mains socket, integral switch and

fuseholder, with fuse 63mATHeatsink type SK59 37.5mm (Fischer,

Dau Components)PCB, order code 980053-4 (see Read-

ers Services page)

above the controller board may needto be adjusted later, so do not mount itsecurely as yet. The rotary switchencoder, S4, is mounted directly on tothe board, but its spindle is not yet cutoff. Later, rectangular clearances are cutin the front panel to allow the LCD tobe viewed, and the push-buttons to bepressed.

It is recommended to use socketsfor IC1 and IC2. All holes in the PCBwith a label printed near it (like A1, T0,Psen, Lock, etc.) are for inter-boardwires. Solder pins are not strictly nec-essary — direct wire connections to theboard are also fine. As with the PSUboard, check your work against ourfully working prototype. This time,refer to the photograph in Figure 9.The board is fitted vertically behind themetal front plate (which has to be pur-chased separately). It is held in positionby a pair of slots moulded on the bot-

tom plate of the case. Several slots areavailable, and the pair you actuallychoose to use should ensure that themetal frame around the face of theLCD is pressed firmly against theinside of the front panel. The threetype ‘D6’ push-buttons should thenprotrude a little from the front panel.

The holes marked ‘In’, ‘Out’ and‘ground’ to the right of preset P1 arefor an optional 3-wire RS232 link to aPC. If you do not require PC control,the MAX232 may be omitted. The prac-tical use of the RS232 interface will bereverted to further on.

V F O / P L L B O A R DAs you can see from the PCB artworkin Figure 10, this is the board with thehighest component density of all four.Care and precision are essential if youwant to avoid a tedious faultfindingsession. Identify and check each part

before fitting it, and double-check itsvalue and position using the Compo-nents List and the component overlay.

As usual, start with the wire links(there are three), so they are not for-gotten or overlooked. Then follow thelow-profile parts and, finally, the verti-cally mounted parts. IC sockets shouldnot be used for the NE592 and theSAA1057 on this board.

The value of the inductors is usuallyprinted on these parts in the form ofcolour bands (like resistors) or dots.

The PLL/VFO board is fitted in atinplate enclosure from Teko. After thesolder work, inspect the board, andcompare yours with our prototypeshown in Figure 11.

22 Elektor Electronics 12/98

COMPONENTS LIST

CONTROLLER BOARD

Resistors:R1 = 22kΩR2,R3,R4 = 4kΩ7R5 = 10kΩ 8-way SIL arrayR6,R8,R10,R12,R14,R16,R18,R20,R22,R

24,R26,R28 = 1kΩR7,R9,R11,R13,R15,R17,R19,R21,R23,R

25,R27,R29 = 3kΩ3P1 =10kΩ preset, H

Capacitors:C1 =1µF 16V radialC2,C3 =33pFC4,C5,C12 =100nF ceramicC6-C10 =10µF 63V radialC11 = 220µF 16V

Semiconductors:T1-T12 = BC557BIC1 =AT89C51-20PC or

SC87C51CCN40 (order code 986515-1)

IC2 = MAX232

Miscellaneous:X1 = 11.059MHz crystalS1,S2,S3 = pushbutton, 1 make con-

tact, ITT type D6-R-RD; cap type D6Q-RD-CAP (Eurodis)

K1 =14 way SIL pinheaderK2 =9-way sub-D socket (female)S4 = rotary encoder, Bourns type

ECW1J-B24-AC0024 (Eurodis)LCD, 2x16 characters, Sharp type LM

16A211 (Eurodis)PCB, order code 980053-3 (see Read-

ers Services page)

Figure 9. Finished con-troller board (proto-type).

980053-3(C) ELEKTOR

9

and the trimmer to the centre of theirtravel. It is assumed that the powersupply board has been tested already(with good results, of course).

After applying power, the first thingto do is set the LCD contrast with pre-set P1. Next, use an oscilloscope tocheck that the VFO/PLL board sup-plies an RF signal to the attenuatorboard.

The output frequency supplied bythe generator may be checked with acalibrated frequency meter, a fre-quency standard (off-air Rugby MSF orsimilar) or a calibrated SW receiver(zero-beat). The relevant adjustment istrimmer capacitor C33.



980053-1

(C) ELEKTOR

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10C11

C12

C13

C14

C15 C16

C17

C18

C19

C20

C21 C22

C23C24

C25

C26

C27

C28

C29

C30C31

C32

C33

C34

C35

C36

C37

C38

C39

C40

D1

D2

D3

D4

D5

D6

D7

D8

D9

D10

D11 D

12

H1 H2

H3H4

IC1 IC2

IC3

L1

L2

L3

L4

L5

L6L7

L8

P1

R1

R2

R3

R4

R5

R6

R7

R8

R9

R10R11

R12

R13

R14

R15

R16

R17

R18

R19R20

R21

R22

R23

R24R25

R26

R27 R

28R

29R

30

R31

R32

R33

R34

R35

R36R37

R38

R39

R40

R41

R42T1T2

T3

T4T5

X1

0

0

T T T

OUTAMFM

+12V

+5V

B5

B4

B3

B2

980053-1

LOCK

AB C

+30V

980053-1

(C) ELEKTOR

10

Figure 10. VFO/PLLPCB design.

A T T E N U A T O R B O A R DThe main point to mind about assem-bling the attenuator board (Figure 12)is that each close-tolerance (1%) resis-tor goes to the right position on theboard. One error in this respect maycause wrong attenuation levels later,with possibly difficult to explain behav-iour of some of the radio equipmentyou may be aligning! Our advice is,therefore: read the Components Listcarefully, check the colour code, use aDMM to measure the value of each

resistor, and then check its position onthe board.

The attenuator board has relativelylarge copper areas to assist in screeningand preventing unwanted signals frombeing generated and picked up by thecircuit. The attenuator board is shownin Figure 11, together with theVFO/PLL board. For RF screening pur-poses, both boards are fitted in Tekotinplate cases.

A D J U S T M E N TThe boards may be wired up experi-mentally for an initial test and a fewadjustments.

To begin with, set the two presets

COMPONENTS LIST

VFO/PLL BOARD

Resistors:R1,R3,R5,R7,R12,R22,R23,R31,R34,R36

,R37 = 10kΩR2,R4,R6,R8 = 390ΩR9,R14,R15,R21,R27,R33,R40 = 1kΩR10,R41 = 330kΩR11,R13,R16,R18 = 100kΩR17,R26 = 100ΩR19 = 2MΩ2R20 = 1MΩR24,R25,R35 = 22kΩR28,R29 = 3kΩ3R30 = 560ΩR32 = 47ΩR38 = 180ΩR39 = 18kΩR42 = 10ΩP1 = 2kΩ multiturn preset, H

Capacitors:C1-C5,C10,C22 = 33nF ceramic

C6,C25,C30 = 2nF2 ceramicC7 = 220nF MKTC8,C9,C16,C23,C24 = 330pF ceramicC11 = 68pF ceramicC12,C18,C38 = 10µF 63V radialC13 =100nF ceramic 5mmC19,C27,C35,C39,C40 =100nF ceramicC14 = 180 p ceramic C15 = 27 p ceramic C17 = 33n ceramic 5mmC20,C32 = 47µF 16V radialC21 = 4n7 ceramic C26,C28 = 2µF2 16V radialC29,C31 = 10nF ceramic C33 = 40pF trimmerC34 = 100µF 10V radialC36 = 1µF MKTC37 = 330nF MKT

Inductors:L1 = 330µHL2 = 100µHL3 = 22µHL4 = 3µH9L5 = 0µH56

L6,L7 = 39µHL8 = 3µH3

Semiconductors:D1,D3,D5,D7 = 1N4148D2,D4,D6,D8 = BA243D9,D10 = BB130D11,D12 = AA113T1,T2,T3 = BF494T4 = BF256BT5 = 2N5179IC1 =NE592N (N14)IC2 = SAA1057 (Philips)IC3 = LM358P

Miscellaneous:X1 = 4MHzTinplate case, Teko, size 160x25x49mmCase, Bopla type Ultramas UM52011

(size 224x72x199mm)Front panel type FP50011 or FPK50011PCB, order code 980053-1 (see Read-

ers Services page)

980053-2

(C) ELEKTOR

D1 D2 D3 D4 D5 D6 D7 D8

H1

H2

H3

H4

OU

T

R1

R2

R3R4

R5

R6

R7

R8

R9R10

R11

R12

R13

R14

R15R16

R17

R18

R19

R20

R21R22

R23

R24

R25

R26

R27R28

R29

R30

R31

R32

R33R34

R35

R36

R37

R38

R39R40

R41

R42

R43

R44

R45R46

R47

R48

RE1 RE2 RE3 RE4 RE5 RE6 RE7 RE8

A8A7A6A5A4A3A2

A1

980053-2

0

T T

C1

C2 C3 C4 C5 C6 C7 C8

980053-2

(C) E

LEK

TOR

12

Figure 12. AttenuatorPCB artwork.

Adjustment of the RF signal level isonly possible if you have an accurateand calibrated RF voltmeter. With theattenuation set to 0 dB, preset P1 maybe adjusted for an output level of630 mVpp into 50 Ω at the generatoroutput. Failing the necessary testequipment, you may leave the multi-turn preset at mid-travel.

W I R I N G A N DM E C H A N I C A L W O R KAlthough there are quite a few wireconnections between the boards, thereare no special precautions in thisrespect. The RF signal connectionbetween the PLL/VFO board and theattenuator board must, of course, bemade in coax cable. The same goes forthe connections between the AM and

FM inputs on the PLL/VFO board andthe associated BNC sockets on thefront panel. If you can get hold of it,use the 3-mm dia. type RG174/U, else,the much thicker RG50/U or /CU is agood alternative.

All other inter-board connectionsare made in light-duty flexible wire orflatcable, although slightly thicker wireshould be used for the 0-V, 5-V and 12-

24 Elektor Electronics 12/98

11

Figure 11. Finished PLL/VFO board (below) and attenuatorboard (above), both fitted in ‘Teko’ ready-made tinplate cases.

V supply wiring. Do not make any ofthe wires longer than necessary to pre-vent digital noise being picked up fromthe controller board.

The wires and the coax cables toand from the PLL/VFO board and theattenuator board should pass throughholes drilled in the short side panels ofthe Teko tinplate cases. Once theseboards are fully operational, the topcovers are fitted for optimum RFscreening.

Guidance for mounting the fourboards into the Bopla enclosure maybe obtained from the photographs inthis article, and in particular, Figure 13.Note that the solder side of the powersupply board is protected by a perspexcover plate cut to roughly the samesize as the board. The VFO/PLL andattenuator boards are screened by tin-plate boxes, and mounted horizontally

on to the bottom plate of the enclosure.As already mentioned, the PSU boardis fitted vertically, using a pair of themoulded PCB slots towards the backpanel. The three holes at the ‘empty’right-hand side of the controller boardare drilled to a diameter of about 8 mmto allow the coax cables to the threefront-panel mounted BNC sockets topass.

The mains voltage is switched onand off by a double-pole switch inte-grated into a mains socket fitted ontothe plastic rear panel of the enclosure.The wires between the mainssocket / swi tchcombination andthe PCB terminalblock on the PSUboard should bemains-rated andproperly iso-

lated. At the PCB side in particular, the‘live’ and ‘neutral’ wires should not bestripped longer than strictly necessary,and they should be inserted into theclamps right up to the insulation.Finally, once the wires are connected,the terminals on the mainssocket/switch combination must beinsulated using heat-shrink sleeving.

The metal front panel is cut, drilledand lettered using the template shownin Figure 13. This front panel foil is notavailable ready-made.

In the (ABS plastic) back panel, youhave to cut rectangular clearances for

the mainssocket /switchc o m b i n a t i o nand, optionally,for the RS232connector (a 9-pin sub-D type).



13

Figure 13. A look inside our pro-totype of the RF Signal Generator.The covers of the tinplate casesof the VFO/PLL board and theattenuator board were removedfor this photograph.

COMPONENTS LIST

ATTENUATOR BOARD

Resistors (all 1%):R1,R5,R21 = 909ΩR2,R6 = 20kΩR3 = 6Ω81R4 =39Ω2R7,R11 = 475ΩR8,R12 = 6kΩ19R9 = 368ΩR10 = 12Ω1

R13,R17 = 243ΩR14,R18 = 2kΩ74R15,R20,R24 = 3kΩ65R16 = 24Ω3R19,R23 = 121ΩR22 = 56Ω2R25,R29,R31,R35,R37,R41,R43,R47 =

75ΩR26,R30,R32,R36,R38,R42,R44,R48 =

825ΩR27,R33,R39,R45 = 3kΩ92R28,R34,R40,R46 = 162Ω

Capacitors:C1-C8 = 100nF SMD

Semiconductors:D1-D8 = 1N4148

Miscellaneous:RE1-RE8 = relay, 2 x change-over, type

V23042-A1001-B101 or V23042-A2001-B101 Siemens (Eurodis, Elec-troValue)

PCB, order code 980053-2 (see Read-

O P E R A T I O N

The instrument is controlled by meansof three pushbuttons and a rotarye n c o d e r, all accessible on the frontpanel. The instrument communicates

with you via an LCDwith two lines of 16c h a r a c t e r s .

The functions ofthe ‘left’ and ‘r i g h t ’pushbuttons are self-evident, we recko n ,because they movethe cursor on the LCdisplay in the direc-tion indicated by thea rrows on the frontp a n e l

From the startingposition (cursor on‘MHz’), the cursormay be moved to theleft on to any of thepost-decimal posi-tions of the fre-q u e n c y. The numberat which the cursora rrives may then bechanged by turn i n gthe rotary encoder.The frequency set inthis way is howevernot actually gener-ated until you pressthe ‘Enter’ pushbut-ton (asynchronousoperation, this isindicated in theupper right-handc o rner of the dis-play). After any fre-quency change, thePLL status is indi-cated by ‘lock’ in theleft-hand bottomc o rner of the read-o u t .From the initial

position to the right,the cursor jumps to‘M0’ (memory 0 ) .This indicates twomemories, M0 andM1, in which fre-quency and attenua-tor settings may bestored. You press theEnter key to changebetween these mem-ories. In this way,you can quicklychange between twopreviously storedsettings, which maybe useful, for ex a m-

ple, for aligning a filter. Altern a t i v e l y,you may use the same frequencytwice, but with two different attenua-tor settings. This facility is useful foradjusting, say, a receiver AGC (auto-matic gain control).

Moving further to the right, the cur-sor jumps on ‘asy’. Here you canswitch to asynchronous operation bypressing ‘Enter’. In synchronousmode, any frequency changerequested by way of the rotary

encoder is immediately passed on tothe VFO/PLL unit. In this mode, the RFoutput frequency is continuouslyadjustable, but only within the selectedrange (one of five). If you turn theencoder to a frequency outside a cer-tain range, the PLL will drop out oflock, and the ‘lock’ indication will dis-appear from the LCD. By pressing anyke y, the PLL is returned to asynchro-nous mode, and the last selected fre-quency is automatically restored. If youthen move the cursor to a decimal digitof the frequency readout, and press theEnter ke y, the generator changes to therelevant frequency range, allowing youto change to synchronous mode againand continue ‘tuning’ again using con-tinuous frequency variation.

One more position to the right, thecursor reaches the ‘dB’ position, indi-cating the currently valid attenuation.The desired attenuation may be setwith the aid of the rotary encoder. Aswith the frequency setting, thedesired attenuation becomes effectiveonly when you press the Enter ke y.This is done to reduce wear and tearon the relays.

O P T I O N A LR S 2 3 2 I N T E R FA C EThe RS232 interface on the controllerboard is an optional extension whosefunction has not been fully developedout by the author/designer. Basically, itwas designed into the circuit to enablethe generator frequency and outputsignal attenuation to be controlled by aP C .

The communication parameters areas follows: 9600 bits/s, 8 data bits, 1stop bit. The communication workswith character strings, and is easilytested with the aid of a terminal pro-gram. To set the frequency you have tosend an ‘F’ (for ‘f r e q u e n c y’), then fivenumbers for the frequency in kilo-hertz, and, finally, a carr i a g e - r e t u rn(CHR$(13)). An additional Line-Fe e d(CHR$(10)) will be ignored. If every-thing is correct (first character is ‘F’, atotal of 6 characters and the frequencyin the right range), the controllerr e t u rns a ‘D’ (for ‘done’), followed bya CR-LF sequence, otherwise, an ‘E’(for ‘err o r’) and a CR- L F.

The attenuation is set by sending an‘A’, two numbers and CR. Again thecontroller answers as described. Themain purpose of the serial interf a c ewas to create a basis for using the gen-erator in an environment like Lab-Vi e wT M.

( 9 8 0 0 5 3 - 2 )

1 4

Figure 14. Suggested front-panel layout. Use it as a tem-plate to drill the metal frontpanel of the instrument, andapply the lettering/symbols.

50Hz

No. 980053

240V

F = 63mA T

ELEKTOR

~

rf signal generator

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