~---

BernhardKokotDieter Schwarzenau

A Home-made RF-Millivoltmeter

Th e authors have designed a RF-millivolt­meter l or measu ring low to medium AF­voltages. The frequency rang&, input im­pedance. and accuracy should be compar­ab le to th ose of commerciall y manufactur­ed RF-millivo llmelers .

Since simple rectifier circuits. and rectifierdiodes provided with a bias current. are notable to futfill these dem ands. the principle ofcompensation mea surement was usee .Fig . 1 showsa photog' aph 01the author's pro­lolype.

1.BLOCK DIAGRAM---- -The operation ct me unit can be seen in theblock diagram given in Figure 2.

The actual compensation takes place in meprobe. II is equipped with a differential recti­fier, wh ich also receives an internally qenerat­eo. low·l requency anematmq voltage. Theoutput DC-voltage is proportional 10 me crtte-

•

,

...... ...:: : ~,._..::: I..... .'-

g, :

Fig . 1: A photograph 01the .u1hor·. profOlype uSIng two AF-modu.....nd. Iog"nlhmic "mplll•• r

66

VHF-COMMVNICATIONS 2/S';'~ j!O,

Fig . 2:Block di.\l r.m of IlleRF-mill ivo ltmllter

"r1 '- f ·ElEl~

Y .- f-EJ-~ '---'renee of the peak values of the lwo alternatingvoltages.

It is ampl ified and fed to an integrator. whichcontrols a control amplifier. The control ampli ­fier varies me amplitude 01the internally gene­rated AC-voltage. This is led via a voltage divi­der to the probe.

In the stationary state. the peak values of thevoltage 10 be measured and the con trolled vol ­lage are identical, since the output voltage ofthe ditterannat recti fier is then zero. and thesignal at the output of the integrator wil l notchange.

The internally generated voltage is, however,higher to the value of the division factor of the

switehable voltage divider and is constant withrespect to frequency. Furthermore, the outputimpedance of the control amplifier is suffi­ciently low so that the rectifier conn ected herecan be constructed relatively simply and onlyneeds 10 be optimized with respect 10 linearity.

The DC-voltage at the output of this rectifier isthen proportional to the peak value of the volt­age to be measured. The meter is connectedto this position

Power supp!y

RF

T l

)--e- OPflHl' ir.g e-

e,= AF module

1- ·nodul... e-

IMo~er Fig . 3:

..~d l vi d ua l modul es of themill ivoltmeter

_ ___ _ _ _ _ ___ _ __ __ _ _ _ _ -2Yl-lf'.COt.lUV.... lCATION$ 2184

2.THE INDIVIDUAL MODULES OF THEUNIT

The basiC verSiOl1 of the AF-milliVOllmewcompuses Irve modules. This can be seen InFig ur. 3. W,th the ellcepllOfl of the probe arldme meier. all modules are constrl.lC1ed on PC­board! of El.lrope an standard see.

II a 19 inChcablnel is used.it is possIble lor theun it to be esteoo ec easily_ A logarithmiCamplifier is availa ble as a sillth module. It canbe connected between the meter and twocompletely co nstructed baSIC vers ions of Ihl!RF-millivolimolfll. This allows gain and aue­r maticn values to be mea sured dire ctly in dB.

Furthprmore. the logari thm ic amp lifier ;90requned If the voltage 10 be rncasuren is to beindicated bneary in dBm .

1----"-' -'"· '1 ~ r t="'rl [>- r.,, 1 11_ _ 1

J•

FI<).': Pnnc:ipiol 01 t.... '"' proton

3.CIRCUIT DESCRIPTION

3." Probe

Thfl probe is equipped With two rectifie rs thatare coupled together, Fig ure 4 shows the prm­ciple 01tms circ uu. r ne rec nner 0 1 is led wit hthe AC-voltagfl to btl measured, and chargesthe capa citor This DC·voltage is supenm-

""

S01I--

0----,0,0,""

I !~ -l4-~'" ,,

,«)1 '"'', rsc "'" 11'501

J UJS02 :.tIS" ~• .T~'",..I 050'

"' ~. s,.: lAIc,"", dlIog._of the 1_ protM OLOH" oct!i

: " L R~O'

l m..OJ~T

--I I: r. ~ -j(-•

~0~ •

l~O1 "- - I L ~ OI,~ I T~• 00, • I e §

''''AIoO) LI,02

_Sob: Clteuil d iagr am Gllhe Ihroug........ probe Ol OHV 00. oJ

,

F.

,

F~

68

VHF·COMMUN ICAT I~!"S 2""""'- _

posed w ith the additional AF-vollage and ISfed to rectifier 0 2. If the peak voltage of 1MAF-signa l is just as great as the cc-vonace.the output voltage of D 2 will be zero.

In all oth er cases, <In ottset voltage will resunThe operation is therefore independent cunemagnitude 01 the two AC-voltages if ui e cha­ractens ucs 01 the two recuue rs are identical.The shape 01 tne cna racrensuc is pnmantyunimportant. HOW(.'V()r, it very tow AF,vollagesare to be measured attentio n should be paidthatthe rectified voltages are greater than thenorse .

Spec ial diodes desig ned for this applic atio n(zero-bi as and pomt-contact diodes) oro.however. velY expensive, For nus reason.expe rim ents were made with conventionalge rmanium diodes. The best results wereobtained using cheap germanium point coo­tact d iodes, type AA 119. It is possible Withthese diodes to increase the mcasurinq rangeof the RF-millivoltmeter down to approximately1 rnV

In orde r 10 avoid thermal voltages Irom theobject to be measured. it is necessa ry lor therecuuer 0 1 to be capacinvely co upled 10 theobject 10 be measured . Furthermore, anentIonsho uld be paid in orde r to obtai n a good , ther­mal stability so that the cnaractens nc c urvesdo not sh ift With respect to another due 10 thedifference in temperatu re.

The complete circuit d iagram of the probe isgiven in Figure Sa. FIgure 5b shows the ci r­cu it diagram 01a through-line probe. The con­suucnon of tout probes is 10 be des cribed Insection 4.1.

The upper frequency limit of the probes ismainly de termined by the capacitance andinductance of the AF·d iodes, or their con nec­tions . Wi th the descri bed construction andgermani um diodes of type AA 119, the cuto ttfrequency is approx. 1500 MHz (-3 dB).

Th is co uld be increased by using specia lpo intco ntact d iodes. or zero-bias diodes.

3.2. Control Module

The co ntrol module consists of a changeoverswitc h fo r up to 12 measur ing ranges, an inputDC·volt age amplifier, and an imped ance con­vert er for the AF-supp ly of the probe. Theseare available in two versio ns:

In the case of the first versio n, the switching ismade mechanically using a swilch . ThisSWitch has three waters with 12 changeovercontacts Since such switches are relat ively

. expensive, an additional version was design­ed that uses digitally cc ntrouec FETs.Both err­cults are to be descr ibed .

3.2.1. Contro l Modu le with MechanicalSWith ing

Figure 6 shows the Circu it d iagram of the con ­trol module with mechan ica l switch ing . Onewill see a DC-vo ltage amplifier with live digi talgain values in the uppe r pari of the circu it ore­gram. Since the signal cominq from the probecan be very low. a chopper amp lifier typele l 7650 W<lSusoo for th is. This integrated en­cun behaves exactly as a normal ope rationalamp lifier Actually. it is two amplifiers belweenwhich a con tinuous switching is ma de.

The amp lifier that is 001In ope ration. is alwaysautomatically alig ned. The offset voltaq e otthe integrat ed circuit is therefore kept very low.

The field effect transistors T 20 1 and T 202serve as over-vonace protection and have alow leakage current. Tho zero-alignment canbe carried ou t with the aid 01P 201.The ope ra­uonat arnpliher I 202 serves as impedanceconvener lo r the AF·voltage div ide r. which isswitctied with the aid of wafer 'c' of the switch .This im pedan ce converter supplies the probevia a transformer,

The transformer is nec essa ry in order toensure that a sufli cic nlly high AF-am plit ude ISavailable 101 the large measuring range. Withwater 'a' of tne switch. a i"'~ (10 dB ) switching iscarr ied out. This works direclly into !he AF­modu le.

69

VHF.cOMMUNICATIQNS 2/84

"

I 203

:-j-f.---1c.2-;f7+-f_''"b-r( 2091 204 (110 (211

i: I 0 1 1 · -Ub

JO~ T T T+ T~009 C210 Ul1

o-rl- - - - --- "

10----.- --- --_ "

r-- - - - - --- - - ,.,.--- - ------ zz

0 06

13)- ---1 MOl

2~C 2 0 ?

11'02"'S201, S20h

T tev T"'"008 ,

'"

"'",m

'",ns

Fig . 6: Cl rc:u it d iagram of t he eonlrol modU le DL OHV 002

T1"""R20 1 R106 ' '''+

iR208 \,<

P 20 t

R201 R209 R210-

i R203 C203

'"0 01~

jR211

R204 V ±OQ50 02 -2Kj ~

- Vb

~, + Vb~

7()

VHF.cOM MU'IliCATlO'llS 2184 ----- - "c•"~ §;~ S~ § ~ ~ ~ ~ •

.,•

n616

T<,tl4

Hl610

, 602

D ~' 2

•

1601

s••l

~•"

."

T6C,

1<0 2

Fig_7: Co ntrol modu le with eteet-emc switching , DLO HV 006

71

3.2 .2 . Conl ro l Module wllhElectronic Switc hing

As can be seen in the circuit diagram given inFigure 7, all switching ccr uacts at the mecha­nica l switching are now made with me aid 01FETs or MOSFET-switches (1607. I 60 8).

These are controlled with the aid 01 a digita lci rcu it, which is mainly tormed by I 602 , abinary 4·bit torwa rd·backward counter. Thecounting impulses drives 1601 that is connect­ed as a dual start-stop-oscillator

In order to obtain an indi vidual co ntrol line foreach measur ing range. the BCD -signal fromI 602 is decoded by I 60 3, a BCD to decimaldecoder. The measuring range is ind icated byLEDs D 602-0 611 which are driven by transi­stors T 608-T 617.

In order to avoid a jumping from the highest tothe lowest measuring range, and vice versa.the actual counting input of I 602 is blockedwith the aid ot NOR-gate (I 604) on reach ingIhe maximum measunng ranges.

Since the lowest measuring range canamount to 3 mY, 1 mY, or 0.3 mY, according tothe selection of the diodes in the probe, thepossibilily was provided of connecting Iheblocking line lor me downward counting tothree different outputs of 1603.

Capacitor C605 togethe r with A 606 prov ide areset signal tor I 602 alter switching on. 11 isthen emcmenceuv switched to the higheslmeasuring range.

In additi on to the described oper ation. theintegrated counling circun also allows Ihe pes­sibili ly 10 load a counter stare direClly. Therequired lines are provided on the connectionstr ip of the module (con nections 4-' 3), Th ismakes it possible to remote-control the unit. orlor it to be used together with a computer inter­lace. The re mote-control mode is indicated onthe control module with the aid of tne LEDD 601.

CMOS-ICs have been selected for all integrat­ed digi tal cncuus. This oeieies me problem 01matching the levels to the rield effect t-enet­stars, and of an additional voltage stabiliza­tion. The follo wing should be noted with

72

VHF-COMMUNICATIONS 2184

respect to Ihe ope rating voltage: Variousmanufacturers give a maximum operatingvoltage of 15 V for CMOS-circuits. Philips(ValVO) allow a maximum voltage 01 18 V fortnerr circuits ' However, no problems wereencountered using circuits Irom omer menu­facturers

3.3. A F-Module

The operation of the AF-module can be seenin Figure 8.

The DC-voltage sign al lrom the controlmodule is fed 10 the converting input of 1 10tThis c oereucnat ampliher is swilcned as aninteg rator. Its output must be able to be drivenclown to tho negative operating voltage, sincethe amplitude of the AF ca nnot be controlleddown to zero. For this reason, an ope ral ionalampli fier type CA3140 was used here.

In the case of I t02, a so-called OTA (Operatio ­na l Transconductance Amplifier) is used. Thisis an operational amplif ier whose gain can beadJusted by prov iding a current in lhe co ntrolinput. This control current supplied by the ou t­put of the integrator via A 102. It controls theampl itu de 01 the AF, whi ch is gen erated in anRC-oscillalor comprising t 103.

The AF-voltage propcmonat to the RF-voltageto be measured , is present <I t the outpu t of theOTA under lock-in cond itions This is fed via C112 to a very linear recuner; which is followedby a RC-Iowpass nner (A 1321C 116), and animpedan ce converter. A sui table meter canthen be co nnected to test point TPA.

The recnrier Circuit seems to be very extensiveat brst, however, il onere a greal advantageover other circuits in that its operat ion is inde­pende nt of the impedance of the meter, and apure DC-VOltage is available at uie output.This means mat one can also carry out mea­surements using more accurate, externa lmeters, if a suitable connection is provid ed .

The controlled AF-signal is fed via C 105 to theamplifier stage equipped wilh I 104. lis gaincan be switched , using the range switch on thecontrol mocu!e, from 1 to ylO (10 dH switch ­ing). This allows one to Obtain a suitable rangeswitching.

VHFoCOMMUNICATIONS 2/84

[ Wi R1C' r1C4I---<

r~1 1 Rl01Rl0" (10 1 Rl 05

( l e I

T T ~1;>100 II il l~ l

~r P ICl \:::1 11 108 011o,

/l l02

---I f- +'" 11 :03 R01 11 I(lOS WI03 0 101·10..

~Jn 1~ 2

,1 1O~ ~t; n,,

<I P 10.. • r, ,n, }!03em

r f- (101 , ' , H, ;kol '" ~ e ~ 13n, U'1 T r-, , I---< ,

:,~: ~\7tsej-"' ( 1(0 8 n 1 10 HO 1~' . " 06

l-I T r-r-

~~

~"Ulft:);r' F,,~I 0 118

'",

~,, ~~lH , 28 ,

'"ce o c115, +I~ 9 , , c 0 S101

m '"H i >0.

jI"" , •n 0 n. ~Hlics >0' r

~ le 107 " IS

" ,120 n, 3~~.Ub

2 ];1'7 Jrt18I cm

>0"

""

""

FIg, 8:CIrCUli d ' agram ol ltle AF-mod" le OL°HV001

T.l

' R10.

"._----s ':,- - - - - -,

j :"i']]:, ~ t ,

I I "22 0 V ~ " , : .i.

' I ' .

T~I~ _ _ : L -l--'

""'"

1

VHF·COMMU NlCATION S 2184

fig . 9 :Circu it diagram 01thepower supply DlO HV007

A second recti fie r is connected to the output 01this amplif ier stage mat is identica l to the pre­viously mentioned one. This is provided forthe connectio n 01a d ig ital voll mc ler. The rect i­ne r equippe d wi th I 106 Always provides thesame voltage tor the lu ll-scale value in eachrange. independent 01 the selected gain 01I 1001 , and therefo re only lwo different scaleswit h fun-scale valuesof 1 and 31 6 arc requiredfor the anatoq me ter.

On the other hand. a DC-voltage 01the corres­ponding value is required for the connection ofa digital voltmeter

The AF·signal is fed from 1104 via C 106 to mevoltage divid er and driver stage 01the controlmodule.

3.5. Mete r

The indicator module comprises only a meter,whose lull -scale oeuecuon is adnrsted to 1 Vwith 'he aid of a dropper resistor It should beprovided with two scales having a full-sca levalue of 10 and rTo = 3.16.

II the measured voltaqe is also to be indi catedin dBm, this will require a further scale.

If a logarithmic amplifier is available. the indiocalion 01dB and dBm will be l inear. Toread 011the voltage in dBm, II is necessary for theupper scale to be additionally provided with amarking from - 10 to 0 in the opposite direc­lion. These scales can be seen in Figur e 10,

Fig, 10: Mete. scales of the author 's pro tot ype

3.4. Power supply

At!described modules are equ ipped with theirown voltage stabilizer circuit. This allows anymterterence on the supply lines to be enecn­vely suppressed.

In addition to this , a relatively simple ci rcu itcan be used as power supply (Figu re 9), andno probl ems are encountered Wllh heal drsst­pation .

The power supply modu le only consists ot atran stormer, recnner, as well as the associatedsmoothing capacitors for the posit ive andnegative voltage.

Two fuses are provided for overload ororec­non.74

• - --- •

VI-'F-COMt.lUr>;I (;ATIO"' S 2;~,- _

•9

:.IIII•

..IIo

. ~ -.. ~ , . .' ;' -'-.'. ' -..-"'7', ..~ ~. ~T--:,.,' ,

.~ ', ....

•

, -' ....... , ... ..._­..•I; ; ; I

~ ... ,o ..­o ,g ;•

•••

Fig . 11, The completed RF-mill' vo.. ....ter in II 19 inch caboncl log"lh~r with on., d ummy pan~1

••CONS TRUCTION

Wllh the exception of the probes and the indi­cator module, all other boards ottne measu r­ing system are built up using cou nte-coatecPc-boarcs 01tne standard EUlopaan formal.

All modules are provided .....uh a St -pm con­nector strip. This allows the un it 10 be accom­modated in a 19inch cabinet.

Enough room is provided in tile cabrne r lorextend ing uie instruments. Tile selected con­struction allows all modules to be easilyaccess-bro. As in the case of the autho r's pro­totype (see Figure 11), all non-used portions01 the cabinet can be covered with a dummypanel .

4.1. Construc t io n c t th e Probes

Figure 12 shows the two through-line probes,as .....el1 as a lest probe.

Fl9.1 2:Tesl p.obes 10. 100RF-millll/Ohmeler

75

k_- . V.,F·CO..,1\1Gf, ICATlOf\S 2184

+

+

Fig . 13:Component locatio n plan 01PC-boardOlO HV004 (throug h-line pr obe)

4.1.1.Through-Li ne Probe

The through- line probe is accommodated in ameta l box, whose dimensions are 37 x 74 x30 mm . One BNC-conneclor is provided oneach of lhe wide sides, The cab le is ted in 10the bottom of the box as shown in Figu re 12,The components of the probe are accommo­date d on a PC-boar d of 11.5 mrn x 34.7 rnm:Ihi s board is double-coaled . and one side isprovided as a continuous ground surface(Figure 13.) This board is designa ted DLOHV00 4.

ettennon should be paid when constructingthis module that the ctup capacitor C 401 issolde red into place di rectly on the board. Theconnections 01 tht! BNC-connectors arc also

directly solder ed to the conductor lane,

A screen ing panel should be provided behindcomponents R 401 and D 401, in order toisolate the AF-side from the AF-circuil. Theinstallation oune other compo nents should bemade accord ing to the component locationpla n given in Figure 13,

Capacitors C 402 to C 404 should . if possible .also be chip-capacitors. The interconn ectioncable should have three cores two of whichmust be screened The components lo r thethrough-line probe are given in Table 1.

4.1.2. Test Probe

A 9 mm x 68 mm narrow board was developedfor accommodating the components 01 tho

Quanl ity', Designation Component

21

1

1

14

2

21

2

76

D 401, D 402R 402R 401R 403C 405

C 401-C 404L 401, l402l 403, L 404

Diode AA 119 (BAT 16, GO 741}

Resistor 47 n

Resistor 4,7 kJ:lResistor 47 kOCapacitor 100 pF JceramicCapacitor 1 nF I ceramicFixed inductance 1 jlH

Fixed inductance 10 ! jH

Metal box 30 x 37 x 74 mmBNC connectorsCable a-core. individuallyscreenedDouble-coated boardDL0HV 004, 34.7 x 71.5 mm

Table I :Li st c t co mponentsfo r th eth roug h_line probe

VHF·COMMUNICATIONS 218~

0504 '00 0503

L501 DO" 02

Fig. 14:PC·bomd DLOHVOOS IOf lhll ' " sl p.o....

test probe,(see FIgure 14). It is double-coatedand has been designaled DLOHV 005. It canbe mounted in any suitable metal box. Atten­tion should be paid that the interconnection ofthe lest probe fa the first rectifier is kept asshort and esicwce pacnance as possible.

In the case of the author's prototype. the com-

Quantity Designation Component

--~

pleted PC-board was built into a brass tubehaving an inner diameter of 9 mm. Spec ialattention was paid to obtain a simple construc­tion.

As was already mentioned in the ooe ranonatdescription , il is very important with respect tome measuring accuracy mat me charac­teristic curves of lhe two diodes used, coin­cide as well as poss ible. The selected diodesshould therefore be from the same productionrun. It this is not possible, it is necessary forthe diodos to be selected - this is also validfor the through-line probeI

Of course, it is possible lor diodes speciallydeveloped for RF-measuring equipment to beused instead of the diodes type AA 119 Suchdiodes are the BAT 16 or GD 741 manufactur­ed by Siemens. These are suitable for higherfrequencies, but also have a far higher price.

Table 2 cor aams a list of the components forthis test probe.

4 .2 . Consl ruc tion 01 the olher Modu les

As has been previously meouonec . all PC­board modu les with the exception of the probeboards, are built in standard Europe an size,and equipped with their own St-pin connectorstrip. Doub le-coated PC-boards are used forthe AF·modu le, control module, and logarith­mic amplifier. A single-coated Pc-boarc wasdesigned for the power supply. Each board isprovided with its own front panel, whosedimensions are 25.4 or 50.8 mm in wid th .

4

22

1

3,2,

O SOt - D 504R SOl , A 503R S02, R 504

C505C SOI- C 503

C504L SOl , L 502

Diode AA 119 (BAT 16. GD 741)

Resistor 180 Q

Resistor 10H~

Chip capacitor 100 pFChip capacitor 1 nFChip capacitor 22 nFFixed inductance 10 11H

Dout ne-coatec boardDL0HV 005,9 x 68 mmCable a-cere, individuallysc.eenec

Casing

Table 2:L.IM or eom"(ln~nte

lot the test probe

77

_ _ ~ VHF-COr,WU NICATIONS 2.'&:

,

'"L

f lg _15' Compo~nl lo<:..lion 5 c t tne AF·module DLOH¥ 001

78

VHF-COMMUNICATIONS 2/84 - - - - -'"4.2 .1. AF -Module

Figure 15 shows the component locations onthe doub le-coated PC-board DLOHV 00 1.Thecompo nen t side of this board is provided wit ha con tinuo us groun d surface, and thi s is drill­ed free for those con nect ions that are not con ­nected to ground. Figur e 16 shows a photo­graph of a comp leted board. The requ iredcomp onents are given in Table 3. The rectifierc ircuit comp rising 1105 need on ly be providedwhen an output tor s. d ig ital voltme ter is requ ir­ed

The AF-module is provided wit h a 1 inch-frontpanel, who se dimensions are given in Figure17. 11 is connected to the Pc.bc aro with the aidof a bracket. It is not necessary to provide anymarkings on the front panel.

If.I

I

1Fig. 17:Front panel of th eAF-m od ule

Fig . 16: Photograph of a co mpleted AF-module

79

~ -,V"H" ·"C,O"M,,MYNICATIONS 2/84

OoJant'ty Designation

, 0 101 _0 1(14

4 0 105 - 0108, t 101, 1 102

2 1 103. 1 1(14, 1 10&, 1 106, ,'", , we4 R 123, R 1:' 4

A 133, R 134

t R 104, A W,a R 106. R 107,

""5 R 1 0~, R 109R 111, R 113

R 114, R 1;>5, A wet R 11;>

9 R 115, R 116R 118, R 120R 121, R 126

A 129- R 131, R 110, A la Ia A 122 , A 132, R 117, R 119

R 127, R 1;>11, C 103. C 1(14

a C 108. C 113, C 102, C 105C 107, C 112

W C 117, C 119, C 101, C 109C 1H

a C 111.C 116, C ice2 C 118

a C 110, C 115

a P 101, P 102

P to, S 10 1,,a

a,

80

Compona<1t

ncee I N 4148Diode HSC H 1001

CA 3140 ECA 3080 ETL 071 ceTL on CP

MC 7808CTMC 79GB CTAEbisto r 100 \l

Rcsistnr 1!lO (I

ResiSl<..- 2.7 enReSIstor 4.1 kO

Resistor 5,6 kllResistor 10 kO

Resi~lOf 12kO

He s'stOf 22 «nRe~i SlOr 30 kll

Rp.sistm 39 kll

RC'S,stor 82 euRes islCw 100 kU

ResistOl' 4/0 kO

Resistor 1 MIl

Cap<lci tor 4,7 nf I f KCCapacitor 10 nF / MKSCa[l'lcitOf 100 ~ f- I MKS

C"~ liC i lO r 100 nF I ceramicCap'lCil<1r 220 n F I MKS

CapacI tor 680 nF I MKSc eoe cao- 1 i' F I MKScaoecno- 101l F, 16 V I tantatu m

Capacitor 47 i' F 16 VI tanlllhmlPotentiOlTl6ttlf 10 kQ(cal l 061313(0)

Pol enl,om ete r 4 ,7 kU(call 061313(0)

MFP 120 Knitter SwitchConnector strip 31·pin, DIN41617

Front panel 1 inch veto F 1V1F

Telephone jacks iJISulatad

Solder p'ns

Screws ano nuts

Mica dis k for TO 220PC·board DL 0 HV 001 ,100 x 160 mrn. dooble-coaTed

To be concluded in ed. 311984

t eeie a:Li st of componenl s10f l he AF-module

VHF·COMMU~ICATIONS 3184

Bernhard Kokot,and Dieter Schwarzenau

A Home-Made RF-MillivoltmeterSecond, concluding part

4.2.2. Mechanically-swi tched Contro lModu le

This double-coated PC-board has been de­signated DLOHV 002 Figure 18 shows thecomponent locations on this board andFigure 19 a photograph of the author's proto­type Table 4 gives all required components.The two inch wide lront panel is drilled asshown in Figure 20.

4.2.3. Electronicall y-switched ControlModul e

This module comprises the electronic boardDLOHV 006a (Euroboard) and a control boardDLOHV 006b (see Figure 22). whose dimen­sions are 40 mm x 100 mm. It is mountedbehind the two men trent panel as shown inFigure 23. A photograph of the author's proto­type is given in Figure 24.

Fig . 18: Componenls of the con l rol mod ule (mechanical) Dl OHV002

146

VHF·COMMUN ICAT ION~--,_"",~ -",

Fig . 19 : Pholo g raph o l the au thor's pr ot ot ype (:011110 1mod ulewitll me chanica l switt.h ing

•

I- ~­-~6- .

s I'" E'

'-91I r 15.~ ­. '

'JEff];"I

All d lm <m slOn s In mm

Fig. 20:FfOnl panel 1o . l heme(:hanica lly-swi lchedconl'Ol module

.L~~~. 0 .

". R617 ~" ",,":: ';"

Fig . 21: Components ol lhe e lect.onic boa.d OLOHV 006.. to. lhe e le(:troniC<'llly-sw lld'led (:onl'Ol module

'4'

Ouant. Designation Component

1 1201 ICL 7650 CPO(Intersil)1 1202 n 071 CP1 1203 MC 7808 CTI 1204 MC 7908 CT

2 T201, T 202 BF 256

2 R 207, R 210 Resistor III

I R 216 Resistor ,onI R 215 Resistor 90 04 A 217 Resistor 100 U, A 214 Resistor 900 02 R 201, R 208 Resistor lknI A 213 Resistor 9 kU2 R 202,R2tt Resistor 10kUI R 212 Resistor 90Hl1 R 203 Resistor tOO kn3 R 204, R206 Resistor 1 Mrl:

R 2091 R 205 Resistor 10 M~}

I C208 Capacito r 1 /IF I MKS2 C 201, C 202 Capacitor t nF /FKC

2 C 206. C207 Capacitor 22 nF I MKS2 C203. C 204 Capacitor 100 nF I MKS7 C205, C2Q9 Capacitor 100 nF I ceram ic

C2 t22 C210 Capac itor 10 JIF I tantalumI P2O ' Potentiometer loo nI TA 201 Potted core 14)(8 . A121 00

(Siemens) wllh con former andmounting plate1.) 300 turns; 2.) 75 turns ena-melled copper wire 0.1 mm dia.

I S 201 Switch SBL 11.3_ 12, BU 201 Connector, 31-pin connector DIN 416 17, Front panel 2 inch I Vero F2V1F1 Knob for 6 mm dia. shaft2 Mica disks for TO 220

Screws and nutsSolder pinsPc-ooard DLOHV OO2.100 _160 mm, double-coated

' 48

VHF·COMMUNICATlorlS 3/8 4

Fig. 22:Componentloc3t lons 01t h~ conlrol b<Ho .d OLOHV006b 101" Ih~

~C<:tron IC .ny-_ 'lched

cont rol module

Table 4;Compon~n l S li st lor th emechanicall y_swi tche d cent re !module

VHF-COMMUNICATIONS 3184

All d'......nsions in mm

sO> Fig.23:Fron t~n.llor the e lOH;tronlc al1v­switch ..:! conlrol modul.

~_.........' .'- -..

"

'Ii"~~)

14'

VHF-COMMUNICATIONS 3/84

Two aluminium blocks of 10 mm per side areprovided with a threaded hole of 3 mm diame­ter and are used for mounting the controlboard DLOHV 006b behind the front panel. Itis fixed to the rear side of the front panel withthe aid of a metal adhes ive. The elect rical con­nect ions to the electron ic board DL 0 HV 006aare made via a te-oore flat cable with DIL con­nectars at each end, and several addit ionalwires. Table 5 gives all required components:

Quant. Designation Compon ent

11 0601 -0611 LED L0 350·4

5 0 612 -0 616 Oiode l N4148

2 T 60 1, T 602 FET BF 256

5 T 603 - T 607 FET BF245 C

10 T608 -T 617 Transisto r BC 237 B

1 1601 NE 556N

1 1602 HEF 40192 BP

1 160 3 HEF 4028 BP

1 1604 HEF 400 1 BP

2 1605, 1606 HEF 4030 BP

2 1607, 1608 CO 4066 BE

1 1609 TL 07 1 CP

1 1610 ICL 7650 CPO

1 1611 MC 7808 CT

1 16 12 Me 7908 CT

2 R 633 , R 634 Resis tor 3.9 0

1 R 626 Resisto r 1011

1 R 625 Resis to r 90 n

2 R 620, R 62 1 Resistor 820 c1 R 624 Resistor 900 0

1 R632 Resistor 1kD1 R623 Resistor 9 kO

8 R 601, Resistor -oscR604 - R 608R 627 - R 631

16 R 609 - R 619 Resis to r 47 kOR 637 - R 641

2 R 602, R 603 Resistor 82 kO

1 R 622 Resistor 90 k~ :l

1 R 630 Resistor 100 kO

150

4.2 .4. Power Su pp ly

As previous ly mentioned, the PC-board for thepower supply is only single-coated and hasbeen designa ted DLOHV 007. Figure 25shows the components on this PC-board ; inthe photograph of the open power supply(Figure 26) one will see the toroid-core trans­former, as wel l as the temporar ily used toroid­core choke, and the charge capaci tors C701and C 702 formed trom two paraltel-connecteccapacito rs. Al l requ ired parts are given inTable 6.

VHFoCOMMUNICATIONS 3/84

3

1

1

22914

21

1

2

2

2

1

1

21

2

A629, R635R 636

A 628C607

C608,C 613C6l 1, C612C610, C 614

C 609C601 - C 604C607, C 615

Tr 601

Bu 601Tasl S 601,Fast S 602

Resistor 1 Mf.l

Resistor 10 MUCapacitor 1 of I FKCcaoa cucr 22 nF I MKSCapacilor 100 nF I MKSCapacitor 100 nF I ceramicCapacitor 1 /IF I MKSCapacitor 2211F, 25 V/ tantalumCapacitor 10 !,F, 16 VI tantalumPotted core 14x8 , AL 2100with coil former and mountingSUPlX'rtpr.: 75 turns; sec.: 300 turns0.1 mm dia. enammelled copperwireconnec torDigil asler, black

OIL sockets is-poreOIL connectors, t e-ccreflat cable, t e-ccre31-pin connector DIN 41617Front panel 2" Vero F2V1FAluminium block 10x tOx 10mmPc -ooaro DLOHV 006,100 x 160 mm , dc ubre-coetecec-toaro 40 x l 00 mm,double-coaledMica disks lor TO 220Solder pinsSCrews and nuts M3

Tabl e 5:Components Iisl lor theelectronicall y -sw Itched co ntro lmodule

0

I I []0

D701 00

,'""D

,0

'"s

H! 701~,

8 EJ,

F~'D7Cl.. / .'0

Fig. 25 :ccmpcnemreeetrene on lhepower supply board DLOHV 007

151

VHF-CO MMU NICATIONS 3/84

Flg .26:Photograph 01the op enpower supply module

",]

Com ponen tDesign ationQua nt.-+- "--------Jf--'----- - - - -

2

2

2

1

1

1

11

1

11

24

Tr70 1

C 701,C702

Si702, Si 703

GI701

S 701

Si701

Pc-ooaro DLOHV 007,100 x 160 mm single-coatedToroid core transto rmator2 x 12V/20 VA

Elektrolytic cap ac ito r4JOOjlFf 40V

Fuse ho lde r lo r PC-boardmounting

Fuses 1 A, ine rt

Rect i1ier B40C3200Two-inch front panel Vero F2V 1F

Power swi tch with lam p

Power conn ector with fuse

Fuse 0.1 A, quick -action31-pin con nector DIN 41617

Cover 100 x 160 mm

Alum inium brecket t nxt sxas mm

Spacers M3 x 40Spa cers M3 x 37

Screws M3 and spr ing washe rsInsulating tub ing

Tabl e 6 :Components list lor the powersupply

152

VHF·COMMU~ ICATIO ,'j S 3.'84

All hal., 3.2 10m don

I:oo

•- 25-

- 15-- - - - -e-

,

;::1-,

Fig 27: Mechanical par ts fo r con structin g l he power supp ly mod ule

A completely insulated constr uction was usedfor feeding in the power sup ply voltage. Theauthor constructed a cover p late and a bracketas shown in Figure 27. The cover plate wasconn ected to the board using fou r spacers of37 mm in length. The power conn ector withbuilt-in fuse is connected to the bracket usingtwo further. 40 mm long spacers . After screw­ing the bracket to the cover pla te, it is possiblefor the interconnection to be made betweenthe power supply and power connector If theinterconnection points are covered with insu­lating tubing, sufficie nt attention will havebeen paid to avoid electrical shOCK .

In order to ensu re that the power connector isaccessible from behind , it is necessary for acutout to be provided in the rear panel of theuni t, whose di mensions are given in Figure28. The 2 inch wide front pane l for the powe rsu pply plug-in is final ly shown in Figure 29.

4.3. Installati o n into the Cabinet

at-core con nectors (DIN 41617) are installedinto the plug-in cabinet at the req uired pos i­tions. At the same time, solder tags are mount ­ed simultaneously with the screws, which arethen con nected to co nnections 1 and 31 inorder to provide the requ ired grou nd connec­tions

. -- - - '

c , - - - - - - - - --,,,

o. _--- - - - - - - -- ..- - - - - - - - - -_ .. _-- - - - - - - ~

Ven t-i :o ~i J n vents '

c

Fig. 28: Cufout on the rear panel of the cabme t

153

Fig. 29:Dimensional drawinglor the ' rool pan..101100~r supply

VHF-COMMUNICATIONS 3184

The output of the AF-module - eit her con nec­tion 24 or 25 acc ord ing 10me position of S 101- is connected to the corr esponding inpu t 01meloganthmic circu it.

5.ALI GNMENT

Finally. the power supply lines are provided inthe cab inet. and soldered to conn ections 3and 29 of eacn connector strip

The following mterconnectmns are requiredbetween the control and AF-mod ule: Connoc ­nons 17, 18. 19. 20, and 22 .

The alignment can be made alte r the describ­ed modules have bee n constructed and wi redtngether.

Firstly, check whether the AC-oscillator 01 theAF-mod ule is operating corre ctly. An AC-volt·aqeot appmx. 1.5 Y AMS should be present attest point TP 102with a frequency 017.5kHz. IIth is vonece is too low. it may be necessary Inch ange the values 01 capaci tors C 103 andC 104, The frequency 017.5 kHz is not critic al

For the actua l aliqn ment. the circu its shouldbe connected as shown in Figure 30, Anyavailable transmitter can be used as signalsource. It should only be able to provide asignal of suffi cient, and constant ampl itude.

The atten uator. or output voltage of the sign alsource should be adjusted so thai a Dc -vou­age of 4 ,47 V is meas ured after the SChotlkydiode.

Att f OOO'...

Fig. 30:Alignmontcircuit

154

VHF·COMMUNICATlONS 3164

Set the RF-m illi voltmeter 10 its 3 V range andconnect a meter to tesfpoint A of the AF·module. Align P 103to obtain a voltage of 1 Vat this testpoint (full scale)

Final ly, an output vo ltage of 0.316 V is alignedwith the aid of P 101 in the 10 V measu ringrange . If the same meter is used for align mentthat is to be used in the indicator module, andil it possesses two sca les. this ind ication wil lcorrespond to the same value, bu lan the othe rscale.

The 10 V measuring range is also sele cted foralignment oune digital vonm eter ou jput. P 102is aligned so thaI the same voltage is presentat tes tpoi nt Das at testp oint A.

6.EXTEN SI ONS

k

s 1U1~"::I

R ~ ~2[1U2)

CD "0,

,,"( tl ~II~- rk,,,

'7 - - -1- --,~

t 1"0" t

(1)"

Fig . 31; P rinciple of the lo ga rithmi c CIrc u it

with the most important comp onents, and theassociated character istic cu rve. Switch S isclosed with a cer tain l ixed Ireq uency in orderto charge capacitor C to a voltage U" alterwhich II opens

SInce U , is greater than U2, the output 01com­parator K w ill become po sitive at the sametime. The capac ito r will now di scha rge itselfv ia resistor R. The voltage can now be ca lc u­lated accord ing to th e lallowing equation :

When the value 01 U2 is reached , the compa­rator WIll switch , and the vol lage al its ou tputwill be negative again

If the corres pond ing values for this conditio nare p laced into the equation . and jj i t is resotv-

155

t

-

nx C

1-

u(t) : U,xe

Figure 31 shows the princip le use d tog ether

6.1. Logarithmic Circuit

The log arithmic c ircuit to be descr ibed rep re­sents a useful e xtension to the AF-mi lli volt­meter. It is pos sible using this mod ule to readall vo ltages or attenuation values d irec tly indBm or dB on the linear sca le of the meter. It isonly neces sary for the meter to be marked inthe opposite d irection from - 10 to 0 lor thedBm-indication .

The logarithmic circuit is conne cted in front ofthe meter. It pos sesses lwo inputs betweenwhich one can switch (voueqe measurement),or which ale both used lor measurem ents 01voltage ratios (attenuation or gain ).

6.1.1. Princi ple of th e Logar it hm ic Circ u it

Most know n logarithmic c ircuits use th e expo­nential characteristics of d iodes or transistorswith the aid of operationa l amplif iers. Thed isadvantages of this p rinc iple are the compli­cated alignme nt, and the large temperaturedril t . For this reason , a dmerent method wasused he re. which uunzes the d ischarge curve01 a capacitor 10 obtain the exponentionalCharacter istic.

",--

-~3

03~

1/4

~

~307

~C3C

1

R30

e~D3C2

TI

304

-"R

316 5

10

0­

P3C

1J

R31

9

• ­N •

'''''C

•B

TC3

0~R

327

R32

4....

.L:..

P30S

7

".

~S

3C2

c..

-nsc

..

4

Fig

.32:

Th

elo

gari

thm

icc

ircu

it

'30'L O

HIV

VHF-COMMUNICATIONS 3184

ed according to the delay time, the followingwill result :

u,t2=RxCxln

u,

The pulse width of the voltage at the output ofthe comparator is therefore a measure of thelogarithmic relationship between the two inputvoltage s. A correspond ing DC-voltage isobta ined for the meter by forming the meanvalue of the signal ,

6.1.2 . Logari th mi c Circuit

The circui t of the logarithmic modu le is givenin Figure 32.

The two inputs are buffe red with the aid ofoperational amplifie rs (I 301 and \ 302). TheLEDs connected to the four comparators(I 303) indicate when the voltages are too highor too low, and that another measur ing rangeshou ld be selected .

It is possi ble tor the various operating modesto be selected with the aid of the tour-positionswitch S 301, In the two upper posit ions, oneofthe input voltages is directly indicated on themeter connected to connec tion 10. In the otherpositions, these voltag es are passed via the

logar ithmic circuit.

The swi tch posit ion should be selected atwhich the lower of the two voltages is presen tat the inverting input of the comparator (1306).With the aid ot S 302, it is possible for th is volt­age to be repla ced by a fixed voltage as provid ­ed by 1308.This is necessary, if individual volt­ages are to be measured in dBm.

The actual logar ithmic circui t diffe rs from thatdescr ibed in the previous sect ion in that it pos­sesses one add itional switch, each , in thedischarge path, and subsequent to the capa­cito r. These are neces sary because the MOS­FET switches used here exhibit a resistancewh ich cannot be neg lected whe n they are con ­duct ing , since this would form a voltage divi­de r in conjunction with the discharge resistor,Further more, this means that the charge timefor the capacito r cannot be kept as short asreq uired , wh ich means that the comparatormust be disconn ected from the capacitordu ring this period.

The switches are controlled from a time r Ie(1305), whi ch is connected as multivibrator. Itqenerates a squa re-wave signal with a keyingratio of 4.5:1 at a frequency 01appro x. 750 Hz.

The output signa l of the comparator is fed via

Fig. 33:Photograph of theauthor's prototypelogarithmicmodule

157

VHF·COMMUNICATIONS 318 4

Fig . 34: Compo ...... ts on thoe logarithm IC: board Dl OHV003

a diode (D 306) 10 a low-pass filler tor lormingthe mean value

The required DC-voItage signal is available erthe ou tpu t of 1307 and is led to a me tes.

6.1.3. Consl rucl ionof Ihe Logarilhmic Circuit

A oocbre-ccaieo PC·board (Eurocard size)has been developed tor accommodating thelogarithmic circuit (Figure 33). This board isdesignated DLOHV 003 and should beequipped according to the component loca­tion plan given in Figure 34. The componentsare listed in Table 7:

6.1.4 . Alignment of th e Logarithm ic Circ uit

For alignment of the logarithmic circuit. thetwo inputs should be connected together andgrounded. The offset of the two input ampli­fiers can be aligned with the aid of trimmerpotentiometers P 301 and P 302 and read offwith the aid of a voltmeter connected to points3 and 4.

158

- lr·-----l. -

Ir''-

_ "_?4'

ee i,, P-I

1,

t- I ~

:lliiiI "I ', ,, I II _

All di me",. i on a in mm

IB

Fig. 35:Frontp~nel 01th~ logari thmi cmodule

VHF-eOMMUt.;ICATIONS " ,Quant , Designation Component

Table 7:4 0 301 -0 304 LED LO 350·4 Compo-1 0306 Diode H$CH 1001 nente list

4 130U 302 TL 071 CP for Ihe

1307, 1308 loga·rithmic, 1303 LM 339 N module, 1304 CD 4066 BE

1 1305 NE 5551 1306 LM 311 N1 1309 MC 7808CT, 1310 MC 7908CT1 A 312 Resistor 62 Q

4 A 301. R 304 Resistor 100 cR31 9.R 320, R 325 Resistor 220 o

4 R 309, R 310 Resistor 470 nR 313, R 314

1 R 324 Resisto r 1.2kU1 R 318 Resistor 3.3 kQ1 R 308 Resistor t.eec1 R 322 Resistor 3 kU1 R 315 ResistOf 3.9kO1 R 321 Resistor 36kO

10 R307.R31 1.R316 Resistor 10kUR 326. R 302, R 303R 305, R 306, R 327

R328, R 317 Resistor 12kU, R 323 Resistor 100 kU1 R 329 Resistor , MO1 C 305 Capacitor '00 nF / MK8

' 9 C301, C 302, C 304 Capaci tor 100 nF / ceramicC 307, C308, C 309

C310, C 3111 C303 Capacitor 150 nF / MK81 C 306 Capacitor 1,uF fMKS2 C312 Capacitor 10l/F, 16 V/ tantalum1 C313 Oapacrtor 3.3 nF / FKC3 P 301. P 302, P 306 Porenuorneter 100 kU, P 303 Potentiometer 2.2 kQ1 P304 Potentiometer 470 n, P 305 Potentlomerer 100 n, 8 301 Switch SBL 1 1/ 1'4~3

1 S 302 Togg le change-over switch M90-3A, at-cere connector DIN 41617, Front panel 1 inch Vero F1V1F2 Telephone connector, insulated, Knob for 6 mm ora. shaft2 Mica disks lor TO 220

Solder pinsScrews and nutsPC-board OLOHV 003, 100x 160 mm,double-coated 159

VHF-COMMUNrCATrONS 3IB4~'-----------=~"""",,""'--'''''

<10% up to SOD MHz

<10% up to 500 MHz

1 MHz - 1.3 GHz10 kHz - 1.3GHz

8.REFEREN CES

< 1%

0 +1 V0 +0.316V11 V

60 '1> 80 kn/2,5 pF

(2) RCA: l ineal jnteqrated CucuusUSA: RCA Corpo ration 19n

(1) U. Tietze; Ch. Schenk :Halb leiterschaltungstectm ik. 5. AuflBerlin , Heid elberg.New York: Sp ring er 1960

Maximum DC-voltage com ponent at themeasuring probe: 40 V

RMS-value indication in the case 01si newavevo ltagesFrequency rang e:Through-probe :Test pro be:

Error limits:Operating module with AF-modute:logar ithmic c ircui t < 0.1 dB

Through -probe(AA 119):

Test probe(AA 119):

Imp edance s:'Thro ugh-probeTest pro be, 10 MHz:

DC-voltage ou tput:For ana log meter'For dig ital mete r;

nanscteotume: 1 s

Testpomt 8 is grounded lor the ottser align­ment 011307; P 306 is then align ed so thai thevoltage at testpomt 9 is sel la 0 V

The next step is to lest wheth er a square-waves ignal with a keying ratio of 4.5:1 and a fre­quency of approx. 750 Hz is availabl e at theou tp ut of I 305 (TP 5).

In switch po sition 3 or 4 (8 301). it is nec essaryfOf the elise! of the com parator to be alignedwith Ihe aid of potentiometer P 303. This ismade by connecunq a DC.vollage of 1 V \0connection 24, and a voltage of 0$ V at con­nection 25. This represents a ga in 01 6 dB anda logarithmic output voltage 01 0.4 V. Anydeviation can be corrected with the aid ofP 304. After this, connection 24 is providedwith a DC-voltage of 300 mV and a voltage of150 mV fed to connection 25.This also corres­ponds 10 a gain of 6 dB P 303 should now bealigned lo r an output vo ltage at TP 9 ot 0.4 V

This procedure should be repe ated alterna te­ly unW no deviat ions are observed . Finally, it ison ly necessary for the output voltage of I 308(TP t1) to be aligned with the aid of P 305 to0.2236 V(OdB ).

Com parative experiments mad e with thislogar ithm ic circuit and another constructedaccord ing to Tietze/Schenk (page 209, Fig.11. 22) show th at high accu rac ies were achiev­ed with both types

However, the describe d ci rcui t exhi bited bettertemperature dri ft characteristics that were atleast one Older of magnitude better. sunn er­more. the alignment is considerab ly easier.

7.SPECIFICATIO NS

(3) Valvo: Integr ierte Digi tal-SChaltungenLOCM OS-ReiheHamburg 'Verlag Boysen + Masch 1980/81

Power Supply: 220 Vl50 Hz

Voltages lo r the modu les: ± 12 V un staotuzeo± 8 V stab ilized

Measuring ranges: 1 rrN10 10V in nine ranges

(4) texas Instruments: Pocket Gu ideBa nd 2 "professional l ineal "Texas mstrumeorsDeutschland GmbH : 1977

160