Tektronix 494AP LOc AMp and Detector Diagram22, Fn9 input signat.at a nominal14 dBm, is applied

o

a

o

o

fn9 input signat.at a nominal_14 dBm, is applied

o

..

through

toroid transformer T1041 to the

Oaii of

t

common-emitteramplifierel040. Gain adjustment

Rl0i!0, in

the emittercircuit.sets the posi Vn impfin"i

o

gain. The outputis transformer

the base of feedback

coupled,by Tt0i0, to

o

include.s

amptifier OiOSO. ihis circuit

emitt€r d€generation

and collector-to-base

through resistorRlOg4

o

feedbackthroulh resistorRl041.

The collectorfeedbackhelps to

prorid"

a well-deftned

o

output impedance

function

of 50 ohms. input impedanceis a

o

of transformer T1040 ana relistor R1042

acrossthe primarywinding.

I

Fro.mthe finalamplifier,

.

the-signalis apptied

I

through

a band-pass filter with about 2

O-einserton toss. The

*6 dBm outputsignatfrom

o

th€ fitteris afptiedthrough

coaxialconnector

J6g2to the Log Amplinii.

I

o

LOc

AMp and DETECTOR

(Diagram22)

I

The

Logarithmic (Log)

Amptifier and

Detector

o

acceptsinput signalsfrom the Vi circuits. Thesignals

are amplifiedso the output is proportional

t

to the 1-oga-

rithmof the input. The outputii ttrenappliedto a linear

o

detectorwhich outputs a video signal. By controlling

the compressioncurve_characteristi-c",

t

in th€ inputsignallevelr€sutts

"""h OBctranfE

in an equal

changein the output. In the 10

incrementof

clBldivmode,

o

eachdivi-

sion of displacementon the screenrepresents

a 10 dB

o

changeof inputsignallevel.

a

Log Amplifier Circuits

t

,

Tlgr". circuitstogarithmicaily

a

amptifythe inputsignat

from the VR circuitsand apply the outputsignalto-the

o

Detectorcircuit. The Log'Amplifier

consistsof seven

ac-coupledamplifierstages. Eachstage has two gain

o

valuesthat dependon signalamplitude.-

In additionlhe

I

first three stag€s have an extra automaticallysel€cted

gain value. The combinedcircuitsprovide

o

highgain for

lowJevel signals and low gain for high-level-ignals.

For.the,outputsignalto be ploportional

o

of

to the log;ithm

the_input,more gain is reguiredfor a chang-efrom

a

-80dBm to -79dBm than a changefrom _1 dBm to

0 dBm. For a givenstage of the cir:cuit,the gainstarts

I

at

approximatelyl0 dB

for

a

low{evel signal and

o

decreasesto unity as the input signallevelincreases.

In

_thefirst three stages, the gain becomestess than

)

unityas th€ signalamplitudeincreases.

a

^ _lnput signaf levels nominalfy range between

-84

dBm and *6 dBm. As the signattev-etincreases.

o

the gain decrease begins with

the final stage and

proc€eds,in succession,_back

o

through the remaining

six stages to the first. Each stageinitiallyproduced

o

approximately

10 dB of gain. That gain

was reduced

unity,so

to

the total gain reductionis Z0 dB. Withfurther

o

increasesin input signaflevel,three more gainchange

o

a a a

Theoryof Operadon- 4g4A/4g4ApService, Vol. 1

st€ps take place. The gain of th€ first three stages is

reducedbetow. unityapproximately7 dB for each itage. This reductionstarts with the first stage and procee-dsto the third, to provide an additionatgain reductionof approximatety20 dB.

- As the Input signal increasesfrom _g4 dBm to

19d?r, the gain throughthe amptifierdecreasestoga_ rithmicallyso that the output signal is exactly propor- tional to the logarithm of the input. This ii accom- plished through a system of seiies diode limiting in each stage,with a second set of diodes for extra timit- ing in eachof the first three stages.

Thefollowingdescriptionof a simplethree-stagelogamplifier,with one gain step in each stage, proviOls ai aid -tounderstandingthe concept of a logarithmic ampfifier.Figures7-g, T-9,and 7-10 show an example amplifierand illustrat€its operation.The gain of each stageis.3.16(10d8) up to an outputlevel-of1Vpeak, then unityfor output tevetsgreaterthan t V peafi tnai is,.each stage uses one breakpoint. That breakpoint voltageis usedfor easeof illustration;the actualbreak- pointvoltageis significanflylower.

The amplifier is shown in Figure 7-g. The source has a step att€nuatorthat allowsthe input signalto be incrementedin 10 dB steps. Table7-5 shows the pro- gressionof gain reductionabove1 V at each ampiifier output. Note that with each input level change of 10 dB, the output changeat point 4 is 0.694V. The gain curv€ for one stage is shownin Figure7-9. Also note that whenthe levelat point 1 is increasedbeyond 1.V, it is beyond the togging range of the ampiifier. Similarly,if th€ inputlevelis decreased10 dB betowthe minimuminpul level,the output incrementis different. A curveof the loggingrangeis shownin Figure7-10.

POINT 1 POINT 2 POINT3 POINT 4

44 16-99

Flgure 7{. Block diagram of a tfrree stage log ampllfier.

7-29