Amplifier-Limiter Chain

TM 11820-695-35

The output of Q7 is coupled, via C37, to limiter stage

CR3 gate is held on by the +15 volt applied through R7.

Q8, which is in the common base configuration.

Inductor L12 is used to tune the collector to the signal

(1) When voltage is applied, one

frequency. L11 provides isolation and 033, C35, and

transistor conducts more heavily due to noise or small

C36 are bypass capacitors.

component variations.

For discussion purposes,

assume that Q4 is initially conducting. Q4 collector

(2) Common base limiter stage Q8

voltage drops from + 16 volts to a lower value. Since

operates with approximately 12.2 volts fixed bias on the

the charge across a capacitor cannot change

base, derived from voltage divider R32 and R33. The

instantaneously, the voltage at the negative plate of C15

emitter current through R34 and R36 produces a

drops causing Q5 base voltage to decrease. Q5

quiescent emitter voltage of approximately 11.5 volts,

conduction decreases, and its collector voltage rises.

so that the quiescent base to emitter voltage is

This increase is coupled to Q4 base through capacitor

approximately 0.7 volt. Thus, when the instantaneous

C23 causing the Q4 collector current to increase further.

signal level during the positive half cycle exceeds about

This loop is repeated until Q5 is driven below cutoff, and

0.7 volt, the transistor cuts off, clipping the positive

Q4 goes into saturation. The above occurs so rapidly

portion at the collector. Transistor Q9 operation is

that the resulting wave is essentially a step function.

identical to that of Q8 and is driven by Q8 through C40.

CR8 is now forward biased and current flows through it

Fixed bias is provided by resistors R37-R40. Signal

and resistors R21, R24, and R25 to ground. As a result,

tuning is provided by' L15. L18 and L14 provide

Q5 collector is at approximately + 14 volts and the drop

isolation and 03-C42 are bypass capacitors.

across R25 is approximately +13 volts. Diode CR7 is

reverse biased and therefore blocks the +t11 volts from

(3) Emitter follower Q10 is driven by

Q4 collector.

common base amplifier Q9 through coupling capacitor

046 and attenuating resistor R45. The transistor is fixed

(2) Since the charge across C16 and the

biased by R43, R44, R46 and R47. The collector is

bias of R26 are aiding, C15 discharges through Q4, R25

bypassed by 047, and decoupled by L1'6, L17, and C46.

and R23, in an exponential fashion. When Q6 reaches

cutoff, it begins to conduct, causing its collector voltage

(4) The final stage in the amplifier-limiter

to drop, pulling down Q4 base. Q4 collector voltage

chain is discriminator driver Q1, which is operated in the

increases, and this increase is coupled back to Q5 base

common base configuration. The drive signal is applied

through C15. The feedback process as previously

at the junction of R51 and R52, through C49. The base

described results, but, in the reverse order, causing Q4

is dc biased at +2.1 volts by voltage divider R48 and

to be cutoff and Q5 to go into saturation. CR7 is now

R49 and bypassed by C48. The quiescent emitter

forward biased and current flows through it and resistors

voltage is approximately + 1.4 volts, giving a base to

R20, 'R24, and R25 to ground. The charge across C23

emitter of 0.7 volt.

discharges through Q5, R25, and R22 Thus, a square

wave is produced alternately at the Q4 and Q5

(5) A metering circuit is also provided at

collectors.

the output of limiter stage Q9. This is a diode detector,

consisting of CR9, C43, C044, R41 and R42. Variable

Amplifier-Limiter Chain. The amplifier limiter

resistor R41 provides output level adjustment. The dc

chain is comprised of transistor stages Q7 through Q11.

output at J3 permits observing the response, and is

The two diode gates, previously described, provide

used, for example, when tuning variable inductors L12

alternate input signal to the amplifier-limiter chain. This

and L15.

results in the 70 MHz frequency modulated signal from

1A8 and the 70 MHz reference signal from the crystal

Discriminator.

The

discriminator

oscillator appearing alternately at the junction of C25

comprised of transformer T1., diode detectors CR10 and

and C26. Since the multivibrator is symmetrical, these

CR11E, diode equalizing resistors R76 and RT7, load

signal samples are of equal duration.

resistors R53 and R54, coupling capacitor C51, tuning

capacitors C62 and C56, temperature compensating

(1) Each signal sample is impressed

capacitor C68, bypass capacitor C54, and decoupling

across gate load resistor R29 at the base of emitter

network L18 and C50. The discriminator operates on

follower Q7, through coupling capacitor C29. Q7 is

the phase shift principle. The output is a square wave in

biased by R28, R30 and R31, the latter also serving as

which the differential level represents the' frequency

the emitter load. C28 and C32 are bypass capacitors

difference between the reference signal from 1A8

and J10 provides decoupling. C54 prevents oscillation.

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