Discriminator Driver

TM 11-5820-695-35

the rf signal. Variable inductor LI tunes the collector

(1) Both the primary and secondary of

circuit of Q2 and resistor R9, connected across L5,

transformer T1 are tuned to the 70 MHz. center

loads the collector circuit thereby providing stability.

frequency.

Tuning is accomplished by variable

The stage output is tapped from inductor L5. The

capacitors C43 and C45, respectively. The signal from

limiting action is obtained by operating Q2 near

Q7 is connected across the primary winding of

saturation. The base is biased at +13.5 volts dc. A

transformer T1, and also to the center tap of the

resistance of 1015 ohms (R11 plus R10) in the emitter

secondary winding of T1 through capacitor C41. With a

circuit results in highly stable operation near the

70 MHz signal applied, the voltages across each half of

saturation region. Under these conditions. the collector

the secondary windings are in quadrature with the

to emitter voltage of Q2 is in the order of 1.0 volt. A

voltage across the primary.

They are of equal

negative signal swing tends to further increase the

amplitude and are 180 degrees out of phase. When the

current of Q2 but, because of the near saturation, the

signal is above resonance, the phase angle between

swing at the collector is limited. With a positive signal

one half of the secondary and the primary is greater

swing, the emitter voltage tends to become more

than 90 degrees, and that for the other half of the

positive, thereby reducing the base emitter voltage.

secondary is less than 90 degrees. Below resonance,

This, in turn, causes the emitter voltage to decrease,

these phase relationships are reversed, the phase angle

due to the reduced emitter current through resistors R10

for the first half of the secondary being less than 90

and R11. Because of these counteracting effects the

degrees, and that for the second, greater than 90

output voltage swing due to positive signal excursions is

degrees.

also limited. The second, third, and fourth limiter stages

(2) The signals at the cathodes of diodes

operate in an identical manner. The output of fourth

CR3 and CR4 are the vector sums of the primary and

limiter stage Q5 is tapped from variable inductor L14

associated secondary voltages. At resonance these

and applied to the first stage of discriminator driver Q6

signals are of equal amplitude; while off resonance, the

through capacitor C33. The output of Q6 is also fed to a

signal associated with the half secondary having the

metering circuit consisting of diode detector CR2,

greater phase difference is the larger. The difference

resistor R30, and associated RC circuit R27 and C81.

between the two voltages is proportional to the

The metering circuit produces a dc signal proportional to

frequency excursion from the resonant frequency. The

the limiter output level. The resultant dc level is made

modulation (video) is recovered by the rectifying action

available for monitoring at Jo (front panel test point TP3

of diodes CR3 and CR4. Since the voltages across

LIM. LEVEL).

these diodes are in opposition, the current through load

resistors R40 and R41 is produced by the difference

c. Discriminator Driver. The discriminator driver

between the impressed voltages. The signal developed

consists of two transistor stages Q6 and Q7. The first

across the load resistors has, as its fundamental

stage Q6, is an emitter follower. The signal enters at the

frequency, the recovered video.

Ripple at the if

base, and the output is taken off at the junction of

frequency is bypassed by capacitor C46.

resistors R81 and R32. Biasing is provided by voltage

(3) Two positive coefficient thermistors, R85

divider R28, R29, and emitter resistors R31 and R32.

and R41, are used in the discriminator to compensate

Capacitors C35 and C37 are bypass capacitors. The

for thermal effects in discriminator driver Q7 and Q8.

input to second stage Q7 is through coupling capacitor

The thermistors have a temperature coefficient of

C38 to the junction of resistors R36 and R37. This stage

approximately +0.7 percent degree centigrade. The

is configured common base, and, unlike the limiter

resistance of R85 in the collector circuit of Q7 increases

stages, is biased in the linear region. Base bias voltage

with increasing ambient temperature.

This action

of +6.76 volts is derived from voltage divider R83 and

compensates for the thermal effects in Q7. Similarly,

R84 and emitter bias from resistors R36 and R37. The

resistor R41 increases the output signal at capacitor

base is bypassed by C39 and the emitter by capacitor

C48,

with

increasing

ambient

temperature,

C44.

compensating for thermal effects in diodes CRS and

CR4, and the video output amplifier, Q8 and Q9.

d. Frequency Discriminator.

The frequency

(4) A metering circuit is provided at the

discriminator recovers the video signal (composite pcm

output of frequency discriminator, consisting of R42,

and orderwire) from the modulated 70 MHz if signal. It

C47, and J4. When the mean if frequency at the input

operates on the phase shift principle, and consists of

of the frequency discriminator differs

transformer T1, diodes CRS and CR4, and associated

parts.

2-111