LOW-PASS FILTER (U1B)

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

approximately 100 ohms as the detector-developed

amplifier. When audio is first applied or the level of

control voltage applied to its gate varies from -12 V dc to

audio is increased, the fast discharge of capacitor C9

0 volt. Through voltage divider action, the audio signal

through resistor R23 sets the attack time at about 4

applied to the noninverting input of amplifier U1A

milliseconds. When audio is removed or the level is

decreases as the detector output voltage decreases

decreased, the slow charge of C9 through R24 sets the

toward zero.

release time at about 150 milliseconds.

4-148. Amplifier. Amplifier U1A provides a voltage gain

4-151. LOW-PASS FILTER (U1B). The low-pass filter

of about 57 V/V. Together, the variable attenuator and

(U1B of figure FO-21) shapes the high-frequency

amplifier provide a variable compression amplifier gain

response of the output signal from the transmit audio

that varies from about 57 to less than 1. The minimum

compression amplifier. The low-pass filter is a 3-pole,

gain depends upon the minimum turn-on resistance of

active filter with unity gain and cutoff frequency of 6 kHz.

FET Q1.

The three poles are determined by resistor-capacitor

networks R12-C10, R13-C11, and R14-C12. In the

4-149. Detector. The detector circuit (U2A, U2B, Q2,

passband, the output varies less than +1 or -2 dB when

and Q3) functions as a peak waveform detector to

referenced to the output at 1000 Hz. In the stopband,

reduce compression amplifier gain whenever positive or

the output is down greater than -10 dB at 10 kHz.

negative audio peaks at the output of amplifier U1A try to

exceed the detector threshold (approximately 0.25 volt).

4-152. HIGH-PASS FILTER (U7B). The high-pass filter

Resistor R22 of voltage divider R21-R22 sets the

(U7B of figure FO-21) shapes the low-frequency

detector threshold by developing a bias voltage of about

response of the output signal from the compression

-0.23 V dc that is applied to the inverting inputs of

amplifier that is passed through the low-pass filter (U1B).

amplifiers U2A and U2B. Each amplifier amplifies the

The high-pass filter is a 3-pole, active filter with a voltage

bias voltage by a voltage gain of about 15.4 V/V to

gain of about 3.9 V/V and cutoff frequency of 300 Hz.

produce a positive output voltage of about 3.55 V dc.

The three poles are determined by resistor-capacitor

The high positive voltage biases transistors Q2 and Q3

networks R62-C28, R63-C29, and R64-C30. In the

into cutoff. With Q2 and Q3 off, -12-V dc power supply

passband, the output varies less than +1 or -2 dB when

voltage applied through resistor R24 charges capacitor

referenced to the output at 1000 Hz. In the stopband,

C9 to about -12 V dc. This voltage, when applied to the

the output is down greater than -10 dB at 100 Hz.

gate of FET Q1, results in maximum FET resistance and

therefore maximum compression amplifier gain. When

4-153. HIGH-PASS FILTER (U3). The high-pass filter

audio is applied to the compression amplifier, the audio

(U3 of figure FO-21) shapes the low-frequency response

signal present at the output of amplifier U1A is coupled

of the wide bandwidth data applied at the data audio

to the inverting input of amplifier U2A through capacitor

input. The high-pass filter is a 3-pole, active filter with a

C4 and resistor R15 and to the noninverting input of

voltage gain of about 9 V/V and cutoff frequency of 16

amplifier U2B through capacitor C5. Each amplifier

Hz. The three poles are determined by resistor-capacitor

amplifies the audio signal by a gain of about 16.2 V/V to

networks R26-C14, R28-C15, and R27-C16. In the

produce an ac output voltage that is superimposed on

passband, the filter response varies less than +1 or -3 dB

the amplified bias voltage applied to Q2 and Q3. When

when referenced to the output at 1000 Hz. Because of

positive peaks of the output audio signal from U1A

the low cutoff frequency, high-pass filter U3 does not

exceed the detector threshold, the ac output voltage from

affect the frequency response of the voice audio signal

U2A forward biases Q2 for the period of time the peak

from high-pass filter U7B. It does, however, amplify the

voltage exceeds the threshold. In the same manner,

voice audio signal by a voltage gain of about 9. Variable

when negative peaks exceed the detector threshold, the

resistor R107 allows the output of the high-pass filter to

ac output voltage from U2B forward bias Q3. When Q2

be set up for proper modulation level when transmitting

and/or Q3 are forward biased, they turn on and allow

wide bandwidth data.

capacitor C9 to discharge through resistor R23. The

ratio of on time to off time determines the average

4-154. PERCENT MODULATION CONTROL. The

negative voltage developed across C9. This voltage,

transmit audio output signal from audio module A4

when applied to the variable attenuator (gate of Q1),

(A4P1-13/A10J13-13) is fed to the percent modulation

controls the compression amplifier gain to maintain the

(% MOD) control located on the front panel of receiver-

peak voltage at the output of U1A at the detector

transmitter.

The percent modulation control allows

threshold. Through this circuit action, the compression

adjustment of the audio level applied to the modulator

amplifier output voltage is held at about 180 millivolts

circuit of power amplifier A8 to set up the modulation

(rms) for the approximate 14- to 250-millivolt range of

percent of the transmitted rf signal.

The percent

audio input signals.

modulation control (figure FO-16) consists of resistors

A10A1R6 and A10A1R7 and variable resistor A10A1R2.

4-150. Resistors R24 and R23 and capacitor C9 deter-

Resistor

A10A1R7

and

mine the attack and release time of the compression

4-29