RECEIVER OUTPUT SECTION (cont)

TM 11-5820-917-13

the spectrum analyzer), and the audio, which is the same bandwidth as the base-

band but offset by 700 Hz to make it easier for an operator to hear when presented

to a loudspeaker. The 200 kHz IF signal is first terminated by R1 (figure FO-

33/1) and then impedance transformed (upward) by T1. Resistor R2, R7 and Q1

form a variable attenuator which is controlled by the AGC circuit. Q1 acts as a

variable resistance to ground - resistance increases as the gate voltage becomes

more negative and decreases as the gate-to-source voltage goes to zero volts.

Q2 and the drain-resonating components L1 and C10, form a tuned amplifier at

200 kHz, while Q3 is a source-follower circuit providing minimum impedance

loading to the preceding tuned circuit and a controlled source impedance to the

following crystal filters. The receiver has two bandwidths, 500 Hz , and 5000 Hz,

controlled by the 6025 system logic. U1-U3 are electronic switches which place

the appropriate filter into the IF circuit. The 500 Hz (BPF #1) crystal filter is

mounted inside the receiver module, and the 5000 Hz bandpass is determined by

the LC tuned circuits in the 200 kHz IF amplifiers. After such filtering, the 200

kHz IF is amplified by several similar amplifier/variable-attenuator circuits.

Q5-7 (figure FO-33/2) form a typical circuit. Q5 is a voltage-variable resis-

tance which, with R30, forms a voltage-variable attenuator to control the ampli-

tude of the signal at the input of the tuned amplifier Q6. Q7 is a source-follower

circuit that presents a high impedance load to the tuned drain circuit of Q6 and a

low-impedance drive to R39, the first element of the next similar circuit. These

amplifiers and attenuators comprise a tuned IF amplifier whose gain can be

changed by the AGC circuit, such that the 200 kHz IF is maintained at approxi-

mately 50 mVrms at Q17. The 200 kHz is converted to baseband by M2 (figure

FO-33/3) which mixes the IF with a locally generated 200 kHz LO. This LO is

generated from the 5 MHz supplied by the sweep synthesizer to J8 of this board.

U10-11 divide the 5 MHz by 25 to yield 200 kHz which drives U12 to modulate M2

through R105 and R106. The baseband output of M2 is amplified by U 14-15 to

approximately 1 Vrms which becomes the baseband output of the sounder receiver.

4-80. The 200 kHz IF is also converted to Ml to audio with a 700 Hz offset.

Y1 and U7 form a 398.6 kHz oscillator whose frequency is divided by 2 in U4 to

199.3 kHz. When mixed with the incoming 200 kHz IF, this signal yields the

receiver bandwidth offset converted to 700 Hz. U5 is an audio amplifier and

low-pass filter which drives the audio volume control mounted on the 4028 front

panel. U 6 further amplifies the audio to power the loudspeaker, also on the

front panel.

4-81. To control the gain of the IF amplifiers, the 200 kHz IF output is detected

in an AGC circuit. The 200 kHz IF, approximately 50 mVrms, is buffered in

unity-gain amplifier U16 and detected in a half-wave rectifier circuit of U17

(figure FO-33/4). This half- wave- rectified signal is further amplified by U 18

and smoothed by R134 and C101. The resulting voltage is amplified by U19 and

presented through Q22 to an RC circuit, R151-152, C107 and CR5. This circuit

determines the AGC time constant which is made to be asymmetrical; that is,

CR5 allows C107 to be charged negative more rapidly than positive. (Note that

voltage on C107 is negative.) This provides an AGC loop with a slow attack-

fast decay time constant necessary for chirp sounding. Relay K1 can switch in

another resistor in parallel with R152 to change the AGC time constant. A

shorter time constant is needed during auto sync to maintain optimum receiver

gain and minimize the change of missing the received chirp signal. The voltage

on C107 is buffered by the unity-gain amplifier U21 and becomes the AGC control

4-37