10.7-MHz CRYSTAL FILTER

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

4-122. 10.7-MHz IF AMPLIFIER. The 10.7-MHz if

The secondary of transformer T3 transforms the 30-MHz

amplifier (U4, U5, and Q16 of figure FO-20) consists of

if signal into two balanced, 180-degree-phase-related

two gain-controlled integrated circuit if amplifiers (U4 and

signals and applies them to the gates of FET's Q10 and

U5) and a temperature-compensated transistor if

Q11. Variable capacitor C61 is adjustable to allow

amplifier (Q16) that together provide about 88 dB of

balancing of the mixer. The 19.3- MHz injection signal

open loop gain. Capacitor C75 couples the 10.7-MHz if

is applied to the sources of FET's Q10 and Q11. The dc

signal to if amplifier U4. AGC voltage developed by the

level from the injection amplifier provides bias voltage to

AGC circuit is applied through resistor R62 to control the

FET's Q10 and Q11 so that when the 19.3-MHz injection

gain of U4. Inductor L18 and swamping resistor R65

signal is applied, the FET's operate in the square law

broadband tune the output of U4 to 10.7 MHz. Resistor

region for mixing action and conversion gain. The

R65 is test selected to adjust the 10.7-MHz if amplifier

second mixer provides about 12 dB of gain to the if

gain to produce an AGC voltage that sets the squelch

signal. The 10.7-MHz difference frequency is devel-oped

threshold at about 3 microvolts when the squelch control

across the secondary of transformer T3 and is coupled

is set for maximum sensitivity.

Capacitor C92,

to the 10.7-MHz crystal filters (FL1). Capacitors C62 and

temperature-compensation network RT2-R66-R67, and

C63 tune the primary of T4 to 10.7 MHz.

capacitor C93 couple the output of if amplifier U4 to the

input of if amplifier U5. AGC voltage applied through

4-121. 10.7-MHz CRYSTAL FILTER. The 10.7-MHz

resistor R63 controls the gain of U5. The output of U5 is

crystal filter (FL1 of figure FO-10) determines the

broadband tuned to 10.7 MHz by inductor L20 and

receiver selectivity. To be compatible with 50-kHz

coupled through capacitor C96 to if amplifier Q6.

channel spacing, the filter provides an if bandwidth of

Capacitor C87 and inductor L21 tune the output of Q16

15 kHz from center frequency with maximum attenua-

to 10.7 MHz. Capacitor C85 couples the output of Q16

tion of 2 dB within this bandwidth. At 18 kHz from

to voltage divider R72-R73 which provides isolation for

center frequency, the filter provides a maximum of 6 dB

the 10.7-MHz if output. The signal level at the 10.7-MHz

of attenuation. At 40 kHz from center frequency, the

if output is about 10 millivolts when loaded externally by

filter provides a minimum of 80 dB of attenuation.

a 50-ohm termination. Capacitor C86 couples the 10.7-

Variable capacitor C64 and test select capacitor C109

MHz if signal to the detector.

Temp-erature-

tune the filter input while variable capacitor C74 tunes

compensation network RT3-R71-C84 in the emitter

the filter output for minimum ripple through the passband

circuit of Q16 and temperature-compensation network

of the filter. Resistors R60 and R61 provide the proper

RT2-R67-R66 between if amplifiers U4 and U5 improve

load to terminate the filter.

closed-loop gain characteristics by making the if gain

less dependent upon temperature. Low-pass filters L17-

NOTE

C77, L19-C94, and C97-R70 decouple each stage of the

10.7-MHz if amplifier from the 12-Vdc line.

To receive wide bandwidth data, the 10.7-

MHz crystal filter can be replaced with a

4-123. DETECTOR. The detector (CR9 of figure FO-

wide bandwidth filter having an if bandwidth

of 25 kHz from center frequency with

20) demodulates the 10.7-MHz if signal and develops the

maximum attenuation of 1 dB. At 30 kHz

detected signal across RC filter network R75-C89. The

detected signal contains the audio signal superimposed

from center frequency the wide bandwidth

on a dc level that is proportional to the carrier level. Bias

filter provides a maximum of 6 dB of

attenuation, and at 60 kHz from center

network R74-CR10-C91 applies a dc voltage equivalent

to one diode drop through inductor L29 to the anode end

frequency the filter provides a minimum of

of detector diode CR9. This biases detector diode CR9

80 dB of attenuation. When installed in the

at the point of conduction to make the AGC voltage

radio set, a special ground pin on the wide-

independent of the diode drop across detector diode

band filter disables the noise blanker circuit.

CR9.

NOTE

4-124. AF AMPLIFIER. The af amplifier (U6A of figure

FO-20) amplifies the detected audio signal and the

To be compatible with 25-kHz channel

carrier derived dc voltage by a voltage gain of about 5.6

spacing, the 10.7-MHz crystal filter can be

volts per volt (V/V). Capacitor C90 couples the amplified

replaced with a narrow bandwidth filter

audio signal through voltage divider R78-R79 to the

having an if bandwidth of 10 kHz from

receiver af output of receiver rf module A3. The audio

center frequency with maximum attenuation

voltage developed across R79 is about 225 millivolts for

of 6 dB. At 25 kHz from center frequency

a 30-percent modulated receive rf input signal. The

the filter provides a minimum of 80 dB of

amplified dc signal is direct coupled to integrator U6B in

attenuation

the AGC circuit where it is used to develop AGC voltage

to control receiver gain

4-25