70 MHz Bandpass Filter 2A6 Circuit Functioning

TM 11-5820-695-35

receiver. Dc amplifiers Q8 and Q9 are connected as a

The gain of 2A5 is controlled by an internal age circuit.

differential amplifier and the age level is coupled from

the wiper of AGC GAIN potentiometer R40 to the base

a. IF Amplifier. The incoming 70 MHz if signal is

of emitter follower Q10. The operation of Q10 was

applied from connector P1, IF INPUT, to the base of

described in paragraph 2-69a.

transistor Q1 through R GAIN potentiometer R2 and

capacitor C2. Cascaded if amplifiers Q1 through Q5

amplify the incoming if signal under the control of an

c. Metering Circuits. The metering circuits provide

age level. The age level is applied to diodes CR1

agc signal level outputs to the meter selector switch on

through CR4 which are connected in the interstage

meter panel assembly 2A15A2, to alarm monitor 2A12,

networks of the cascaded if amplifiers. These diodes

and to Indicator, Antenna Alignment ID-1708/GRC. The

present a minimum ac impedance at high dc currents

age output derived from peak detector diodes CR5 and

and low dc, currents the diodes present maximum ac

CR6 is applied to the carrier if meter circuit on 2A15A2

impedance. This provides automatic gain control under

through the voltage divider consisting of R32 and R37

the influence of the amplified agc voltage level

and pin P3-10. The age output of emitter follower Q10

established at the emitter of emitter follower Q10. A

is applied to the agc meter circuit on 2A15A2 through

high level of age voltage at the emitter of emitter

R43 and P3-9, and to the ID-1708/GRC through R52

follower Q10 will cause a high dc current flow through

and P3-11. The references for age metering and

the diodes CR1 through CR4 producing minimum

antenna alignment indicator outputs are applied through

attenuation of the incoming if signal. The rf signal is

the meter adjust circuit of R46, R47, and R48. METER

detected by diodes CR6 and CR6 and the resulting

ZERO control R47 is used to set the meter on 2A15A2

detected dc voltage is amplified by dc amplifiers Q8 and

to electrical zero. The age output of emitter follower

Q9 which controls the agc voltage applied to emitter

Q10 is also applied to the carrier if monitor circuit in

follower Q10. Each cascaded if amplifier is forward

alarm monitor 2A12 through R55 and R56 and P3-12.

biased by a resistive divider network in the base circuits;

The reference for the carrier if monitor signal is applied

the cascaded if amplifiers are tuned by variable

through the carrier level alarm adjust circuit, R49, R50,

capacitors C4, C8, C12, C16, and C21 in the emitter

CR8 and CR9. The alarm adjust circuit produces a 1.5

circuits. Additional degenerative feedback is provided

to 7 vdc reference for the alarm circuit in 2A12.

by unbypassed resistor R57 in the emitter circuit of the

input transistor Q1. The amplified if signal is coupled

through buffer Q6 to the base of output amplifier Q7

2-70.

70 MHz Bandpass Filter 2A6

Circuit

through capacitor C24. The if output signal is taken

Functioning (figs. 2-25, 2-25.1 and 2-25.2)

from a center tap on autotransformer T1 to provide an

output impedance of 75 ohms.

The 70 MHz bandpass filter 2A6 is used in the signal

path between amplifier-mixer 2A7 and 70 MHz if

amplifier 2A5 to reduce if noise components, to reject

b. AGC Circuits.

Full wave detection of the

undesired signals and to aid in shaping of the pcm

amplified if signal is provided by peak detector diodes

pulses. The 70 MHz bandpass filter 2A6 is a five pole

CR5 and CR6 in conjunction with capacitors C28 and

bandpass filter with a center frequency of 70 MHz, a

C31. The detected agc level is developed across series

bandwidth of 5.0 MHz is tuned to approximately

resistors R32 and R37. AGC SEL switch S1 allows

Guassian shaping. The source and load impedances of

selection of the source of age applied to the base of dc

the filter are 75 ohms, respectively. The maximum

amplifier stage Q8. When AGC SEL switch is in the DIV

insertion loss at 70 MHz is 2.5 dB. The frequency

OFF position, the detected agc level is applied to the

response of 2A6 (reference 0 dB at 70 MHz) is as

base of Q8 through contact 3 of section SIA. Placing

follows:

the AGC SEL switch in the TEST position disables the

age function and places the if gain A1 a constant

maximum value by grounding the base of Q8 through

contact 1 of section SIA. The DIV ON position is not

used because the R1467/GRC-144 is a single channed

Change 6

2-112.1/(2-112.2 blank)