Amplifier-Mixer 2A7 Circuit Functioning

TM 11-5820-695-35

crystal diodes CR1 and CR2. A positive dc voltage is

Frequency (MHz)

Attenuation (db)

developed across resistor R1 which is proportional to the

68.20 and 71.81................................ ...... 1.2 to 1.6

de current through crystal diode CR1. This positive dc

67.64 and 72.54................................ ...... 2.6 to 3.4

voltage is sent to the meter selector switch through pin 8

66.58 and 73.59................................ ...... 6.4 to 6.6

of plug P1.

Similarly, a negative dc voltage is

65.15 and 75.19................................ ...... 10.8 to 13.2

developed across resistor R2 which is proportional to the

63.52 and 77.10................................ ...... 17.0 to 23.0

dc current through crystal diode CR2. This negative dc

voltage is also sent to the meter selector switch via pin 9

For Receiver, Radio R-1467(P)A/GRC-144(V), 10,6

of plug P1. Inductors L1 and L2 are used to block the if

MHz bandpass filter assembly 956417 (fig. 2-25.1) is

signal from the diode monitoring circuits, and capacitors

used for 9 Mb/s data rate and equalizer-filter assembly

C1 and C4 provide filtering. A filter in each circuit (C6,

956462 (fig. 2-25.2 is used for 18 Mb/s data rate.

L4, C9, and C8, L6, C13) provides additional filtering of

the if signal. Resistors R3. R23, and R4, R22 and

meter decoupling resistors. Resistors R22 and R23 are

2-71 Amplifier-Mixer 2A7 Circuit Functioning (figs.

variable and are used to adjust meter deflection.

2-26 and 2-26.1)

Amplifier-mixer 2A7 converts the 4.4-5.0 GHz rf input

b. First Preamplifier. The input circuit for first

signal to the 70 MHz if signal. This conversion is made

preamplifier stage Q1 is a parallel resonant circuit.

by mixing the 4.4-5.0 GHz rf input signal with the 4.33 to

Therefore, the resonant frequency and bandwidth of the

5.07 GHz local oscillator signal. The B+ supply (15 vdc)

parallel resonant circuit determines the band of

is provided by decoupling capacitors C5, C10, C14, and

frequencies coupled to the base of first preamplifier

C20. Inductors L5, L8, and L10 block the if signal from

stage Q1. The parallel resonant circuit is formed by L3,

the B+ input. Test point TP2, + 15v, can be connected

C7, the input capacitance of Q1, and the output if

to the meter on meter panel assembly 2A15A2 with a

capacitance of crystal mixer 2A7Z1. The resonant

test lead and resistor R21 is a meter decoupling resistor.

frequency is set at 70 MHz by adjusting IF ADJ control

L3. Q1 amplifies the 70 MHz if signal passes by the

input circuit, and the amplified 70 MHz if signal is

a. Crystal Mixer 2A7Z1. The 4.4 to 5.0 GHz rf

coupled by C11 to the base of second preamplifier stage

input signal is applied to input of 2A7Z1 through

02 through capacitor C11. Resistors R5 and R6 provide

connector J1 RF IN and the 4.33 to 5.07 GHz local

fixed dc basis for Q2, and resistor R7 is its collector

oscillator signal is applied to input jack J2 LO IN.

load. Emitter resistor R8 provides negative dc feedback

Crystal mixer 2A7Z1 consists of a matched pair of IN23

to temperature stabilize first preamplifier stage Q1, and

WEMR crystal diodes CR1 and CR2 which mix the rf

capacitor C12 provides the if bypass and tuning.

input signal with the local oscillator signal. Mixing the

two signals produces numerous frequencies, of which,

the four predominant frequencies are: (1) the original

c. Second Preamplifier. Second preamplifier stage

carrier frequency, (2) the original local oscillator

Q2 also amplifies the 70 MHz if signal, and the output is

frequency, (3) the sum of the two frequencies, and (4)

coupled by C15 to the base of third preamplifier Q3.

the difference between the two frequencies (70 MHz).

Resistors R9 and R10 provide fixed dc bias for this

Only the 70 MHz difference frequency is applied to the

stage and inductor L7 is its collector load. Resistor R11

input circuit of first preamplifier Q1, through coupling

provides negative ac feedback to stabilize second

capacitors C2, C3, and C7. All other frequencies

preamplifier

produced by the mixing process are rf bypassed to

ground in crystal mixer 2A7Z1. Additional outputs are

provided which permit monitoring the dc current through

Change 6

2-113