Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 117

CIRCUIT DESCRIPTION WJ-8718-19/FE HF RECEIVER
The 10.7 MHz IF signal is input at pin 13 and coupled through CI to the
base circuits of Ql, Q2, and Q3. If Ql is on, the signal is amplified and coupled to
FL1. This filter has a 200 ohm input impedance and a 3 dB bandwidth of 3.2 kHz.
The filtered IF signal is applied to amplifier Q4 through level-adjust potentiometer
R26. The amplified IF signal is output at pin 57. If 6.0 kHz bandwidth is selected
the IF signal is routed through Q2, FL2, and Q5. If any other bandwidth is selected'
the IF signal is routed through Q3, attenuator R22, R23, R24, and Q6. The gain of the
three signal paths is equalized by R26, R28, and R30 to approximately 14 dB. The
circuit has nominal input and output impedances of 50 ohms.
3.4.12 10.7 MHz/455 kHz CONVERTER (A4A2) (794254)
The 10.7 MHz/455 kHz Converter mounts on the IF Motherboard, A4, (see
paragraph 3.3.4). Figure 3-17 is a detailed functional block diagram of the
10.7 MHz/455 kHz Converter which should be referred to in the following circuit
description. Figure 6-16, 10.7 MHz/455 kHz Converter Schematic Diagram, may be
referred to for greater component level detail, if desired.
10.7 MHz IF signals from A4A1 drive the input to A4A2. In all modes
but DF, the DF select pin 45 is low, causing U2-1 to be low and U2-7 to be high.
The IF signals then pass through Q2 and Q4 to the input of mixer Ul. In DF mode
pin 45 is high, causing U2-1 to be high and U2-7 to be low. The IF signals then pass
through Ql, filter FL1 and Q3 to the input of mixer Ul. FL1 restricts the bandpass to
2.2 kHz.
Mixer Ul converts the 10.7 MHz IF signals to 455 kHz. The 3rd LO
signal is input at the fixed frequency of 11.155 MHz and a level of approximately
-6 dBm, and is amplified by transistor Ql and its associated circuitry to +7 dBm before
entering the mixer. The amplifier operates as a common emitter stage with some
unbypassed emitter resistance to stabilize its gain and reduce distortion. The
pi-network, C7-L2-C8, serves as an impedance transformer and low-pass filter further
reducing distortion of the LO signal.
Low-pass filter C9, L3, C10, L4, and Cll filters out undesired components
above 500 kHz from the mixer output and matches impedances between the mixer and
the following circuits. The nominal input impedance of the 3rd Mixer is 50 ohms and
the output impedance is 1000 ohms.
3-34