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For narrow (51 kHz) bandwidths, A4A6Al Up Converter Assembly converts the 3 MHz IF
signal from A4A7 3 MHz Filter to a 21.4 MHz IF signal and sends it to A4A5 Step Gain. For
wide (23 kHz) bandwidths, the 21.4 MHz from A4A6A2 Down Converter is amplified and
sent to A4A5.
Variable Gain Amplifier @
The Variable Gain Amplifier has about 6 dB of gain to compensate for the loss incurred by
the 50 Ohm Driver in A4A6A2, which converts the 21.4 MHz IF signal into a 50 Ohm system.
21.4 MHz IF Switch @
For wide (23 kHz) bandwidths, the SWITCH line goes to +15 V, which turns QS off. This
forward biases CR3 (the current is sunk through the LO Driver and Buffer Amplifier) and
reverse biases CR2, allowing the 21.4 MHz IF signal to pass through CR3 and then to A4A5.
For narrow (51 kHz) bandwidths, the SWITCH line goes to 1 V, turning QS on. The
collector of QS goes to about +15 V, which turns the LO Driver on, forward biases CR2, and
reverse biases CR3, breaking the path from the Variable Gain Amplifier.
LO Driver and Buffer Amplifier @
This circuit is only on for bandwidths _<l kHz. It consists of an emitter follower (Q7) driving
a common base stage (Q8) which in turn drives a common emitter stage (Q9). The voltages
and signal levels are indicated on the schematic.
Mixer @I
The Mixer is double balanced and has about 6 dB conversion loss. It mixes the 3 MHz IF
signal from A4A7 with the 18.4 MHz LO signal from the LO Driver and Buffer Amplifier to
produce a 21.4 MHz signal through the 21.4 MHz Bandpass Amplifier to A4A5.
21.4 MHz Bandpass Amplifier @
This is a common emitter amplifier with about 7 dB of gain. It is preceded by a wide
(BW z 5 MHz) bandpass filter (R61, C32, and L2) and followed by a narrow (BW z 10 kHz)
and the narrow filter attenuates the fundamental of the 18.4 MHz LO. 18.4 MHz NULL
adjustment C31 compensates for the case capacitance of the crystal. The crystal Yl and
inverting transformer Tl are driven by an emitter follower Q4. The output emitter follower is
turned on and off by the SWITCH line.
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