HP 85662A, Руководство по устранению неисправностей и ремонту

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For narrow bandwidths, Up Converter Assembly converts the 3 MHz IF
signal from 3 MHz Filter to a 21.4 MHz IF signal and sends it to Step Gain. For
wide bandwidths, the 21.4 MHz from Down Converter is amplified and
sent to
Variable Gain Amplifier
The Variable Gain Amplifier has about 6 of gain to compensate for the loss incurred by
the 50 Ohm Driver in which converts the 21.4 MHz IF signal into a 50 Ohm system.
21.4 MHz IF Switch
For wide bandwidths, the SWITCH line goes to V, which turns 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
For narrow bandwidths, the SWITCH line goes to 1 V, turning on. The
collector of goes to about 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 It consists of an emitter follower (Q7) driving
a common base stage (Q8) which in turn drives a common emitter stage The voltages
and signal levels are indicated on the schematic.
Mixer
The Mixer is double balanced and has about 6 conversion loss. It mixes the 3 MHz IF
signal from
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 Amplifier to
21.4 MHz Amplifier
This is a common emitter amplifier with about 7 of gain. It is preceded by a wide
(BW 5 MHz) filter (R61, C32, and L2) and followed by a narrow (BW 10
L8, and R60). The wide filter attenuates harmonics of the 18.4 MHz LO,
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 and
inverting transformer are driven by an emitter follower Q4. The output emitter follower is
turned on and off by the SWITCH line.
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