HP 85662A, Troubleshooting And Repair Manual

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For resolution bandwidths Down Converter converts the 21.4 MHz IF signal
from A20 Third Converter down to a 3 MHz IF signal. For resolution bandwidths
the 21.4 MHz is not converted but is sent through Up Converter to Step Gain.
Switch and Ohm Driver
For bandwidths the SWITCH line is V and Q7 is off. R38 in the LO Driver
and Buffer Amplifier pulls the collector of Q7 to less than 0 V. This forward biases CR3
and reverse biases CR5, passing the 21.4 MHz IF signal to the 50 Ohm Driver, where it is
converted to a 50 Ohm system (6 loss) and sent to
For bandwidths the SWITCH line is 0 V, turning Q7 on. The collector Q7 goes to
almost V, which turns CR5 on (shorting any signal that might pass through CR3) and
CR3 off. Q7 also turns the LO on.
LO Driver and Buffer Amplifier
This circuit is only on for bandwidths It consists of an emitter follower driving
a common base stage Q5 which in turn drives a common emitter stage Q2. The proper bias
voltages and signal levels are shown on the schematic.
Mixer and Driver
is an emitter follower driving a step-down transformer. The turns ratio is about to
1, which yields a 12 loss and drives the Mixer from a low impedance. The Mixer has an
additional 5 loss.
3 MHz Amplifier
This amplifier compensates for the losses of the transformer and the mixer The filter
at the output, consisting of L5, C22, and R52, has less than 1 of loss at 3 MHz.
2