Detailed Discussion Of The A5; To 28Db Amplifier In 4Db Steps; To 20Db Amplifier In 2Db Steps; Front Panel Meter - HP 35601A Operating And Service Manual

Model 35601A
Service
SERVICE GROUP THREE
A5 BOARD: PHASE LOCKED LOOP CONTROL
6-60. DETAILED DISCUSSION OF THE A5 BOARD
6-61. 0 to 28dB Amplifier in 4dB Steps
The phase error signal from the High Frequency Board (A4) is amplified in two adjustable
gain stages. The first amplifier includes U13 as the amplifier. The combination of U2 and a
resistor network comprise the feedback to provide the desired gain. The output of U13 is
clamped by CR2 and CR4 to ensure that the voltage at this point never exceeds 6 volts; this is
done to avoid damaging the CMOS multiplexer U2. The three control lines, pins 9, 10, and
11 of U2, select the feedback resistance for U13. The gain is set in 4dB steps from 0 to 28dB;
eight steps are possible. Following U13 is a 6dB attenuator consisting of two resistors, R94
and R13.
6-62. 6 to 20dB Amplifier in 2dB Steps
U14 can have a gain ranging from 6-20dB in 2dB steps. The output of U14 is fed back via
R22; this provides 6dB attenuation to the junction of CR7 and CR8. This point has been
clamped at ± 6 volts to protect the CMOS Multiplexer U3. Pins 9,10, and 11 of U3 select the
feedback resistance for U14. This provides eight gain steps, from 6dB to 20dB, in 2dB steps.
6-63. 5 Volt Detector and Integrator
Following the 6-20dB Amplifier, a comparator circuit detects if the input or output of the in­
tegrator U15 exceeds ±2.1 volts. If the voltage at the input or output of U15 exceeds 2.1
volts, the Out-of-Lock Flip-Flop will be set; this closes a switch around the integrator
capacitor on U15.
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(i.e., The integrator becomes a unity gain amplifier when the 2.1 Volt
Detector trips.) It is assumed that if the voltages at this point have exceeded ±2.1 volts, the
loop is either out of lock or on the verge of being out of lock. Therefore, if the voltage ex­
ceeds ±2.1 volts, the Out-of-Lock Flip-Flop is set and the the LED on the front panel label­
ed "OUT OF LOCK TRIPPED" is turned on.
6-64. Front Panel Meter
Following the Integrator is a meter which is mounted on the front panel. This meter in­
dicates whether or not the control voltage being sent to an external oscillator is in an accept­
able range. In particular, it warns the user when one of the oscillators is drifting (before the
phase locked-loop unlocks). The meter also indicates a quadrature phase relationship be­
tween two signals sent into the input mixer when Phase Noise measurements without voltage
control are being made.
6-65