HP 83522A Service Notes page 224

Service Model 83522A
Miscellaneous front panel controls must pass through the A1 and A2
assemblies. The RPG produces pulses when rotated, and sends them directly
back to the 8350A Sweep Oscillator to be decoded and processed to adjust the
power. The FREQ CAL adjustment is used to fine-tune the R F output frequency
to correct for drift or error in the frequency of the A16 Cavity Oscillator. The
EXTIMTR ALC CAL adjusts the absolute power level when external detector or
power meter leveling is used.
FREQUENCY CONTROL
The Frequency Control section of the plug-in is responsible for determining the
actual RF output frequency. Based on the tuning voltage VTUNE and digital
data, the correct current is developed to tune the A12 YIG Oscillator. Frequency
modulation is also processed in these circuits.
A6 YO Driver
A9 Reference Resistor Assembly
The A6 YO Driver and A9 Reference Resistor assemblies scale and offset the
tuning voltage from the 8350A Sweep Oscillator, converting it into a current for
controlling the A12 YIG Oscillator frequency.
The tuning voltage, W U N E , is buffered and inverted before being scaled, offset
and summed with various correction signals to produce the tuning current for
the A12 YIG Oscillator. The full 0 to 10V VTUNE must tune the oscillator from
3.81 to 6.2 GHz.
The Scaling and Offset DACs are also used to compensate for small differences
in oscillator sensitivities. The amount of scaling and offset can be set by the
Frequency Cal switches. At power-on or Instrument Preset, the status of the Cal
switches is read by the 8350A and stored in RAM. This information is then used
to program the DACs. The -lOV Ref generates a stable voltage source used as a
reference on both the A6 YO Driver and A4 ALC assemblies.
The +20V Tracking circuit monitors the +20V supply, producing an output
which follows this voltage. The current through the YO is referenced to this
supply, this prevents power supply drift or noise from creating frequency errors.
The summing junction adds together the scaled tuning voltage, offset, +20V
tracking voltage, and offset compensation, plus the front-panel FREQ CAL. The
Delay Compensation from the A7 Marker assembly and LO FREQ FM from
the A5 FM Driver assembly (both described below) are also added. The result is
the YO DRIVE V, a signal proportional to the YO frequency.
The remainder of the A6 circuits and the A9 components convert the YO
DRIVE V to a current to control the YO frequency. The final current drive
transistor (A9Q1) is controlled by the A6 assembly. The current through this
transistor, and hence the YO, generates a proportional voltage across the
Reference Resistor, which is monitored and compared to the YO DRIVE V. Any
errors between the two are corrected in a closed loop, producing a current
proportional to the YO DRIVE V. Compensation elements (Comp) correct for
nonlinearities in the YO. If the YO is replaced, this section of circuitry may also
require changing.