HP 83522A Service Notes page 225

Model 83522A Service
In CW mode, a relay connects a large capacitor across the YO'S coil. The
capacitor resists changes in the YO current to reduce residual FM noise.
The Freq Cal Switches/Status block has two functions. During INSTR
PRESET , the Freq Cal Switches, set when the plug-in is calibrated, are read for
use in setting the Scale and Offset DACs. This information sets frequency end-
point accuracy. This section also reads the sweep status and unleveled condition
for use by the microprocessor.
A7 Marker
The Delay Compensation circuit on the A7 Marker assembly produces a signal
to compensate for time delay in the YIG Oscillator response. The coils in the
YO are used to set up a strong, controlled magnetic field to control the RF
frequency. Due to inductive and magnetic delays of the electromagnets, there is
a delay between the applied voltage and resultant current flow through the
coils. The Delay Compensation circuitry monitors the scaled tuning voltage,
and from its amplitude and slope produces a signal added to the YO DRIVE V
to compensate for swept frequency errors that would occur because of the
response delays.
The Oscillator Bias section produces the bias voltage needed by the A12 YIG
Oscillator. The YO'S correct bias pointis dependent on its frequency, so the YO
DRIVE V is used to make these frequency-dependent adjustments. The L
RFON line will turn off the bias and shut down oscillations altogether when the
RF is turned off.
A5 F M Driver
The A5 FM Driver assembly splits the external FM signal, passed through the
mainframe, into two paths. One is added to the main coil tuning voltage; the
other is routed to a separate coil inside the YO, dedicated to high-frequency FM.
One FM path is lowpass filtered, removing high-frequency components; the
other is highpass filtered, removing low-frequency components. The filters are
matched in stop-band response, such that one picks up where the other leaves
off. Both paths are amplified, and sent through Sensitivity Select circuits which
determine the FM sensitivity (i.e. MHz of deviation per volt) and select either
cross-over or direct coupling. The LO FREQ FM is eventually added to the YO
DRIVE V, and modulates the output frequency through the YO'S main coils.
However, the main coil cannot respond to fast deviations due to inductive and
magnetic delays. Hence, a completely separate, small, but fast-acting FM coil is
built into the YIG Oscillator. The HI FREQ FM is sent to this coil, allowing
limited high-frequency FM.
ALC / POWER CONTROL
The A4 ALC assembly, and parts of the A5 FM Driver assembly, are responsible
for power level control. Power leveling is accomplished by detecting the output
RF power level, comparing it to a fixed reference voltage, and adjusting the R F
modulator to correct for power errors. This results in constant RF power level
across the entire sweep. The absolute R F power is digitally controlled, and can
be set between +13 and -2 dBm. (Instruments with Option 002 use an RF Step
Attenuator to achieve power control down to -72 dBm. However, this is not part
of the leveling loop.) The power sweep and power slope functions are obtained
by adding a scaled voltage ramp offset to the reference power level.