HP 83522A Service Notes page 221

Model 83522A
Frequency Control
Service
The Frequency Control block is responsible for converting the tuning ramp
(VTUNE) from the 8350A Sweep Oscillator into a drive current controlling the
YIG Oscillator (YO) frequency. The tuning voltage is digitally scaled and offset
to yield a voltage proportional to the YO'S frequency. A delay compensation
signal is summed in with the scaled tuning voltage to compensate for response
delays in the YO. Lastly, low-frequency components of external frequency
modulation (FM) are filtered and also summed in to produce a total YO control
voltage. However, the YO is current controlled, so a Current Driver converts the
control voltage to a drive current for the YIG Oscillator.
The high-frequency FM components cannot be summed in with the drive
current due to the limited dynamic response of the YO'S main tuning coil.
Instead, they are filtered off and sent to a separate coil built into the YO to allow
smaller but faster frequency modulation.
The Sweep Interrupt block, used in other multiband RF plug-ins, monitors the
tuning voltage (VTUNE) when the R F plug-in is performing a sweep requiring
multiple bands. When a tuning voltage corresponding to the end of the band is
sensed, these circuits temporarily stop the sweep ramp and interrupt the 8350A
microprocessor. The microprocessor then prepares the plug-in for the new band,
including new scaling and offset values, and continues the sweep. This portion
of the frequency control circits is not used in the 83522A.
Power Leveling (ALC)
The Power Control circuits determine the RF output power level, and ensure
that the power is constant across the sweep. A feedback loop detects the R F
power level, compares it with a reference voltage, and adjusts a PIN modulator
in the RF path to correct for amplitude errors.
The power level is digitally programmed from the 8350A Sweep Oscillator. A
scaled sweep ramp to provide the power slope or power sweep function is added,
yielding a reference power level.
An RF detector provides a voltage proportional to the actual R F power level.
This is then compared to the desired reference power level voltage to produce an
error voltage. The error is then amplified to drive a PIN modulator and correct
the output power level.
Marker Generation
The Marker Generation circuitry produces markers at 1, 10, and 50 MHz
intervals for R F frequencies up to 2.4 GHz (below 1 Ghz for 1 MHz markers).
A coupler samples the R F from the frequency R F path. This is combined with
the frequency comb of a crystal oscillator to produce "marker birdies" when the
RF frequency is an integer multiple of the crystal oscillator frequency. The
birdies are then discriminated to produce centered marker pulses of uniform
width. These pulses can be routed to the ALC loop to produce amplitude
markers, or sent to the 8350A Sweep Oscillator to provide Z-axis intensity
markers.