HP 83522A Service Notes page 227

Model 83522A Service
The Power Sweep DAC adds a ramp voltage to the power reference signal when
the Power Sweep or Power Slope functions are activated. Its input, VSW, is a
sweep ramp that essentially tracks the tuning voltage but always runs from 0 to
10 Vdc. A digitally programmable multiplying DAC scales this voltage
according to the dB/SWP or dB/GHz value selected. (If these functions are
disabled, the DAC is set to its minimum value.) This ramp is added to the ALC
Compensation signal described above, and added to the Power Ref signal on the
A4 assembly.
MARKER GENERATION
The 83522A features both amplitude and intensity markers at multiples of 1
MHz up to 1.0 GHz, and multiples of 10 or 50 MHz up to 2.0 GHz. They are
derived from a crystal, and hence are extremely accurate and stable.
A8 Sampler Assembly
A crystal-stabilized 50 MHz oscillator on the A8 Sampler assembly provides the
reference frequency for the markers. This squarewave is divided by five, then
divided by ten again, making squarewaves at 50 MHz, 10 MHz, and 1 MHz
available. A switch selects one of these outputs to be used, depending on the
front panel function selected. The squarewave passes through a comb generator,
making a pulse train containing many harmonics at integer multiples of the
input frequency. This is then mixed with the RF output sampled by the DCl
Directional Detector, producing many mixing products at the sum and
difference frequencies of the RF frequency and each harmonic of the comb
generator.
The external marker is produced by mixing the RF output with an externally-
produced signal (EXT MKR) below 2.4 GHz in a second mixer. The sum and
difference frequencies are then processed just as the harmonic sum and
difference frequencies.
The mixing products are passed through a lowpass filter with a programmable
cutoff frequency. This filters off the high-frequency mixing products and allows
only the low-frequency product to pass. When the RF output frequency is being
swept, the resulting string of "birdies" reach peak amplitude when the mixing
product frequency approaches zero.
The "birdies" are buffered by an amplifier with a controllable gain. A Gain
Shaping circuit monitors the reference power level and adjusts the gain to
compensate for varying levels of SAMPLED RF. This maintains uniform
"birdie" amplitude as power level changes.
The "birdies" pass through a switch opened by the LPULSE line, disabling the
marker circuits when the RF power is pulsed. This prevents the power dropouts
from producing false markers. A second switch, actuated by the buffered
"birdies," transforms the analog signals into a TTL rectangular wave for
processing on the A7 Marker assembly.