HP 8560A, Service Manual

Page: 376 / 450

Unlocked Roller Oscillator PLL
Operation
The Roller Oscillator is used in the HP 8560A as a reference for the 1st LO phase-lock loop.
It provides the 25 Hz start-frequency resolution for the 1st LO, and is the means by which the
1st LO is swept in LO spans of 1 MHz or less (Roller spans). It is composed of the following
three separate phase-lock loops:
Offset Oscillator PLL
Transfer Oscillator PLL
Main Oscillator PLL
These three operate together to produce an output frequency in the range of 65 MHz to
104 MHz selectable in 25 Hz increments. The output frequency can be swept (increasing or
decreasing) over a selectable 2.5 to 1 MHz range.
To determine the Roller frequency for any given center frequency, press
(CAL), MORE ,
FREQ DIAGNOSE,
and
MAIN ROLLER.
The MAIN ROLLER frequency displayed is the
frequency that will be measured at A14J304 ROLLER TST with the HP 8560A in zero span.
Note Because the three Roller loops are all interdependent, an error message
displayed on screen might not be the primary cause of the failure. Be sure to
query the error message buffer for all encountered errors by pressing
MORE, RECALL ERRORS
and using the knob or step keys to scroll through all
encountered errors. The frequency displayed for the Offset Roller using the
above procedure is the Offset Roller
During the LO ADJUST sequence performed at power-on, pretune-DAC-values for Roller
Oscillator frequencies, spaced 2 MHz apart, are determined and stored. Pretune-DAC-values
for frequencies locked during instrument operation are interpolated from these calibration
values. Tuning sensitivities for the Main and Offset Rollers are determined and used to adjust
the Roller Span Attenuator DAC
to improve span accuracy.
Confirming an Unlocked Condition
1. Set the HP 8560A to the following settings:
CENTER FREQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz
2. Connect ROLLER TST to the input of a synthesized spectrum analyzer and view
the Roller Oscillator output at 94.7 MHz.
Note If a synthesized spectrum analyzer is not available, connect A14J304 to the
input of a 20
gain amplifier, such as an HP 84473. Connect the output of
the amplifier to the input of a frequency counter.
Synthesizer Section