150 E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
Reference/Synthesis Loop Description
The A6 Frac–N can contribute to phase noise between 5 and 10 KHz. This
overlaps with the frequencies over which the A7 Reference contributes to
phase noise. There is no easy way to identify if a phase noise problem in the
overlapping area is generated by the A6 Frac–N or A7 Reference, other than to
try new assemblies.
FM Mode
FM rates up to 230 Hz are accomplished by combining the FM signal and the
A6 Frac–N VCO tune signal to drive the YIG Driver main coil.
FM rates above 230 Hz are accomplished using the FM drive circuit on the A9
YIG Driver to drive the FM coil in the YIG Oscillator.
Sweep Mode
The A6 Frac–N maintains phase lock in sweep mode by providing the
frequency correction voltage; in CW mode the A5 Sampler provides the
frequency correction voltage.
On the A6 Frac–N, the RF signal is downconverted to 5 MHz using fixed and
programmable dividers. The RF 5 MHz is compared to a 5 MHz reference
developed by dividing 10 MHz from the A7 Reference by 2. The difference in
phase is integrated and a correction voltage is applied to the crossover circuit
on the A9 YIG Driver. By using programmable dividers, the RF side of the
comparator can be kept at 5 MHz while the RF frequency sweeps. Because the
instrument can maintain phase lock while sweeping over a finite frequency
range, the A6 Frac–N divide numbers are changed at filter switch points and
band crossings.
A7 Reference
Clean and stable reference signals are essential for low phase noise. In this
case, two reference signals are provided:
— 1 GHz (to the A5 Sampler and the A8 Output, for down conversion)
— 10 MHz (to the A6 Frac–N for VCO phase coherency, and the A9 YIG Driver
for DDS clock and baseband generator accuracy).
The standard A7 Reference uses a 1 GHz STW Oscillator and 10 MHz OCXO.
The low phase noise A7 Reference (Option UNR/UNX) uses a 100 MHz VCXO
that is multiplied by 10 to achieve 1 GHz, and a high performance 10 MHz
OCXO.
The A7 Reference contributes to phase noise between 100 Hz and 10 KHz. See
the “A6 Frac–N” assembly description on page 149, for more information on
phase noise.
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