Fluke 6080A Service Manual page 140

TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
Status code 244, appearing without status code 242, might indicate a marginal
break-up condition. To check the Sub-Synthesizer across the band, move the jumper
on the Sub-Synthesizer VCO (A3) from TP1-TP2 to TP1-TP3. This allows the
Sub-Synthesizer VCO frequency, not divided, to appear at A3-J2. Connect this output
to a spectrum analyzer. Program the signal generator to 800 MHz. There should be a
stable signal at 160 MHz displayed on the spectrum analyzer. Step the signal generator
in 200-kHz steps, while stepping the spectrum analyzer in 4-MHz steps. At each point,
a stable signal should be displayed on the spectrum analyzer. If the signal shows
evidence of breaking up, there is a problem with the Sub-Synthesizer and/or
Sub-Synthesizer VCO.
If there is a status code 242, check to see if the VCO control voltage is stuck high or low.
A good way to do this is to measure the DC voltage at TP27. This test point can be
accessed without removing the module cover. If the DC voltage is around 1.5V, the
problem is in the circuitry that supplies the 1-MHz reference or in the phase detector
circuit; if it is around 23V, the problem is associated with the whole phase lock loop
(VCO, SSB mixer, divider).
Table 6C-1 shows the characteristics of the signals at the various test points on the
Sub-Synthesizer PCA. The table gives the range of the signal and the expected value
for a typical instrument state. The values in the typical apply when the signal generator
is programmed to 804.001499 MHz. In this troubleshooting procedure it is useful to
have the undivided Sub-Synthesizer VCO signal available. On the Sub-Synthesizer
VCO (A3), move the jumper from TP1-TP2 to TP1-TP3.
If the voltage at TP27 is approximately 1.5 volts, check TP22. There should be a
1-MHz TTL square wave. If the signal is missing or the frequency is incorrect, work
backwards from this point. If this frequency is correct, the problem is probably in the
phase detector (U30-32) or loop amplifier (U34, etc.). At U21-9 there should be a
10-MHz TTL square wave. The input, U21-11, should be a 20-MHz TTL square wave.
At U21-3 there should be a 40-MHz TTL signal. There should be a 40-MHz ECL signal
at both U20-2 and U20-14. The frequency input, J6, from the Coarse Loop (A2),
should be approximately a 600-mV, p-p 40-MHz signal.
If the voltage at TP27 is around 23V, remove the shorting jumper connecting
TP40-TP41 and connect a variable power supply to TP41, being careful not the short
to TP40 which could destroy U34. This opens the loop and allows the frequency of the
Sub-Synthesizer VCO to be controlled directly. Use a spectrum analyzer or counter
connected to the undivided Sub-Synthesizer VCO output (A3-J3) to monitor the
frequency. Adjust the power supply so that the frequency tunes from approximately
160 to 320 MHz. If the frequency cannot be adjusted, the problem is probably in the
Sub-Synthesizer VCO (A3). Set the frequency to approximately 240 MHz with the
variable power supply.
Using a spectrum analyzer and the low impedance probe, with the 10X attenuator,
measure the level at the output of U51. A good place to measure this is at the input to
the coupler, U52. Note that the low impedance probe should be grounded as closely as
possible. PCA hold-down screws and the walls of the plate provide good grounds. The
level at this point should be approximately -10 dBm as measured on the spectrum
analyzer. Troubleshoot the RF section (U50, U51, etc.) if this level is not correct.
6C-10