HP 8560A

3. If the Roller Oscillator measures a stable 94.7 MHz, the Roller Oscillator PLLs are
probably locked.
4. If the Roller Oscillator is off-frequency or unstable and no error message is displayed, check
Roller MUX U305, Roller MUX Amplifier U328, and MUX Gain Control Switch U328B.
Refer to function block AG of Al4 Frequency Control Schematic (sheet 5 of 5).
Note
Although J304 is the output of the Main Roller Oscillator, an unstable signal
here does not necessarily mean the Main Roller Oscillator is unlocked; the
Main Roller Oscillator will track the frequency movements of the other two
Roller
PLLs.
Offset Oscillator PLL
The Offset Oscillator PLL provides a synthesized frequency in the range of 189 MHz to
204 MHz in 2.5
kHz steps. This frequency is divided by 100 to provide a 1.89 MHz to 2.04
MHz signal with 25 Hz resolution to the Main Oscillator PLL. It is swept in LO spans 5100
kHz.
1. Confirm an output from the Offset Oscillator at the points listed below. Refer to function
block AE of Al4 Frequency Control Schematic (sheet 5 of 5) in the Component-Level
Information binder.
A14TP302 (A14AlOl)
189 MHz to 204 MHz
U102 pin 2
189 MHz to 204 MHz (>-3 dBm)
2. Measure the Offset Amp and Offset Ox Bias voltages at the following test points:
A14J302 pin 5 (Offset Amp Bias)
A14J302 pin 2 (Offset Osc Bias)
between $2 and $4 Vdc
between +7 and +8 Vdc
3. The oscillator’s power level, measured with an active probe at TP302, should measure
-15 dBm.
4. Set the HP 8560A to the following settings:
CENTER FREQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
SPAN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz
5. Measure the unlocked Offset Oscillator’s frequency at
A14TP302. If the frequency is not
200 MHz f10 MHz, short U416 pin 14 to ground. Refer to function block AD of Al4
Frequency Control Schematic (sheet 5 of 5)
in the Component-Level Information binder.
6. The Offset Oscillator’s frequency should measure 200 MHz
fl MHz. If the frequency is
not in this range, measure the pretune voltage at J302 pin 15.
7. If the pretune voltage is not -5.25 +/-1.75 Vdc, troubleshoot U126B, U119B, and the
offset pretune DAC address decoder U302. Refer to function block AF of Al4 Frequency
Control Schematic (sheet 5 of 5) and function block B of Al4 Frequency Control
Schematic (sheet 2 of 5)
in the Component-Level Information binder.
8. Remove the short from U416.
Synthesizer Section
lo-33