Main Oscillator Pll - HP 8560A Service Manual

Spectrum analyzer
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Main Oscillator PLL

The Main Oscillator PLL provides an output signal in the range of 63 MHz to 106 MHz. This
signal is the reference to the YTO PLL. The Sampler IF signal from the Al5 RF assembly is
compared to this signal in the YTO Loop providing an error voltage to phase-lock the YTO.
The Main Oscillator PLL's sample-and-hold function is performed digitally using two DACs,
after retrace and before the start of a Main Roller span, these DACs are adjusted to zero.
When the PLL error voltage is zero it provides no frequency correction and can thus be
disconnected (via switch U116A) without affecting the Main Roller frequency. The span ramp
is then applied to sweep the Main Roller over the desired range.
Assuming the DACs and their address decoder are working properly, suspect a bad calibration
performed in the power-on sequence, an unstable Main Roller loop or Oscillator, or an
intermittent failure in the Main Roller PLL circuitry.
Note
Before proceeding with the following steps, confirm that the Roller Oscillator
PLL's Transfer Oscillator PLL and Offset Oscillator PLL loops are locked.
1. Measure the signal power at A145304. The signal should measure greater than -33 dBm.
2. Measure the signal at A14TP305. The signal's nominal power should measure -16 dBm.
3. If the output of A14TP305 is not correct, confirm that the voltage at J303 pin 1, Out
Amp Bias, is between +3 Vdc and +6 Vdc. Refer to function block A0 of Al4 Frequency
Control Schematic (sheet 5 of 5) in the Component-Level Information binder.
4. Confirm that the voltage at J303 pin 2, Main Osc Bias, is +8 Vdc fl V.
5. Set the HP 8560A to the following settings:
C E N T E R F R E Q
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
6. Confirm that the voltage at A14A103 pin 14, TUNE+, is t12.6 Vdc +/-1.4 V. Refer to
function blocks A0 and AN of Al4 Frequency Control Schematic (sheet 5 of 5).
7. If TUNE+ is within range, short A14A103 pin 12 to ground.
8. Measure the frequency at C345 (lead closest to mixer U117) with a frequency counter and
a x 10 oscilloscope probe.
9. If the frequency is not within 94.7 MHz *l MHz, the Main Roller Oscillator, A14A103, is
probably defective.
10. Set the HP 8560A to the following settings:
CENTER FREQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .300 MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
11. Measure the PLL's IF amplitude at U109 pin 1 with an oscilloscope and a x10
oscilloscope probe. A 50 mV signal at approximately 2 MHz should be observed.
12. If the PLL's IF is not correct, check for Transfer Oscillator drive at U117 pin 1 (-13 dBm
nominal drive).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
Synthesizer Section lo-37

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