HP 8562E Service Manual page 406

If the output frequency is wrong by less than 1 MHz, the phase locked loop is not unlocked
but still requires repair. Continue with the "Fractional N Oscillator PLL" section.
The fractional N PLL provides a synthesized frequency in the range of 60 MHz to 96 MHz.
The 800 MHz to 1020 MHz voltage controlled oscillator (VCO) in the loop is divided down
to lock with the 2.5 MHz reference. Simultaneously, the VCO is divided by two and then by
either 5, 6, or 7 to generate the 60 MHz to 96 MHz output.
The prescaler (function block AR) supplies the clock signal for the fractional divider and
is required for the fractional divider to operate. At the start of a fractional N sweep, the
fractional divider is set to a value for the start frequency and a sweep rate. It then sweeps
for long as HSCAN is high. Use the following procedure to troubleshoot unlocked loop
as
problems or problems of locking to the wrong frequency (by less than 1 MHz):
1. Check the two LEDs on Al4 frequency control assembly. If either LED is lit, the
fractional N phase locked loop is not locked.
2. The 10 MHz reference is required for fractional N operation. It is divided by four to
2.5 MHz in the reference divider circuitry, block AN. It is used to lock the divided voltage
controlled oscillator (VCO) frequency. Check that the 10 MHz reference is present at
assembly.
3. Change the spectrum analyzer from the fractional N span to 0 Hz.
4. Check the frequency at A14TPl. It should equal the value found by pressing a,
5. Check the tune voltage at R240 in function block AQ.
6. Look up the expected problem area in Table 11-11 with the information from steps 4 and
5. Go to the appropriate troubleshooting steps.
1 l-32 Synthesizer Section
Table 1 l-10. Postscaler Divide Numbers
Divide
Number
0
7
1
6
1
5
output
Input
Range (MHz) Range (MHz)
to 973 to 69.5
840 60.0
to 987.96 69.5 to 82.33
834
to 960 to 96.0
823.2 82.33