HP 8560E, Service Manual

Page: 549 / 665

548 Chapter 10
Synthesizer Section
Unlocked Fractional N PLL
6. If neither LED is lit, but the output frequency is wrong by more than
1 MHz, check the postscaler, function block AV.
7. Check that the postscaler is dividing properly. The frequency at
A14J304 should be equal to the frequency at A14TP4 divided by
either 5, 6, or 7. Refer to
Table 10-11 on page 548 . To keep the divide
number at a constant value set the spectrum analyzer to:
Span ................................................................................. 0 Hz
Trigger ............................................................ SINGLE, EXT
(with no external trigger connected)
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.
Fractional N PLL
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 as
long as HSCAN is high. Use the following procedure to troubleshoot
unlocked loop problems or problems of locking to the wrong frequency
(by less than 1 MHz):
1. Check the two LEDs on A14 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 A14J301.
The 10 MHz reference is derived from the 600 MHz reference on the
Table 10-11 Postscaler Divide Numbers
Divide
Number D11 D10 D9 Input Range
(MHz)
(A14J304) Output
Range
(MHz)
(A14TP4)
7 0 0 1
840 to 973 60.0 to 69.5
6 0 1 0
834 to 987.96 69.5 to 82.33
5 0 1 1
823.2 to 960 82.33 to 96.0