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Table 1 l-10. Postscaler Divide Numbers
Divide
Number
7
6
5
Dll
DlO
I
0
1
1
D9 Input
Range (MHz)
(A14J304)
output
Range (MHz)
(A14TP4)
840 to 973
834
to 987.96
823.2 to 960
60.0 to 69.5
69.5 to 82.33
82.33 to 96.0
-7
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 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
A14J301. The 10 MHz reference is derived from the 600 MHz reference on the Al5 RF
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,
MORE
1
OF
2,
FREXj
DIA$ZNOSE
, and
R$W
OSC
FREq
.
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