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Unlocked Offset Lock Loop (Sampling Oscillator)
Operation
The offset lock loop drives the
A15UlOO
sampler. The offset lock loop sampling oscillator
tunes to one of sixteen discrete frequencies between 285 MHz and 297.222 MHz. Refer to
Al5 schematic. Mixer A15U400 mixes the oscillator output with 300 MHz from the reference
PLL, producing a 3 MHz to 15 MHz IF signal. The 3 MHz to 15 MHz signal is compared
in the phase/frequency detector with the divided-down 300 MHz from the reference PLL.
The phase/frequency detector drives a voltage-to-current diode switch which drives the loop
integrator. Loop bandwidth switches vary the loop bandwidth to minimize noise sidebands.
The sampling oscillator must produce low noise because the
A15UlOO
sampler multiplies noise
by a factor of approximately 24.
Table 11-6 lists the prescaler and postscaler divide numbers in the offset loop reference divide
chain, for each of the 16 discrete frequencies to which the offset lock loop may be set. It also
indicates what the reference frequency into the phase/frequency divide chain is. Refer to
function block AN on the RF schematic.
Troubleshooting
Check loop references (steps 1 and 2)
1. Use an active probe and spectrum analyzer to confirm the presence of the following
reference to the offset lock loop input.
A15TP404
300 MHz at
+5
dBm
2. If this signal is not correct, refer to “Unlocked Reference PLL” in this chapter.
Check levels into mixer (steps 3-13)
3. Set the spectrum analyzer to the following settings:
Center frequency
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
Span
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
Trigger
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single
4. Force the PLL to unlock by shorting A15X201 pin 1 to A15X201 pin 5 with a short length
of wire. Then connect a dc power supply to A15J200 pin 16.
5. Monitor A15TP201 with an active probe/spectrum analyzer combination. Vary the dc
supply until the frequency of the sampling oscillator is 296 MHz.
6. The voltage required to tune the oscillator should measure between
+15
Vdc and
+19
Vdc. If the voltage is out of this range, perform the sampling oscillator adjustment in
Chapter 2, “Adjustment Procedures.”
7. Vary the voltage to tune the sampling oscillator to 296 MHz.
8. Use an active probe/spectrum analyzer combination to measure the 300 MHz LO signal at
the following test point:
A15TP402
+7
dBm
1 l-20 Synthesizer Section