Chapter 11 547
Synthesizer Section
Unlocked Offset Lock Loop (Sampling Oscillator)
Unlocked Offset Lock Loop (Sampling
Oscillator)
Operation
The offset lock loop drives the A15U100 sampler. The offset lock loop
sampling oscillator tunes to one of sixteen discrete frequencies between
285 MHz and 297.222 MHz. Refer to A15 schematic (sheet 3 of 4). The
oscillator output and the reference PLL 300 MHz signal is mixed by
A15U400 to produce 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 A15U100 sampler multiplies noise by a factor of
approximately 24.
Table 11-4 on page 548 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 of A15 RF schematic (block 3 of 4).
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. Refer to
function block AM of A15 RF schematic (sheet 3 of 4).
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 ..................................................... 300MHz
Span .................................................................................0Hz
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.
A15TP404 300 MHz at +5 dBm
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