Unlocked Offset Lock Loop (Sampling Oscillator); Operation; Troubleshooting - HP 8560E Service Manual

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. 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
A15U100 sampler multiplies noise by a factor of approximately 24.
Table 10-7 on page 535
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
2. This signal is not correct, refer to "Unlocked Reference PLL" in this
chapter.
Check levels into mixer (steps 3-13)
3. Set the HP 8560E/EC to the following settings:
Center frequency ..................................................... 300 MHz
Span ................................................................................. 0 Hz
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
534
lists the prescaler and postscaler divide
300 MHz at +5 dBm
Chapter 10