Unlocked Offset Lock Loop (Sampling Oscillator)

Operation

The Offset Lock Loop drives the A15A2 Sampler. The Offset Lock Loop’s sampling oscillator tunes to one of fifteen discrete frequencies between 280 MHz and 298 MHz. Refer to Al5 Schematic (sheet 3 of 3). M ixer A15U400 mixes the oscillator’s output with 300 MHz

from the Reference PLL, producing a 2 MHz to 20 MHz IF signal. The 2 MHz to 20 MHz signal is divided down to 2 MHz or 2.5 MHz and compared in the phase/frequency detector with the divided-down 10 MHz from the Reference PLL. The phase/frequency detector drives a voltage-to-current (V/I) d’10 de 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 A15A2 Sampler multiplies noise by a factor of approximately 24.

Table lo-15 lists the divide ratios and control-line settings for each of the 15 discrete frequencies to which the Offset Lock Loop may be set.

Troubleshooting

1.Use an active probe and spectrum analyzer to confirm the presence of the following references to the Offset Lock Loop’s input.

A15TP404

300 MHz at +5 dBm

A15TP408

10 MHz TTL-level

2.If either of these signals is not correct, refer to “Unlocked Reference PLL” in this chapter.

3.Force the PLL to unlock by shorting A15X201 pin 1 to A15X201 pin 5 with a short length of wire.

4.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 280 MHz.

6.The voltage required to tune the oscillator should measure between +2 Vdc and +8 Vdc. If the voltage is out of this range, perform the Sampling Oscillator Adjustment in Chapter 2.

7.Vary the voltage to tune the sampling oscillator to 280 MHz.

8.Use an active probe/spectrum analyzer combination to measure the 300 MHz LO signal at the following test point:

A15TP402

+7 dBm

9.If the signal is not measured near the indicated power, troubleshoot the Offset Lock Loop Buffer (function block AM of Al5 RF Schematic sheet 3 of 3).

Synthesizer Section lo-39