A4 Assembly’s Cal Oscillator Circuit

The Cal Oscillator on the A4 assembly supplies the stimulus signal for Automatic IF adjustments. Normally, the oscillator operates only during retrace (for a few milliseconds) to adjust part of the IF. (All IF parameters are to be readjusted about every 5 minutes.) With continuous IF adjust on, a group of IF parameters are adjusted during each retrace period (non-disruptive). If continuous IF adjust is off, the most recent IF calibration data is used.

The IF parameters adjusted include step gains, log amplifier gain and offset, bandwidth centering, 3 dB bandwidth, bandwidth amplitude, and crystal-filter symmetry.

The A4 Cal Oscillator provides three output frequencies (all -35 dBm):

n10.7 MHz

n9.9 to 11.5 MHz in 100 kHz steps

nFrequency sweeps from 20 kHz to 2 kHz centered at 10.7 MHz (lasting 5 to 60 ms respectively)

The signals perform the following functions:

nAdjust gains, log amps, and video slopes and offsets.

nAdjust 3 dB bandwidth and center frequencies of LC resolution bandwidth filters (30 kHz through 1 MHz).

nAdjust 3 dB bandwidth, symmetry, and gain of the crystal resolution bandwidth filters (300 Hz through 10 kHz).

The cal oscillator uses a phase-locked loop (PLL). T he oscillator (function block X) is locked to the instrument 10 MHz reference. The reference divider (function block U) divides the reference and delivers a 100 kHz logic signal to the phase detector (function block V). The divide-by-N circuitry (function block Y) divides the oscillator output of 9.9 MHz to 11.5 MHz (by 99 to 115) resulting in a 100 kHz output to the phase detector. When the cal-oscillator PLL is locked, narrow positive and negative of equal width pulses occur at the phase detector output. Since the phase detector drives a low-input impedance at the loop integrator, observe the positive pulses at A4CR808 anode and negative pulses at A4CR809 cathode.

The loop integrator acts as a low-pass filter that filters the pulses and inverts the result. If the anode of A4CR808 is more positive (with respect to ground) than the cathode of A4CR809 is negative, the loop integrator output should saturate to approximately -13 V. Conversely, if the anode of A4CR808 is less positive than the cathode of A4CR809 is negative, the integrator should saturate to a positive voltage.

Note

If error messages 581 AMPL or 582 AMPL appears, refer to error message 582

 

AMPL in Chapter 6 and perform the procedure provided.

 

 

1.The oscillator output frequency should exceed 11.5 MHz if the CAL OSC TUNE line, A4U804 pin 14, exceeds +9 V. The oscillator frequency should be less than 9.9 MHz if CAL OSC TUNE is below -9 V. The oscillator only operates when CALOSC-OFF is low (0 v>.

2.If the Cal Oscillator remains locked (no error code 499 displayed) but does not have the correct output level, troubleshoot the output leveling circuitry (function blocks AA, AB, and AC) or output attenuator (function block AD).

8-34 IF Section