IF Amplifier @

The 60 MHz low-pass filter rejects unwanted mixing products from the mixer. Q4 and U18B amplify and limit the IF signal and give it the proper levels to run the following ECL circuitry.

Phase/Frequency Detector @

The outputs of M and N dividers are compared in Ul. When they are in phase, the outputs of Ul are narrow, coincident pulses. For unlock conditions, the output pulses are of varying widths.

Preamplifier @

Ql and Q2 are a low-noise differential pair preamplifier. Their outputs are combined in the integrating amplifier of A7A4.

Phase Lock Indicator @

U2A and U2B are voltage comparators which compare each tune line to the average plus a small offset provided by R29. If the loop unlocks, one of the preamplifier outputs is higher than the comparison voltage, and the phase lock indicator goes high, indicating the unlock condition.

A7A3 M/N Phase Detector, Troubleshooting

The A7A3 M/N Phase Detector mixes the A7A4 M/N VCO output (355 to 395 MHz) with 400 MHz from the A7A2 100 MHz VCXO. The product of this mixing and the 20 MHz reference from the A7Al are then divided by the M and N dividers respectively, and their resultant outputs are compared by a phase/frequency detector. The phase error is used to tune the A7A4 VCO.

The two dividers (M and N) are identical in operation and their waveforms can be compared for troubleshooting purposes. The M and N numbers can be set to the same value and various test points within each divider compared to reveal a malfunction, providing one of the dividers is known to be operating properly.

M Divider @ and N Divider @

To display the M and N divide numbers on the HP 8566B CRT, key in the following settings:

@TGGGHz)

(Gii?Tj (MKR*REF L E V E L ) (KSR)

(-1 [BW) (KSF)

[FREQUENCYSPAN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz

(CENTERFREQUENCY) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.77GIIZ

SWEEP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...(~) The third line of the diagnostic (KSR) should read as follows:

1 20 20

2 A7A3