Filter
Top Products
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
LEVEL)
(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