Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
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Table 5-19
U2025 OUTPUT LINES
Line
High Low
Q1
Window disabled (OS low) Wide window (OS low)
Q2
Lock (connected FM coil) Unlock (disconnected FM coil)
Q3
Search (wide loop gain response) Narrow loop gain response
Q4
Strobe enabled
Strobe disabled
05
Narrow window Wide window (with Q1 low)
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Search Amplifier. This circuit consists of amplifier
U2048 and surrounding components. The ERROR 1 signal
from the Phase Gate Detector and Error Amplifier is applied
through LOOP GAIN adjustment R3082 to the inverting in
put of U2048. The signal (ERROR 1) is a result of the com
parison of the 1st Local Oscillator frequency and the
nearest multiple of the STROBE signal from the Strobe
Driver circuit. The ERROR 1 signal varies from zero to
about 500 kHz, and is up to four volts peak-to-peak in am
plitude. The LOOP GAIN adjustment is set for best sensitiv
ity with minimum hunting.
Amplifier U2048 is connected to operate as a low-pass
filter/integrator for the incoming ERROR 1 signal. During
search operation, however, the inverting input side of the
amplifier causes the stage to operate as a Wien-bridge oscil
lator at about 25 Hz. At this point, the U2037 outputs are in
the following states: Q1 line is high (contacts open) because
the phaselock system is in search mode; 0 2 is high (con
tacts closed, which allows U2048 to drive the FM coil); 03 is
high, allowing the lock bandwidth to be over 100 kHz wide
(thus, the Wien-bridge circuitry has sufficient positive
feedback to oscillate); 04 is high, which enables the strobe;
and Q5 is low because the system is in search mode. (01
and 05 remain open during this part of the search operation
to hold the window comparator disconnected.)
As the Strobe signal frequency is changed to be nearer
the 1st LO frequency, the ERROR 1 signal decreases in
frequency. Since the inverting side of U2048 is a low-pass
filter, the decreasing frequency receives more amplification,
until enough feedback occurs on the inverting side to sup
press the oscillations on the non-inverting side (i.e., the neg
ative feedback exceeds the positive feedback that normally
sustains oscillations).
The SEARCH signal (once locked) is now essentially a dc
level so the Phaselock Control circuits indicate to the
microcomputer that lock has occurred; it in turn causes line
Q3 to move low, closing the feedback path for the inverting
side of U2048. This decreases the bandwidth, ensuring that
the amplifier cannot break into oscillation until phaselock is
broken. It also improves the close-in noise performance of
the phaselock loop.
Window Comparator. This circuit consists of U1015 and
the associatd components, and is used to sense when
U2048 has approached its operating limits. When the
microcomputer causes the 0 2 signal to close the path from
U2048 to the FM coil, U2048 begins to furnish current to the
coil which causes the 1 st LO to track the stable strobe sig
nal. That is, each time the 1st LO frequency drifts, the ER
ROR 1 signal changes and U2048 shifts the FM coil current
to bring the 1st LO back to its original frequency. At the
same time, the microcomputer causes lines Q1 and A5 to be
low, closing the contacts that connect the output of U2048
to the input of the window comparator through a divider
network. Now, as the 1st LO frequency drifts, the search
amplifier will compensate for the drift. If the drift is exces
sive, however, U2048 will approach its design limits and will
be unable to furnish any more current to the FM coil.
Window comparator U1015 is a dual comparator stage
that senses a deviation of ± 15 mV. For instance, if a fre
quency shift forces U2048 to move positive enough (ap
proximately 3 V), the upper half of the comparator
conducts, and the UP line goes high. This triggers the ser
vice request circuits on the Phaselock board, which in turn
alerts the microcomputer, which then begins adjusting the
TUNE voltage from the Center Frequency Control circuits. If
the output drifts negative, the other half of U1015 conducts,
causing reverse action to occur.
Ordinarily, the input to the window comparator is attenu
ated by R2043, which reduces the voltage applied to U1015
to 0.3% of the output from U2048. This allows U2048 to
drift up and down without immediately triggering either com
parator. When R2043 is in the circuit, it is called “wide win
dow" operation. When phaselock is de-selected, the micro
computer selects narrow window (which bypasses R2043).
The Center Frequency Control circuit is then instructed by
the microcomputer to move the 1 st LO frequency until the
window comparator indicates that the FM coil current is
near zero. This prevents the 1st LO frequency from shifting
too far from the lock point when phaselock is cancelled.
Error Signal Filter. This circuit, which consists of active
lowpass filter U2065 and Schmitt trigger U1035, filters and
REV AUG 1981
5-75
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