Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
squares the incoming ERROR 1 signal for application to the
Phaselock Control circuits. The ERROR 1 signal is applied
through C2067 to an RC filter network that is a 500 kHz
low-pass filter. After filtering, the signal is applied through
ERROR COUNT BREAKPOINT adjustment R1061 to the
input of U1035, a Schmitt trigger circuit. The squared output
signal is then applied to the Phaselock Control circuits
where it is used by the microcomputer for determining the
relationship between 1 st LO frequency and the strobe line.
CONTROLLED OSCILLATOR, OFFSET
MIXER, AND STROBE DRIVER <^>
Controlled Oscillator
The Controlled Oscillator is a voltage-controlled crystal
oscillator whose frequency is controlled by the output of the
Error Amplifier. The oscillator generates a reference signal
that is used to stabilize the 1 st LO frequency.
Refer to the block diagram adjacent to Diagram 42. The
control voltage from the Error Amplifier, which is a function
of the difference between the microcomputer controlled N
signal and the Offset Mixer difference frequency, is applied
to the Controlled Oscillator to regulate its frequency of oper
ation. The circuit has two outputs: the first, which is part of
the inner loop of the phaselock circuits, is fed to the Offset
Mixer, where it is used to derive the difference frequency
that is compared against the -f- N signal. The second out
put, which is part of the outer loop, is fed to the Strobe
Driver circuits, where it is divided down to become the
STROBE signal that is compared against the 1 st LO signal
in the Phase Gate.
The Controlled Oscillator consists of five major circuits,
four of which are connected in a positive feedback loop to
sustain oscillation. These circuits are the resonator stage,
the differential amplifier, the bandpass filter, the isolation
amplifier, and the output amplifier. The resonator stage op
erates at a frequency of 25.032 MHz to 25.094 MHz. It
presents a high impedance to ground at resonance, which
reduces as the operating frequency moves away from the
resonant point. The output signal from the resonator is fed
to the differential amplifier, which splits the signal and sends
it to the output amplifier and the bandpass filter. The output
amplifier sends the signal to the Offset Mixer and the Strobe
Driver, and reduces loading of the feedback loop. The
bandpass filter strips the signal of any spurious responses
or harmonics and feeds the signal to the isolation amplifier.
This stage furnishes the positive feedback drive to the reso
nator stage and isolates the bandpass filter from the reso
nator stage.
The resonator stage consists of crystal Y1012, varactor
diodes CR1011 and CR1012, and related components. The
stage operates within a frequency range of 25.032 to
25.094 MHz, controlled by the voltage applied to varactor
diodes CR1011 and CR1012. Feedback energy for sustain
ing oscillations comes from the isolation amplifier by way of
coil L1025.
The resonator output signal is applied to a differential
amplifier Q2033 and Q2041. The Q2033 side drives the out
put amplifier and serves to isolate the output load from the
feedback loop. Gain from this side is less than one. The
signal is fed from the collector of Q2041, following amplifica
tion, into the bandpass filter.
The bandpass filter consists of passive components, and
is used to strip the signal of any frequency components
more than about 40 kHz away from the center operating
frequency, which is approximately 25.06 MHz. Capacitors
C1041 and C1042 are adjusted at the factory to set the
bandwidth and center frequency of the filter. The signal from
the filter is sent to the isolation amplifier.
Transistor Q1028 and related components make up the
isolation amplifier. The amplifier is a common-base configu
ration, in order to match the impedance of the filter to the
resonator. Output current from the stage furnishes positive
feedback for the resonator.
The output amplifier consists of transistors Q2025 and
Q2026, which are connected as a differential pair. The sig
nal from the collector of Q2026 furnishes the signal that
drives one side of the Offset Mixer; the signal from the col
lector of Q2025 drives the input of the Strobe Driver circuit,
for eventual application to the Phase Gate circuits.
Offset Mixer
The Offset Mixer consists of a ring diode mixer circuit, a
differential amplifier, and a phase/frequency detector. For
explanatory purposes, assume that the Controlled Oscilla
tor frequency is at 25.06 MHz and the η- N signal is 50 kHz.
The 25.06 MHz signal from the Controlled Oscillator en
ters the board at pin N of the Offset Mixer assembly. It is
applied to the base of transistor Q2 02 1 which drives trans
former T2010. The output of T2010 is connected across the
ring diode mixer. The 24 MHz reference frequency is applied
at pin K of the Offset Mixer and coupled through T1010 to
the ring diode mixer. The four frequency components are
picked off at the center tap of T2010. The two fundamental
frequencies and the sum are blocked by a pi filter, and the
5-76
REV AUG 1981
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