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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
filter (that is, signals exiting the mixer via the RF port) is
reflected back to the mixer by the filter. If the cable between
the filter and the mixer is the correct length, the most signifi
cant reflected signals (that is, the image and the sum) can
be returned to the mixer in phase and converted into addi
tional energy at the intermediate frequency. This technique
is called "image enhancement mixing” and typically im
proves conversion loss by approximately 3 dB at the design
frequencies.
The image frequency in this instance is very near the RF
frequency. A very sharp cut-off filter is thus required to pass
the RF, yet reflect the image. The Four-Cavity Filter per
forms this function.
The 2nd Local Oscillator-to-Mixer LO input cable: The
image and sum products are also present at the LO port of
the mixer. These signals leave the mixer via the cable to the
2nd LO and are reflected back to the mixer by the LO. The
oscillators resonator appears highly reflective to the image
and sum signals because it is tuned to the LO frequency.
Again, the length of the cable from the LO to the mixer LO
port is adjusted so the image and sum signals are reflected
back to the mixer in the proper phase for re-conversion to
supply additional energy at the IF frequency.
2182 MHz Microstrip Oscillator such that the difference fre
quency exactly matches the frequency of the 18 MHz refer
ence VCO.
Sweep and tune signals from the Span attenuator and
Center Frequency Control circuits tune the 18 MHz VCO.
The output voltage from the phase/frequency detector
forces the Microstrip Oscillator to tune the same amount.
2182 MHz MICROSTRIP OSCILLATOR
B
This oscillator consists of a printed 1/2 wavelength reso
nator driven by a common-emitter feedback amplifier
(Q1021). The base of Q1021 is capacitively tapped into the
resonator. The resonator serves as a tuned phase inverter
and impedance transformer, connected between the base
and collector of Q1021. Part of the base feedback capaci
tance is provided by a bendable tab (C1021). This allows
fine adjustment of the total feedback. This feedback RF sig
nal is detected, by the base-emitter junction of Q1021, to
produce a change in bias voltage that is related to the
amount of feedback. The base voltage can be monitored at
TP1015 with a high impedance voltmeter without signifi
cantly disturbing the oscillator.
2182 MHz PHASELOCKED 2ND LO
General Description
The 2182 MHz Phaselocked 2nd LO assembly contains
a tunable microwave oscillator, frequency reference circuit
ry, and phaselock circuitry, within a two-section housing.
Microwave circuitry is packaged within the machined alumi
num portion of the housing. Low frequency phaselock cir
cuitry is within the mu-metal compartment.
The dc collector voltage and current for Q1021 is regulat
ed by an active feedback circuit containing transistor
Q2021. Voltage at the junction of R2023 and L2023 is a
function of Q1021 collector current. This voltage is sensed
by Q2021, which alters the base current to Q1021 thereby
regulating the collector current and maintaining +10 V dc
on the resonator. Decoupling and control of bias loop
dynamics are provided by C2104. Resistor R2016 swamps
the negative base resistance of 01021 to provide stabiliza
tion. Resistor R2015 protects the base-emitter junction of
Q1021 from excessive reverse bias in the event the
+ 12 volt supply fails.
In the microwave or LO portion of the assembly, the
2182 MHz Microstrip Oscillator generates 2182 MHz for the
2nd converters and the 2nd LO internal reference circuitry.
The 2200 MHz Reference circuit receives a 100 MHz drive
signal from the 3rd converter crystal oscillator and produces
100 MHz harmonics. The 22nd harmonic or 2200 MHz is
mixed with 2182 MHz from the microstrip oscillator in the
2200 MHz Reference Mixer circuit. The difference frequency
of 18 MHz is then fed to the phaselock side of the module.
A phase/frequency detector, on the 14-22 MHz
Phaselock circuit board, compares the 18 MHz difference
frequency with a signal from a linearized tuning 18 MHz volt
age controlled oscillator. The detector output tunes the
5-10
The oscillator is tuned by varactor diode CR1028, con
nected to one end of the resonator. Decoupling for the
varactor is provided by the low-pass elements in the tune
line. Bendable tab C1022 can be used to fine tune the oscil
lator center frequency.
Three output taps are coupled to the resonator through
printed capacitors under the resonator. One output supplies
2182 MHz through a 6 dB attenuator to the Harmonic Mixer
in the 829 MHz 2nd Converter. The other two output taps
couple LO power through 6 dB attenuators to buffer amplifi
ers Q1031 and Q1011. The amplifiers provide approximate
ly + 10 dBm to the 2072 MHz 2nd Converter and +8 dBm
to the Reference Mixer.
REV FEB 1983
I
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