Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
divided down from the 100 MHz oscillator output supplied
from the 3rd Converter. This comparison is done by the
phase/frequency detector circuit. Its output is a correction
voltage that is applied to the VCO to drive the frequency in
the required direction to maintain the nominal output fre
quency at 719 MHz. This completes the loop that causes
the VCO to track the 2182 MHz reference.
Because the third harmonic of 719 MHz oscillator fre
quency is locked to the 2182 MHz reference, the tuning
range of the 719 MHz oscillator is only one third of the
swept range of the reference. Since that swept range is
8 MHz, the 719 MHz oscillator range need be only 719
±1.33 MHz.
The 719 MHz VCO, Q2014, uses a Colpitts configuration
with a printed circuit quarter-wavelength transmission line
resonator to achieve high spectral purity and good thermal
stability. Correction voltage is applied to varactor diode
CR1011 (which is connected at the midpoint of the trans
mission line resonator) to vary the resonant frequency of the
transmission line over a 2.66 MHz range. A tunable trans
mission line (also printed) adjacent to the printed resonator
compensates for variations in component tolerances and
resonator dimensions. This adjustable transmission line is
cut at factory calibration to the correct length for proper
VCO operation. A scale with minor divisions every 2 MHz is
printed next to the adjustable line to aid in calibration. The
output from the oscillator is extracted near one end of the
quarter-wavelength line through two printed inductors and
applied to output amplifiers through a power splitter.
Note that the 719 MHz VCO is enabled or disabled under
microprocessor control, dependent upon the frequency
band being analyzed. When the oscillator is disabled, the
719 MHz signal is no longer available for conversions with
829 MHz RF. This is controlled by the IF SELECT signal
from the RF Interface through connector C231. If this signal
is low, transistor Q2017 is cut off, which cuts off transistor
Q2016. This, in turn, cuts off transistor Q3015 (which is the
current source for oscillator transistor Q2014), thus cutting
off the 719 MHz oscillator.
From the oscillator, the +6 dBm 719 MHz output signal
is applied to isolation amplifier Q1021 through a power di
vider that consists of resistors R1021, R1022, and R1020.
(From the other side of this power divider, the signal is ap
plied to an output amplifier for transmission to the 829 MHz
2nd Converter Mixer circuit.) A second isolation amplifier
(Q3021) identical in configuration, provides the necessary
isolation between the 719 MHz oscillator output and
undesired harmonic mixer products.
The harmonic mixer produces not only the required
25 MHz difference frequency, but also many other higher
order products. Two in particular, those at 744 MHz and
694 MHz, are separated from the 719 MHz oscillator fre
quency by only 25 MHz. Were it not for the isolation pro
vided by amplifiers Q1021 and Q3021, these two products
could be converted in the 829 MHz mixer and would thus
appear as real signals on the screen. The isolation amplifiers
provide sufficient attenuation in the reverse direction to pre
vent this occurrence.
To provide maximum reverse attenuation in each amplifi
er circuit, external RF feedback is kept to a minimum. An
output matching LC network, consisting of capacitor C1025
and a printed inductor for Q1021, and capacitor C3021 and
a printed inductor for Q3021, presents an optimum load im
pedance to the collector of each transistor to allow maxi
mum power transfer to the attenuator that precedes the
harmonic mixer. An input LC matching network consisting
of capacitors C1023 and C1022, plus a printed inductor for
Q1021 and capacitors C3023, C3022 plus a printed inductor
for Q3021, establishes the 50 Ω input impedance to each
transistor.
A 3 dB attenuator consisting of resistors R3021, R3022,
R2021, and R3023 at the output of the second isolation
amplifier (Q3021) provides a non-reflective source imped
ance to the mixer. Without the attenuator, mixer conversion
loss could vary from unit to unit.
The harmonic mixer, consisting of diode CR2021, induc
tor L2014, and a half-wavelength (at 2182 MHz) transmis
sion line, produces the difference frequency between the
third harmonic of the 719 MHz oscillator frequency and the
2182 MHz reference frequency (nominally 2157 MHz). Note
that the 2182 MHz signal is supplied from the 2182 MHz
2nd Local Oscillator through coaxial connector P237 and
the power divider consisting of resistors R1021, R1023, and
R1022 to the half-wavelength transmission line. The VCO
input to the mixer switches diode CR2021 at a 719 MHz
rate. The 2182 MHz reference acts as the RF and is applied
to the diode from the transmission line. The resultant
25 MHz intermediate frequency is diplexed from the mixer
through the 100 MHz low-pass filter consisting of capacitor
C3014 and inductor L3014. (Diode CR2021 is mounted in
printed circuit board cut-outs to relieve any necessity of
bending the diode leads. Lead bending may fracture the di
ode case.) Inductor L2014 provides a bias return path to
allow the diode to switch at a 719 MHz rate.
From the harmonic mixer, the signal is applied through
the above mentioned lowpass filter to cascaded amplifiers
U1053 and U1044B. These amplifiers boost the —32 dBm
mixer output signal to a level appropriate to drive the
phase/frequency detector. Amplifier IC U1053 contains two
differential amplifiers in cascade; amplifier IC U1044 con
5-18
R EV FEB 1983
