Service manual
CIRCUIT DESCRIPTION
6-4
Month 2000
Part No. 001-2004-601
Second Mixer/Detector
As shown in Figure 6-2, U201 contains second
oscillator, second mixer, limiter, detector and RSSI
circuitry. The 52.95 MHz IF signal is mixed with a
52.5 MHz signal produced by TCXO Y401, tripler
Q204 and amplifier Q203. The 17.5 MHz (±1 PPM)
output of Y401 is fed through C275 to tripler Q204.
The tripler passes the third harmonic at 52.5 MHz to
amplifier Q203. Amplifier Q203 amplifies the 52.5
MHz signal for the oscillator input of U201.
Biasing of Q204 is provided by R258, R259 and
R260. RF choke L222 blocks the flow of RF through
R261. An AC voltage divider formed by C280/C281
matches Q204 to the highpass filter. L223, C282 and
L224 form a high pass filter to attenuate frequencies
below 52.95 MHz. C283 and C284 match the output
of the filter to U203. The third harmonic of the TCXO
frequency is lightly coupled to amplifier Q203 through
C270, R262 and C265. Biasing of Q203 is provided
by R254, R255, R256 and R257. The amplified 52.5
MHz output is passed to U201 OSC B input through
C271.
The 450 kHz second IF is then fed to ceramic fil-
ter Z203/Z204, then into the IF amplifier. The center
frequency of Z203/Z204 is 450 kHz with a bandwidth
of 15 kHz used to attenuate wideband noise. The lim
-
iter amplifies the 450 kHz signal 92 dB which
removes any amplitude fluctuations.
From the limiter the signal is fed to the quadra-
ture detector. An external phase-shift network con-
nected to U201, pin 8, shifts the phase of one of the
detector inputs 90° at 450 kHz (the other inputs are
unshifted in phase). When modulation occurs, the fre
-
quency of the IF signal changes at an audio rate as
does the phase of the shifted signal. The detector, that
has no output with a 90° phase shift, converts the
phase shift into an audio signal. Z205 is adjusted to
provide maximum undistorted output from the detec
-
tor. The audio signal is then fed out on U201, pin 9.
Wideband Audio Amplifier
U202B amplifies the detected audio and data sig-
nal. R212/R213/R214 set the gain of the amplifier and
R215/R216/R217/R218 and RT202 provide a DC ref
-
erence level. C226 bypasses the 450 kHz IF signal
and C227 bypasses other frequencies. The output sig
-
nal is adjusted by R220 and fed to J205, pin 1. Install
jumper plug P205 on J205, pins 1-2 to select the
25 kHz audio to be routed to J201, pin 6.
RSSI Amplifier
U201, pin 13 is an output from an internal RSSI
(receive signal strength indicator) circuit which pro
-
vides a current proportional to the strength of the
450 kHz IF signal. The RSSI output is buffered
through U202A and the level is adjusted by R219.
The DC output signal is then fed to J204, pin 1. Install
jumper plug P204 on J201, pins 1-2 to select the 25
kHz RSSI to be routed to J201, pin 7.
6.1.6 VCO (A006)
The Voltage-Controlled Oscillator (VCO) is
formed by Q802 circuitry and a resonator consisting of
Z801. The VCO oscillates in a frequency range from
433-572 MHz. Biasing of Q802 is provided by R805,
R806, R807 and R808. AC voltage divider C806,
C807 and C808 initiates and maintains oscillation and
matches Q802 to the tank circuit. The ceramic resona
-
tor is grounded at one end to provide shunt inductance
to the tank circuit.
The VCO frequency is controlled in part by DC
voltage across varactor diode CR802. As voltage
across a reverse-biased varactor diode increases, its
capacitance decreases. Therefore, VCO frequency
increases as the control voltage increases. The control
line is RF isolated from tank circuit by choke L802.
The amount of frequency change produced by CR802
is controlled by series capacitor C804.
6.1.7 ACTIVE FILTER
Q801 functions as a capacitance multiplier to pro-
vide filtering of the 12V supply to Q802. R803 and
R804 provide transistor bias, and C812 provides the
capacitance that is effectively multiplied by the gain of
Q801. If a noise pulse or other voltage change appears
on the collector, the base voltage does not change
because of C812. Therefore, the base current does not
change and transistor current remains constant. R805
decouples the VCO output from AC ground. L803 is
an RF choke and C810, C811, C813 and C814 provide
RF bypass.