User Manual
CIRCUIT DESCRIPTION
4-6
June 1997
Part No. 001-3492-001
4.3.4 CRYSTAL FILTER, FIRST IF SECTION
Z220 and Z221 form a 2-section, 4-pole crystal
filter with a center frequency of 87.85 MHz and a
-3 dB passband of 8 kHz (12.5 kHz BW) or 15 kHz
(25 kHz BW). This filter establishes the receiver
selectivity by attenuating the adjacent channel and
other signals close to the receive frequency. C223,
C224, and L221 adjust the coupling of the filter.
L222, C226, C227 and R223 provide impedance
matching between the filter and U230.
4.3.5 SECOND LO/MIXER/DETECTOR
Oscillator and Mixer
As shown in Figure 4-3, U230 contains the sec-
ond oscillator, second mixer, limiter, detector, and
squelch circuitry. The control line from synthesizer
U800 is on U230, pin 4. R250/C252/CR240 stabilize
the base current of the oscillator control line. C243/
C245 provide the feedback for the Colpitts oscillator.
The 87.4 MHz output of the oscillator is on U230, pin
3 and is coupled to buffer Q240. Bias for Q240 is pro-
vided by R242/R243/R245. The output of Q240 is
coupled to the auxiliary input of U800, pin 10 to main-
tain the control line. The 87.85 MHz IF signal is
mixed with the 87.4 MHz second LO to produce the
450 kHz Second IF.
Figure 4-3 U230 BLOCK DIAGRAM
Second IF Filter
The output of the internal double-balanced mixer
is the difference between 87.85 MHz and 87.4 MHz
which is 450 kHz. This 450 kHz signal is fed out on
pin 20 and applied to second IF filters Z230 and Z231.
These filters have passbands of 9 kHz (12.5 kHz BW),
or 20 kHz (25 kHz BW) at the -6 dB points and are
used to attenuate wideband noise.
Limiter-Amplifier
The output of Z230/Z231 is applied to a limiter-
amplifier circuit in U230. This circuit amplifies the
450 kHz signal and any noise present; then limits this
signal to a specific value. When the 450 kHz signal
level is high, noise pulses tend to get clipped off by
the limiter; however, when the 450 kHz signal level is
low, the noise passes through the limiter. C233/C234
decouple the 450 kHz signal.
Quadrature Detector
From the limiter stage the signal is fed to the
quadrature detector. An external phase-shift network
connected to U230, pin 10 shifts the phase of one of
the detector inputs 90° at 450 kHz (all 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 input. The detector,
which has no output with a 90° phase shift, converts
this phase shift into an audio signal. L230 is tuned to
provide maximum undistorted output from the detec-
tor. R232 is used to lower the Q of L230. From the
detector the audio and data signal is fed out on pin 8.
The audio/data output of U230, pin 8 is applied to
J201, pin 13.
Audio/Data Amplifier
The audio/data output of U230, pin 8 is applied
to J201, pin 13.
Receive Signal Strength Indicator (RSSI)
U230, pin 5 is an output for the RSSI circuit
which provides a voltage proportional to the strength
of the 450 kHz IF signal. The RSSI voltage is applied
to J201, pin 12.
1
2
4
MIXER
5
15
14
11
RSSI
87.85 MHz
C228
C237
20
OSCILLATOR
87.4 MHz
+
-
RSSI OUTPUT
9
V
REG
6
V
CC
7
8
+
-
10
QUAD
AUDIO OUTPUT
L230
12
13
3
CONTROL
LIMITER IN
LIMITER
IF AMP OUT
IF DEC1
IF DEC2
IF AMP IN
MIXER OUT
LIMITER DEC1
LIMITER DEC2
LIMITER OUT
IF AMP
16
17
18
19
RSSI FB
AUDIO FB