User Manual
CIRCUIT DESCRIPTION
4-6
June 1997
Part No. 001-3412-002
4.3.2 MIXER (U211)
First mixer U211 mixes the receive frequency
with the first injection frequency to produce the
52.95 MHz first IF. Since high-side injection is used,
the injection frequency is 52.95 MHz above the
receive frequency. The RF signal is coupled to the
mixer through C215
4.3.3 FIRST LO AMPLIFIER (Q301)
The first injection frequency from the VCO is
coupled to the first local oscillator amplifier Q301
through C301. L301/C302 match Q301 to the VCO.
Bias for Q301 is provided by R301, R302 and R303,
R306 and C307. Impedance matching to the mixer is
provided by L302, R305 and C306 decouples RF sig-
nals.
4.3.4 BUFFER (Q211), CRYSTAL FILTER (Z221/
Z222), IF AMP (Q231)
The output of U221 is coupled to buffer Q211.
C213, R213 and Q211 match the 50 ohm output of
U221. Bias for Q211 is provided by R211 and R213.
The output of Q211 is matched to crystal filter Z221
via L211, C214 and R212.
Z221 and Z222 form a 2-section, 4-pole crystal
filter with a center frequency of 52.95 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. C221,
C222, and L221 adjust the coupling of the filter.
L222, C223 and C233 provide impedance matching
between the filter and Q231.
IF amplifier Q231 amplifies the 52.95 MHz IF
signal to recover filter losses and improves receiver
sensitivity. Biasing for Q231 is provided by R231,
R232, R233, R234 and R235 and C232, C235 decou-
ple RF signals. The output of Q231 is coupled to the
detector by C234.
4.3.5 SECOND LO AMP/TRIPLER (Q401)
The input frequency to Q401 is 17.5 MHz from
TCXO Y801 coupled through C402. Bias for Q401 is
provided by R401, R402, R403 and R404. C403,
C404 decouple RF from the amplifier. L401, L402,
C405, C406 and C407 pass the third harmonic
(52.5 MHz) to the input of U241, pin 4. The output of
the amplifier is coupled to U241, pin 4 by C408, C409
and L403 form a 450 kHz notch.
4.3.6 SECOND MIXER/DETECTOR (U241)
Oscillator and Mixer
As shown in Figure 4-3, U241 contains the sec-
ond oscillator, second mixer, limiter, detector, and
squelch circuitry. The 52.95 MHz IF signal is mixed
with a 52.5 MHz signal produced by second LO
amplifier/tripler Q401 from TCXO Y801.
Figure 4-3 U241 BLOCK DIAGRAM
Second IF Filter
The output of the internal double-balanced mixer
is the difference between 52.95 MHz and 52.5 MHz
which is 450 kHz. This 450 kHz signal is fed out on
pin 20 and filtered by IF filter Z251. The filtered sig-
nal is fed back into U241 on pin 18 to an internal IF
amplifier. After amplification the signal is fed out on
pin 16 where it is filtered by Z252 and then fed back
into U241, pin 14 to the limiter.
1
2
4
MIXER
5
15
14
11
RSSI
52.95 MHz
C234
C267
20
OSCILLATOR
52.5 MHz
+
-
RSSI OUTPUT
9
V
REG
6
V
CC
7
8
+
-
10
QUAD
AUDIO OUTPUT
L253
12
13
3
NC
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