User Manual

CIRCUIT DESCRIPTION
4-8
Part No. 001-3422-003
This removes the bias from Q202 and disables the RF amplifier in transmit mode. This prevents noise and RF from
being amplified by Q202 and fed back on the first injection line.
Additional filtering of the receive signal is provided by a three pole discrete LC varactor tuned bandpass filter
composed of filter L212-L214, L221-L224, CR283-CR285, C214-C217, C221-C223. L211 and C213 provide
impedance matching between Q202 and this filter. Resistor R205 is used to lower the Q of L211 to make it less
frequency selective. The same control voltage that adjusts to two pole filter on frequency adjusts this filter as well.
The inductors are factory tuned to align the filter tracking and should not be adjusted.
4.3.2 MIXER (U231), AMPLIFIER (Q231)
First mixer U231 mixes the receive frequency with the first injection frequency to produce the 21.45 MHz
first IF. Since high-side injection is used, the injection frequency is 21.45 MHz above the receive frequency. The
RF signal is coupled to the mixer through C232.
The output of U231 is coupled to buffer Q231. C232, R233 and Q231 match the 50 ohm output of U231. The
output of Q231 is matched to crystal filter Z231 via L231 and C234. This filter presents a low impedance to 21.45
MHz and attenuates the receive, injection, and other frequencies outside the 21.45 MHz passband.
4.3.3 FIRST LO AMPLIFIER/BUFFER (Q301, Q302)
The first LO amplifier provides amplification and buffering of the receive first injection. R305-R307 form a 3
dB 50 ohm pad. C303 couples the signal to C304 and L301 which match Q302 to 50 ohms. L302 and C307 match
Q302 to the mixer Q232. Q301, R301-R304 provide biasing for Q302. R308 enhances the stability of Q302.
C302 and C306 provide RF decoupling.
4.3.4 CRYSTAL FILTER (Z231/Z232)
The output of Q231 is matched to the crystal filter, Z231/Z232 by L231, C234 and C237. This filter presents
a low impedance to 21.45 MHz and attenuates the receive, injection and other frequencies outside the 21.45 MHz
passband.
Z221 and Z222 form a 2-section, 4-pole crystal filter with a center frequency of 21.45 MHz and a
-3 dB passband of 8 kHz (15 kHz BW) or 15 kHz
(30 kHz BW). This filter establishes the receiver selectivity by attenuating the adjacent channel and other signals
close to the receive frequency. C241, and C242 adjust the coupling of the filter. L242, C244, C245 and R243
provide impedance matching between the filter and U241.