Motorola PRO Series, Detailed Service Manual

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Section 7: 2-2 Low Band Receiver Front-End
6881091C63-F
There are two 2-pole 10.7 MHz crystal filters in the high-IF section and two switched pairs of 455
kHz ceramic filters in the low-IF section to provide the required adjacent channel selectivity. The
second IF at 455 kHz is mixed, amplified and demodulated in the IF IC. The processing of the
demodulated audio signal is performed by an audio processing IC located in the controller section.
2.2 Front-End Bandpass Filters and Pre-Amplifier
The received signal from the radio’s antenna connector is first routed through the harmonic filter and
antenna switch, which are part of the RF power amplifier circuits, before being applied to the
receiver 5-pole antenna filter (L1001-L1005 and associated components). This filter configuration
provides more rapid attenuation above the passband to provide better rejection of the half-IF
spurious response. A dual hot carrier diode (D1001) limits any inband signal to 0 dBm to prevent
damage to the RF pre-amplifier.
The RF pre-amplifier is an SMD device (Q1001) with collector-base feedback to stabilize gain,
impedance, and intermodulation. Transistor Q1002 compares the voltage drop across resistor
R1005 with a fixed base voltage from divider R1006 and R1007, and adjusts the base current of
Q1001 as necessary to maintain its collector current constant at 25 mA. Operating voltage is from
the regulated 9.3V supply (9V3). During transmit, 9.1 volts (9T1) turns on both transistors in U1001,
turning off Q1003 and therefore Q1001-2. This protects the RF pre-amplifier from excessive
dissipation during transmit mode.
A second 5-pole fixed-tuned bandpass filter provides additional filtering of the amplified signal. This
filter configuration also provides steeper attenuation above its passband for best half-IF attenuation.
2.3 First Mixer and High Intermediate Frequency (IF)
The signal coming from the front-end is converted to the high-IF frequency of 10.7 MHz using a
passive double-balanced mixer (U1051). The high-side injection signal (RXINJ) from the frequency
synthesizer circuitry is filtered by a 7-pole low-pass filter (L1012-14 and associated circuitry) which
removes second harmonic content from the injection signal and improves half-IF rejection. The
50-ohm output of the first mixer is applied to the input of the high-IF circuit block.
2.4 High Intermediate Frequency (IF) and Blanker Switches
The first mixer IF output signal (IF) is applied to diplexer network consisting of L1101, L1111 and
associated components. This network has three functions: it terminates the mixer output at
frequencies other than 10.7 MHz into 51-ohm resistor R1101; it matches the 50-ohm mixer output to
the first IF amplifier (Q1101) input; and it provides bandpass filtering at 10.7 MHz to prevent the
5.35 MHz half-IF component of the mixer output from creating a second harmonic at 10.7 MHz in
Q1101, which degrades half-IF rejection.
The IF amplifier Q1101 uses AC and DC feedback to stabilize gain and quiescent current
(approximately 28 mA). Operating voltage is from the regulated 9.3V supply (9V3). Its output is
applied to a 10.7 MHz ceramic filter FL1101 which has a 3 dB bandwidth of 270 kHz and provides a
time delay of 2.6 usec. This delay allows enough time for the
“Extender” to respond to impulse noise
present at the input of Q1101 and operate the blanker switches Q1102 and Q1103, muting the IF
signal for the duration of the noise pulse. L1104 and L1105 also provide additional selectivity and
time delay. Operation of the
“Extender” circuit is explained in Section 2.6 below.