Midband (66-88Mhz) Specific Circuit Description; Receiver Front-End; Mixer And Intermediate Frequency (If) Section - Motorola GM-950 Manual

5C.3
8.0

Receiver Front-End

The receiver is able to cover the MB range from 66 to 88 MHz. It consists of four major blocks: front-
end, mixer, first IF section and IF IC. Antenna signal pre-selection is performed by two varactor
tuned bandpass filters. A double balanced schottky diode mixer converts the signal to the first IF at
21.4MHz.
Two crystal filters in the first IF section and two ceramic filters in the second IF section provide the
required 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.
8.1 Front-End Band-Pass Filter and Pre-Amplifier
A two pole pre-selector filter tuned by the dual varactor diode D2301 pre-selects the incoming signal
(PA RX) from the antenna switch to reduce spurious effects to following stages. The tuning voltage
(FE CNTL VLTG) ranging from 0.5 volts to 8 volts is controlled by a Digital to Analogue (D/A)
converter (U0731-11) in the controller section. A dual hot carrier diode (D2303) limits any inband
signal to 0dBm to prevent damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q2301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA, drawn from the
voltage 9V3 via L2302, is controlled by a current source composed of Q2302, R2302, R2300, and
R2311 - R2313. In transmit mode the high K9V1 signal fed through diode D2300 switches off the
current source and in turn the pre-amplifier. In receive mode K9V1 must be low to switch on the
current source. A 3 dB pad (R2306 - R2308 and R2316 - R2318) stabilizes the output impedance
and intermodulation performance.
A second two pole varactor tuned bandpass filter provides additional filtering to the amplified signal.
The dual varactor diode D2304 is controlled by the same signal which controls the pre-selector filter.
If the radio is configured for a base station application, R2318 is not placed and TP2301 and TP2302
are shorted.
8.2

Mixer and Intermediate Frequency (IF) Section

The signal coming from the front-end is converted to the first IF (21.4 MHz) using a double balanced
schottky diode mixer (D2331). Its ports are matched for incoming RF signal conversion to the
21.4MHz IF using high side injection. The injection signal (VCO MIXER) coming from the mixer
buffer (Q2770) is filtered by the lowpass consisting of (L2333, L2334, C2331 - C2333) and has a
level of approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T2301 pin 2 is fed to the first two pole crystal
filter Y2201. The filter output in turn is matched to the following IF amplifier. The IF amplifier Q2201 is
actively biased by a collector base feedback (R2201, R2202) to a current drain of approximately 5
mA drawn from the voltage 5V STAB. Its output impedance is matched to the second two pole
crystal filter Y2202. A dual hot carrier diode (D2201) limits the filter output voltage swing to reduce
overdrive effects at RF input levels above -27 dBm.
Theory of Operation
Midband SPECIFIC CIRCUIT DESCRIPTION
5C.3-19