216-246Mhz Specific Circuit Description; Receiver Front-End; Mixer And Intermediate Frequency (If) Section - Motorola GM950E Service Manual

6.3
8.0

Receiver Front-End

The receiver is able to cover the range from 216 to 246 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
45.1 MHz.
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 D3301 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 2 volts to 8 volts is controlled by a Digital to Analog (D/A) converter
(U0731-11) in the controller section. A dual hot carrier diode (D3303) limits any inband signal to
0dBm to prevent damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q3301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA, drawn from the
voltage 9V3, is controlled by a current source composed of Q3302, R3302, R3300, and R3311 -
R3313. In transmit mode the high K9V1 signal fed through diode D3300 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 (R3306 - R3308 and R3316 - R3318) 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 D3313 and D3314 are controlled by the same signal which controls the pre-
selector filter.
If the radio is configured for a base station application, R3318 is not placed and TP3301 and TP3302
are shorted.
8.2

Mixer and Intermediate Frequency (IF) Section

The signal coming from the front-end is converted to the first IF (45.1 MHz) using a double balanced
schottky diode mixer (D3331). Its ports are matched for incoming RF signal conversion to the
45.1MHz IF using high side injection. The injection signal (VCO MIXER) coming from the mixer
buffer (Q3770) is filtered by the lowpass consisting of (L3333, L3334, C3331 - C3333) and has a
level of approximately 10 dBm.
The mixer IF output signal (RX IF) from transformer T3301 pin 2 is fed to the first two pole crystal
filter Y5201. The filter output in turn is matched to the following IF amplifier.
Theory of Operation

216-246MHz SPECIFIC CIRCUIT DESCRIPTION

Receiver Front-End
6.3-17