Uhf (403-470Mhz) Specific Circuit Description; Receiver Front-End; Front-End Band-Pass Filter & Pre-Amplifier; Front-End Band-Pass Filter And Pre-Amplifier - Motorola GM1200E Service Manual

Receiver Front-End

The allowable internal alert tones are 304, 608, 911, and 1823Hz. In this case a code contained
within the SPI BUS load to the ASFIC sets up the path and determines the tone frequency, and at
what volume level to generate the tone. (It does not have to be related to the voice volume setting).
For external alert tones, the µP can generate any tone within the 100-3000Hz audio band. This is
accomplished by the µP generating a square wave which enters the ASFIC at U0201-C3.
Inside the ASFIC, this signal is routed to the alert tone generator. The output of the generator is
summed into the audio chain just after the RX audio de-emphasis block. Inside U0201 the tone is
amplified and filtered, then passed through the 8-bit digital volume attenuator, which is typically
loaded with a special value for alert tone audio. The tone exits at U0201-J4 and is routed to the
audio PA like receive audio.

UHF (403-470MHz) SPECIFIC CIRCUIT DESCRIPTION

8.0

Receiver Front-End

The receiver is able to cover the UHF range from 403 to 470 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 shottky 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 & Pre-Amplifier
A two pole pre-selector filter tuned by the varactor diodes D5301 and D5302 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 Analogue
(D/A) converter (U0731-11) in the controller section. A dual hot carrier diode (D5303) limits any
inband signal to 0 dBm to prevent damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q5301) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the
voltage 9V3 via L5302 and R5302. A 3dB pad (R5306 - R5308 and R5317 - R5319) stabilizes the
output impedance and intermodulation performance.
A second two pole varactor tuned bandpass filter provides additional filtering to the amplified signal.
The varactor diodes D5304 and D5305 are controlled by the same signal which controls the pre-
selector filter. A following 1 dB pad (R5310, R5314, R5316) stabilizes the output impedance and
intermodulation performance
If the UHF radio is configured for a base station application, R5319 is not placed and TP5301 and
TP5302 are shorted.
3.1-18 Introduction/Theory of Operation