Receiver Section; Vhf Band Radios; Front-End Section; Back-End Section - Motorola ASTRO Digital XTL 5000 Detailed Service Manual

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2.4

Receiver Section

This section discusses the receiver section components and basic operation for each band.
2.4.1

VHF Band Radios

The VHF (136–174 MHz) receiver consists of a front-end and a back-end sections.

2.4.1.1 Front-End Section

The primary function of the receiver front-end is to optimize image rejection and selectivity while
providing the first conversion. The front-end uses discreet filters and LNAs. The first filter reduces
the IF spur, image frequency response and limits some of the out-of-band interference. The second
filter following the second low-noise amplifier (LNA) provides additional image rejection and half IF
spur.
The receiver front-end signal is fed to the monolithic mixer IC where it is down converted to an IF of
109.65 MHz. The mixer is driven by the receiver injection buffer that provides 20 dBm to the mixer.
The VCO performs high side injection for the VHF band. The design maintains temperature stability,
low insertion loss, and high out-of-band rejection.

2.4.1.2 Back-End Section

The crystal filters provide IF selectivity and out-of-band signal protection to the back-end IC. Two 2-
pole crystal filters centered at 109.65 MHz that are isolated from one another by a stable, high-gain
low noise amplifier are used to meet the receiver specifications for gain, close-in intermodulation
rejection, adjacent-channel selectivity, and second-image rejection.
The output of the IF circuit is fed directly to the Abacus III digital back-end IC. The ABACUS III is an
IC with a variable-bandwidth bandpass Sigma-Delta architecture. It is capable of down-converting
analog, as well as digital, RF protocols into a baseband signal transmitted on the Synchronous Serial
Interface (SSI) bus. The ABACUS III IC converts the 109.65 MHz signal from the IF section down to
2.25 MHz using a second LO frequency of 107.4 MHz or 111.9 MHz. The second LO VCO is tuned to
107.4 MHz (low side), but can be modified to 75.6 MHz (high side injection) depends on known
spurious interference related to the programmed received frequency.
2.4.2

UHF Range 1/UHF Range 2 Band Radios

The UHF Range 1 (380–470 MHz)/UHF Range 2 (450–520 MHz) receiver consists of a front-end
section and a back-end section.
2.4.2.1 Front-End Section
The receiver front-end consists of a switchable high pass filter and LNA, a preselector, a switchable
attenuator, a second switchable low noise amplifier (always in for UHF Range 2), image filter, mixer,
and injection amplifier. The preselector filter is varactor-tuned and is aligned at the factory. The radio
tuner software can also be used to check and re-align the preselector. The switchable stage provides
AGC capability.
The signal from the preselector is amplified by the low noise amplifier, then filtered by the image filter
before it is sent to the mixer. The mixer uses the LO signal, amplified by the injection amplifier, to
convert the RF signal to IF frequency of 109.65 MHz. This signal is then sent to the IF and back-end
circuits.

2.4.2.2 Intermediate Frequency and Back-End

The Intermediate Frequency (IF) consists of a crystal filter, amplifier, a second crystal filter, and a
switchable attenuator. This provides selectivity at the IF while attenuating out-of-band signals and
protecting the back-end (BE) IC.
May 25, 2005
Product Overview: Receiver Section
6881096C74-B