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

Product Overview: Transmitter Section
The back-end is primarily the ABACUS III digital IC. The ABACUS III IC uses a variable-bandwidth
bandpass sigma-delta architecture. It is capable of down-converting analog as well as digital RF
protocols into a baseband signal, which is then transmitted over the Synchronous Serial Interface
(SSI) bus to the DSP and microprocessor.
2.4.3 700–800 MHz Band Radios
The 700–800 MHz receiver consists of a front-end section and a back-end section.

2.4.3.1 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 ceramic-filter technology and includes a wideband,
monolithic amplifier. The first filter is a dual-switched filter that reduces the image frequency
response and limits some of the out-of-band interference. The second filter following the monolithic
low-noise amplifier (LNA) provides additional image rejection.
The receiver front-end signal is fed to the monolithic mixer IC where it is down converted to an IF of
73.35 MHz. The mixer is designed to provide low conversion loss and high intermodulation
performance. The mixer is driven by the receiver injection buffer, a two-stage discrete IC design used
with the receiver VCO to efficiently drive the mixer over a wide temperature range with minimum
power variation. The injection buffer provides 15 dBm to the mixer. The VCO performs low-side
injection for the 800 MHz band and high-side Injection for the 700 MHz band. The design maintains
temperature stability, low insertion loss, and high out-of-band rejection.

2.4.3.2 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 73.35 MHz that are isolated from one another by a stable, moderate-
gain 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 73.35 MHz signal from the IF section down to
2.25 MHz using a second LO frequency of 71.1 MHz or 75.6 MHz. The second LO VCO is tuned to
71.1 MHz (low side) or 75.6 MHz (high side injection). The choice of frequency depends on known
spurious interference related to the programmed received frequency.
2.5

Transmitter Section

This section discusses the transmitter section components and basic operation for each band.
2.5.1

VHF Radios

The VHF (136–174 MHz) transmitter consists of an RF power amplifier (RFPA), output network
(ON), and power control. See
sections.
2.5.2

UHF Range 1/UHF Range 2 Radios

The UHF Range 1 (380–470 MHz)/UHF Range 2 (450–520 MHz) transmitter consists of an RF
power amplifier (RFPA), output network (ON), and power control. See
below for an overview of the transmitter sections.
6881096C74-B
2.5.3 700–800 MHz Radios below for an overview of the transmitter
2.5.3 700–800 MHz Radios
2-9
May 25, 2005