Front End; Back End - Motorola ASTRO XTS 2500 Service Manual

VHF Detailed Theories of Operation: Receiver

4.1.4.1 Front End

For the purposes of this discussion, the receiver front end is defined as being the circuitry from the
antenna switch to the output of the IF crystal filter. The VHF front end converts the received RF
signal to the 1st IF frequency of 44.85 MHz, while at the same time providing for spurious immunity
and adjacent channel selectivity. A review of the interstage components of the front end follows, with
emphasis on troubleshooting considerations.
The received RF signal is passed through quarter wavelength components C180, L176, and C178,
(which are anti-resonant at the radio's transmitter frequencies). Both PIN diodes D170 and D171
must be reverse biased to properly route the received RF signal.
Next is the varactor-tuned preselector filter made up of C1, C2, L5, D4, D3, L7, C9, D11, D10, L12,
L8 and L16. The preselector is positioned right after the antenna switch to provide the receiver
preamp with some protection against strong signal, out-of-band signals, and first-image suppression.
The stage following the preselector is a step attenuator (U2). This attenuator provides 15 dB step
attenuation to protect the receiver from strong RF signals. The attenuator is controller by a
DSP-based algorithm that continuously monitors signal strength. When the ON threshold is
exceeded (approximately -95 dBM), the attenuator is activated via a DAC in the PCIC by the host.
The attenuator remains activated until the signal drops below the OFF threshold (approximately -115
dBM including the 15 dB attenuation). Hysteresis and timer functions are included in the algorithm to
enhance performance. The algorithm controlling the attenuator is enabled via the CPS for each
personality. When the algorithm is disabled, the attenuator is essentially a short circuit from input to
output.
After the step attenuator, the received signal is applied to the receiver preamp, Q29. The preamp is
a transistor, which has been biased and matched for optimum intermodulation (IM), noise figure
(NF), and gain performance. Components L21 and C20 match the input of the amp to the step
attenuator output. The preamp is supplied by a 5V analog regulator and is biased at approximately
0.75Vdc at the base and 2Vdc at the collector.
The output of the amp is matched to a second two-pole preselector filter of the type previously
discussed. The match is via C35 and L31. The subsequent stage in the receiver chain is the 1st
mixer U1, which uses high-side injection, to convert the RF carrier to an intermediate frequency (IF)
of 44.85 MHz. Since high-side injection is used, the LO frequency is offset above the RF carrier by
44.85 MHz, or Flo = Frf + 44.85 MHz. The mixer utilizes GaAs technology in a double-balanced,
Gilbert Cell configuration.
A balun transformer (T51) is used to couple the RF signal into the mixer. The primary winding of T51
is matched to the preceding stage by C62, C63, C64 and L61. The secondary winding of T51
provides a differential output. The center tap pin is grounded via an 100-ohm resistor that sets the
mixer bias current. The differential signal is applied to the mixer through pin 4 and 5.
The final stage in the receiver front end is a two-pole crystal filter (FL51). The crystal filter provides
some of the receiver's adjacent channel selectivity and intermodulation. The crystal filter provides at
least 75dB of second image protection and improves IM distortion in the ABACUS III IC.

4.1.4.2 Back End

In the ABACUS IC (U401), the first IF frequency is amplified and then down-converted to the second
IF frequency (2.25 MHz). At this point, the analog signal is converted into two digital bit streams by a
sigma-delta A/D converter. The bit streams are then digitally filtered, mixed down to baseband, and
filtered again. The output data stream is then sent to the Patriot IC, where it is decoded to produce
the recovered audio.
6816985H01-B
4.1-3
August 16, 2007