Receiver; Front End - Motorola XTS 2500 Service Manual

UHF2 Detailed Theories of Operation: Receiver

4.3.4 Receiver

The receiver consists of the following:

• Front end

• Back end
4.3.4.1 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 UHF2 front end converts the received RF
signal to the 1st IF frequency of 73.35 MHz, while at the same time providing for spurious immunity
and adjacent channel selectivity. A review of the inter-stage 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 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.
The step following the step attenuator is the varactor-tuned preselector filter made up of C1, C2, C3,
C4, C5, L1, L2, C63, C64, CR1 and CR2. 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.
After the preselector filter, 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 L3 and C7 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 73.35 MHz. Since high-side injection is used, the LO frequency is offset above the RF carrier by
73.35 MHz, or Flo = Frf + 73.35 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 L57, C51 and C90. The secondary winding of T51 provides a
differential output. The center tap pin is grounded via an 75-ohm resistor that sets the mixer bias
current.
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.
6816985H01-F
4.3-3
February 14, 2012