Receiver; Front End - Motorola XTS 2500 Service Manual

UHF1 Detailed Theories of Operation: Receiver

4.2.4 Receiver

The receiver consists of the following:

• Front end

• Back end
4.2.4.1 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 receiver front end converts the incoming RF
signal into Intermediate Frequency (IF) of 73.35MHz while providing spurious rejection and 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 L176, C178 and C192,
(which are anti-resonant at the radio's transmitter frequencies). Both PIN diodes D170 and D171
must not be biased to properly route the received RF signal.
Next is the varactor-tuned preselector filter made up of C2, C9, C61, C62, C63, C64, C65, C83, C84,
D3, D10, L5, L12, L61, R13 and R14. 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 C20, C71 and L63 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, C41 and L74. The subsequent stage in the receiver chain is the
1st mixer U1, which uses low-side injection, to convert the RF carrier to an intermediate frequency
(IF) of 73.35 MHz. Since low-side injection is used, the LO frequency is offset below 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 C51 and L57. 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 differential signal is applied to the mixer through pin 4 and 5.
The final stage in the receiver front end is a three-pole crystal filter (FL51). The crystal filter provides
some of the receiver's adjacent channel selectivity. The input to the crystal filter is matched to the 1st
mixer using components L72, L73, L54, and C57. The output of the crystal filter is matched with
components C58 and L55. The crystal filter provides at least 80dB of second image protection and
improves IM distortion in the ABACUS III IC.
6816985H01-F
4.2-3
February 14, 2012