Receiver Detailed Functional Description - Motorola MCS 2000 Service Instructions Manual

Receiver
Detailed Functional
Description
Receiver Front End
(All Kits)
Receiver Intermediate
Frequency (IF)
(Kits FLF5591A, B, C;
FLF5592A, B, C;
HUF1190A; HUF1191A)
Revision B, 5/98
The portion of the receiver function that is not part of the controller
section of the radio is composed of three sections: receiver front end;
receiver If, and receiver back end.
The following discussion is based on the schematic diagram for the
receiver front end section on page 37. The received RF signal (RX_IN)
from the antenna switch in the PA section of the radio enters the first
bandpass filter FL6250. The first bandpass filter has three poles, a 938-
MHz center frequency, a 6-MHz wide passband, and a 45-dB ultimate
rejection for frequencies outside the passband.
After the first bandpass filter, the received RF signal goes to a pair of
hot carrier limiting diodes (CR6250). The hot carrier diodes limit
strong signals to prevent them from over driving and damaging RF
preamplifier Q6271.
The main purpose of RF preamplifier Q6271 is to set the noise figure
of the receiver. Q6271 is actively biased through Q6272. During
transmit, the RF preamplifier is shut off by the K9.1 line via switch
Q6250 and bias transistor Q6272. After the signal leaves the RF
preamplifier, it enters second bandpass filter FL6251, which is
identical to FL6250.
When the RF signal leaves the second bandpass filter, it goes into
mixer U6251. The mixer is the double balanced active Gallium
Arsenide type. The RF signal is applied to the mixer through balun
transformer T6251. The first injection local oscillator is applied to the
mixer via balun transformer T6252 and is 73.35 MHz below the RF
signal frequency. The bias for the mixer is set by resistors R6285,
R6286, and R6287. The IF output signal from the mixer, which is at a
frequency of 73.35 MHz, is fed to the receiver IF section as IF_OUT
through transformer T6253.
The following discussion is based on the schematic diagram for the
receiver IF section on page 39.
The IF_OUT signal from the receiver front end section enters the
receiver IF section as the IF_IN signal. The first circuit in the receiver
IF section is a resistive pad (R6376, R6377, R6378, R6392), which
stabilizes the impedance presented to the output of the mixer in the
receiver front end section. It also stabilizes the impedance presented to
the input of the first 73.35-MHz crystal filter, Y6376.
From the resistive pad, the signal passes through to the first 73.35-
MHz crystal filter, Y6376 whose surrounding components match it to
50-ohms and to the input of IF amplifier Q6388. A matching network,
which follows Q6388, matches the IF amplifier output impedance to
the input impedance of the second 73.35-MHz crystal filter, Y6377.
Both crystal filters have two-poles and have a bandwidth of 13kHz to
accommodate the bandwidth requirements for digital data. Matching
elements, which follow the second crystal filter, match the output of
the second crystal filter to the input of the receiver back end section.
The signal out of the receiver IF section (IF_V_OUT) passes through
Theory of Operation 9