Motorola MCS 2000 Service Instructions Manual page 18

Receiver IF
(Kits FLF5604A,
FUF5752A, FLF5952A,
FLF5606A)
10 Theory of Operation
connector J6400 and jumper plug J6401 to the input of the receiver
back end section.
The following discussion is based on the schematic diagram for the
receiver IF section on page 41.
The IF output signal (IF_OUT) from the receiver front end section
enters the receiver IF section as IF_IN and passes into three automatic
gain control (AGC)/attenuator stages. The first AGC/attenuator stage
is formed by diodes CR6251 and CR6253 and the associated circuitry,
the second stage by CR6252, CR6254, and the associated circuitry, and
the third stage by CR6255, CR6256 and the associated circuitry.
The three attenuators are controlled by the radio signal strength
indicator (RSSI) signal, from the zero intermediate frequency
integrated circuit (ZIF IC), in the receiver back end section, via a DC
shift and integrator circuit formed by two of the operational amplifiers
in U6202. For the lowest level IF_IN signals, the attenuator stages
attenuate minimally. For the highest level signals, they attenuate
approximately 25dB per stage. The attenuator stages are designed for
minimum VSWR, which means that for any level of operation they
present a matched port to the surrounding circuitry.
The principle of operation for the attenuator stages, taking the first
stage as an example, is the followings: The main components in the
attenuator are the two diodes, CR6251 and CR6253. The attenuator is
biased by two lines, a constant +5V line, connected to the anode of
diode CR6253, and a control line driven by the RSSI signal via the
operational amplifiers in U6202.
For a high signal level, the attenuation is at maximum value because
the control line is at 0V, which causes CR6251 to be zero biased while
CR6253 is maximum biased. This makes the IF signal flow to ground
through R6366, C6367 (which acts as a DC block), CR6253, and C6372
(which is also a DC block). The input signal sees a 50-ohm match,
which is roughly the sum of the impedances formed by the above
mentioned components.
For low signal levels, the attenuation is at minimum value because the
control line is at +5V and CR6251 is maximum biased while CR6253
is zero biased. The DC return for the control line is through R6368 and
R6370. The large values (470K) of these two resistors makes them also
function as RF blocks.
The control line is driven by the RSSI signal line from the ZIF IC in the
receiver back end section. The RSSI signal is amplified and DC shifted
through one of the operational amplifiers of U6202 and the associated
circuitry formed by resistors R6342 through R6345. This then passes
though the inverting integrator formed by another operational
amplifier of U6202 and C6362, R6358, R6359, R6360 and R6361. The
result is that for the lowest RSSI voltage, the control line is +5V and for
the highest RSSI voltage, the control line is 0V.
The inherent insertion loss of these attenuators ensures that a good
match is presented to the output of the front-end mixer.
Revision B, 5/98