Motorola MTM700, Detailed Service Manual

Page: 66 / 328

4.2 - 10 MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (800 MHz)
Detailed Circuit Description
Receiver Path, Detailed Circuit Description
For the Receive Path Block Diagram see Figure 2.
Antenna Switch
The signal coming from the antenna is routed to the receiver section via the harmonic filter, which is
comprised of L5461, L5462, and C5462 to C5467. The signal continues to flow through quarter
wave transformer L5452 and C5555 to the Rx section toward pre-selector filter. The limiter (D5002)
function is to prevent entrance of signals with power higher than 30dBm, which may damage the
receiver front-end.
Pre-selector Filter
The signal is passed to the first pre-selector filter (FL5002) and from there routed to Low Noise
Amplifier (LNA) (U5003). The first pre-selector filter attenuates incoming spurious RF frequencies,
especially the image frequency.
LNA
The Low noise amplifier (LNA) U5003 amplifies the incoming received signal. It maintains a large
gain and small Noise Figure. When LNA “enable” (pin 3, VPD) receives a supply of 1.9 V from DC
switch (Q5002), the amplifier is enabled. From the LNA the signal passes to the second pre-selector
filter (FL5001).
Second Pre-selector
The second pre-selector filter (FL5001) provides further attenuation of incoming image and half-IF
spurious response signals, as well as noise at the image frequency that may be generated by the
LNA. From the output of the second pre-selector filter the received signal is sent to the RF input port
of the first mixer.
Mixer
Mixer U5002 down-converts the incoming RF signal to the 73.35 MHz first IF, utilizing a first local
oscillator signal tuned 73.35 MHz below the desired receiver operating frequency.
An impedance
matching network (L5012, C5037, C5036, L5201, C5203, L5011, L5018) is placed between the
mixer output and IF filter U5201.
IF Filter, Amplifier, Dynamic Range Extension
The IF signal is routed via IF filter (FL5201) to IF amplifier Q5201. From there it is routed to the
Dynamic Range Extension circuitry which comprises Q5101, its bias components, and D5121. This
circuitry is required to maintain good receiver linearity over a wide range of incoming signal levels,