Detailed Circuit Description; Receiver Path, Detailed Circuit Description; Antenna Switch; Pre Selector Filter - Motorola MTM800 User Manual

4.1 - 10

Detailed Circuit Description

Receiver Path, Detailed Circuit Description

For the Receive Path Block Diagram see Figure 4.1-2 on page 4.1-5.

Antenna Switch

The signal coming from the antenna is routed to the receiver section via the harmonic filter, which is
comprised of L5461, L5462, C5461, C5462, C5464 and C5466. 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.
THEORY OF OPERATION (TRANSCEIVER)