Motorola MTM700

4.1 - 10 MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (400 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 AGC_0 attenuator and pre-selector fil-
ter.
AGC_0 Attenuator
The switchable AGC_0 attenuator (D5025) provides a defined attenuation above a defined field
strength to protect the digital IF section from overdrive. If the processor detects a field strength
above a defined limit, it selects the high attenuation state.
Pre Selector Filter
From the AGC_0 Attenuator the signal is passed to the first pre-selector filter (D5011, D5012) and
from there routed to Low Noise Amplifier (LNA, Q5031). The first pre-selector filter is a varactor
tuned band pass filter, which is set by the microprocessor via the PCIC and signal FECNTL_1, to
pass the receiver frequency and reject any spurious signals. Limiter diode D5013 protects Q5031
from destructive RF input levels
LNA and AGC_1 Attenuator
The Low noise amplifier (LNA) Q5031 amplifies the incoming received signal. It maintains a large
gain and small Noise Figure. In receive mode, a low K9V1 signal enables current source Q5001,
which enables the LNA. In transmit mode, the current source and LNA are disabled to provide fur-
ther isolation of the transmitted signal from the antenna switch towards the mixer stage. From the
LNA the signal passes to the AGC_1 attenuator pin diode D5031. From D5031, the signal passes to
the second pre-selector filter (D5041, D5042).
The Automatic Gain Control (AGC) attenuator provides a low attenuation
(0.2 dB) and a high attenuation state (18 dB). This attenuation state is dependent on the state of the
AGC_1_FE signal from the digital signal processor. If the processor detects a field strength above a
defined limit, it selects the high attenuation state. In the low attenuation state, pin diode D5031 is
ON, and pin diode D5018 is OFF. In the high attenuation state, pin diode D5031 is OFF, and pin
diode D5018 is ON
Second Pre-selector
The second pre-selector filter (D5041, D5042), also a varactor tuned band pass filter, provides fur-
ther attenuation of incoming spurious response signals. The microprocessor controls the filter via
the PCIC and signal FECNTL_1 for low attenuation at the receiver frequency and high attenuation at