Limiter; Front Filter; Lna; Second Pre-Selector - Motorola MTH500 Service Manual

Theory of Operation
frequency by approximately 38dB. The attenuation of wanted signal is typically
0.7dB.

Limiter

The limiter (D500) function is to prevent entering of signal with power higher then
30dBm, which may damage the receiver front-end.

Front Filter

The signal arrived from the limiter is flowing via the front filter (FL500) and rout-
ed to the Low Noise Amplifier (LNA) (U500) via C576.
The front filter is also called -1st pre-selector and is used to attenuate the incoming
parasitic RF frequencies, especially the image frequency (25dB). The Insertion
Loss (IL) is typically 2dB.

LNA

The LNA module is the RF signal amplifier. The LNA gain (U500) including out-
put matching circuit, C1048, L518, C521, R965, L500, C532, R580 and C507, is
typically 23dB. Because the LNA is the first amplifier in the line, it is important
that it will maintain large gain and small Noise Figure - 1.8dB.
The LNA has an "enable" pin (pin 3) which is called VPD. When this pin receives
a supply of 1.8 V from DC switch (Q504) - the amplifier is enabled. From the
LNA the signal passes to second pre-selector, LC filter (FL501). The IIP3 of the
LNA is between -5dBm to -2.5dBm.

Second Pre-selector

The 2nd Pre-selector (FL501) attenuates the non-linear products of the LNA and
also the image product by 25dB. Its IL is typically 2.5 dB. From the output of the
LC filter the received signal is routed via a Pi-attenuator to the Mixer RF input.

Attenuator

The purpose of the resistor Pi-attenuator (R519, R517, R945), is to protect the
mixer (U504) input from saturation. The pad attenuates RF power by 2.5dB

Mixer

The Mixer (U504) function is to down convert the incoming RF signal into IF sig-
nal (109.65 MHz). The Conversion Loss (CL) is typically 6.2dB and its IIP3 is
typically 20dBm.
The RF signal enters the Mixer at pin 1, and the IF is produced at pin 6. The LO
(from the main VCO) signal passes via the matching circuit (C522, L505, R943
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