Preselector Filter; Switchable 15 Db Attenuator; Low-Noise Amplifier (Lna, U5302); Image Filter - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Receiver Front-End

3.10.3.2 Preselector Filter

The front-end operates in the 450 to 520 MHz band. The front-end's primary function is to optimize
half IF rejection, image rejection, and selectivity while providing the first conversion. The front-end
uses a varactor-tuned filter that is tuned by the controller. The tuning signal is a DC control voltage
between 0 and 9V that come from the PA power control section. Low voltages are for lower
frequencies and higher voltages correspond to the higher frequencies. This filter is aligned in the
factory and can also be aligned using the Tuner software.

3.10.3.3 Switchable 15 dB Attenuator

This circuit block can provide 0 dB or 15 dB of attenuation in the signal path. Normally, it is set for
0 dB and does not affect the received signal. When strong signals are detected, the radio controller
can choose to activate this attenuator to provide protection to the back end circuits, and to enhance
high level intermodulation performance. Proprietary algorithms are used to control the switching.

3.10.3.4 Low-Noise Amplifier (LNA, U5302)

A diode (D5281) located after the varactor preselector and before the LNA protects the receiver from
strong RF signals by limiting the signal amplitude going into the amplifier. The LNA is a low-noise
monolithic IC providing ~ 15 dB of gain to the receiver. It is biased with 5 V.
The UHF receiver also has a second LNA based on Q5252 that can be activated or bypassed by the
radio software. This amplifier is protected by D5280 and provides 11 dB of gain.

3.10.3.5 Image Filter

Following the LNA (U5302), the signal goes through a bandpass filter before it is sent to the mixer.
The passband is from 450 to 520 MHz with an insertion loss of about 2 dB, while the image rejection
is 55 dB. There is a trap on the input side of this filter to attenuate the 109.65 MHz IF.

3.10.3.6 Mixer

A passive double-balanced diode ring mixer is used to down-convert the received signal to an
Intermediate Frequency (IF) of 109.65 MHz. The mixer is designed to provide low conversion loss (<
7.0 dBm) and high intermodulation performance and requires a strong injection signal. The mixer is
driven by the receiver injection buffer, a two-stage LDMOS IC design, that amplifies the +3 dBm
high-side injection signal from the Frequency Generation Unit (FGU) to +24 dBm.
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June 15, 2005