ATR4255P
3. Functional Description
The ATR4255P implements an AM up/down-conversion reception path from the RF input signal
to the AM-demodulated audio frequency output signal, and for FM reception, a double-conver-
sion reception path from the RF input signal to the FM-demodulated multiplex signal (MPX). A
VCO and an LO prescaler for AM are integrated to generate the LO frequency for the 1st mixer.
Automatic gain control (AGC) circuits are implemented to control the preamplifier and IF stages
in the AM and FM reception path.
For improved FM performance, an integrated IF filter with adjustable bandwidth, a soft mute fea-
ture, and an automatic multipath noise cancellation (MNC) circuit are fully integrated. A powerful
set of sensors is provided for receiving condition analysis and stop signal generation.
Several register bits (bit 0 to bit 93) are used to control circuit operation and to adapt certain cir-
cuit parameters to the specific application. The control bits are organized in two 8-bit and three
24-bit registers that can be programmed by the 3-wire bus protocol. See Section “3-wire Bus
Description” on page 19 for the bus protocol and the bit-to-register mapping. The meaning of the
control bits is described in the following sections.
3.1
Reception Mode
The IC can be operated in four different modes; modes AM, FM, Weather band (WB), and
Standby are selected by means of bits 92 and 93 as shown in Table 3-1.
In AM mode, the AM mixer, the AM RF-AGC and the 1st IF AM amplifier at pin 33 are activated.
The input of the 2nd IF amplifier is connected to pin 28 and the output of the 2nd IF amplifier is fed
to the AM demodulator. The output of the AM demodulator is available at MPX output pin 11.
In FM mode, the FM mixer, the FM RF-AGC and the 1st IF FM amplifier at pin 38 are activated.
The bandwidth of the output tank at pins 23 and 24 is increased and the input of the 2nd IF
amplifier can be switched between pins 23, 24 and 28. The output of the 2nd IF amplifier is fed to
the integrated bandfilter and FM demodulator. The output of the FM demodulator is available at
MPX output, pin 11.
WB mode is similar to FM mode, but the input of the 2nd IF amplifier is fixed to pin 28, the range
of the bandwidth control of the integrated band filter is shifted to lower bandwidth, and the gain
of the FM demodulator is increased.
In Standby mode, the mixers, IF amplifiers and AGC circuits are deactivated to reduce current
consumption.
Table 3-1.
Operating Mode
AM/FM/Weather Channel
Bit 93
Bit 92
Standby
0
0
1
1
0
1
0
1
FM
AM
Weather band (WB)
5
4883B–AUDR–01/06
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