U4255BM-M Datasheet, PDF (4/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

Test Mode
VCO
FM RF-AGC
In FM mode the FM mixer, the FM RF-AGC and the 1^st IF FM amplifier at Pin 38 are
activated. The bandwidth of the output tank at Pin 23, Pin 24 is increased and the input
of the 2^nd IF amplifier can be switched between Pin 23, Pin 24 and Pin 28. The output of
the 2^nd 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 2^nd 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 1. Operating Mode
AM/FM/Weather Channel
Standby
FM
AM
Weather band
Bit 93
0
0
1
1
Bit 92
0
1
0
1
A special test mode is implemented for final production test only. This mode is activated
by setting Bit 9 = 1. This mode is not intended to be used in customer application.
For normal operation, Bit 9 has to be set to 0. Bit 18 to Bit 30 are deactivated in normal
operation mode.
Table 2. Test Mode
Mode
Normal operation
Testmode
Bit 9
0
1
An oscillator circuit is implemented to build a VCO as proposed in the application sche-
matic. The VCO frequency is used to generate the LO frequency of the 1^st mixer stages.
The control voltage of the VCO is usually generated by the PLL circuit U4256BM. The
VCO signal is provided at the buffered output Pin 16 to be fed to the PLL circuit.
The FM RF-AGC circuit includes a wideband level detector at the input Pin 1 of the FM
mixer and an in-band level detector at the output of the FM IF amplifier (Pin 30). The
outputs of these level detectors are used to control the current into the pin diode (see
Figure 3) in order to limit the signal level at the FM mixer input and the following stages.
The maximum pin diode current is determined by R115 and the time constant of the
AGC control loop can be adjusted by changing the value of C111.
The AGC threshold level at the input of the FM mixer can be adjusted by Bit 64 and
Bit 65 according to Table 3. The in-band AGC threshold referred to the FM mixer input
(Pin 1, Pin 2) depends on the gain of the FM IF amplifier and can be adjusted by Bit 89
to Bit 91.
4 U4255BM-M
4519C–AUDR–07/02