ATR4258 Datasheet, PDF (16/38 Pages) ATMEL Corporation – AM/FM RECEIVER IC

Field Strength Sensor
The field strength sensor provides a DC voltage at pin 9 which represents the logarith-
mic field strength of the signal in the reception band.
The field strength information can be retrieved either from a level detector at the input of
the 2^nd mixer (pin 26) or from the IF-AGC depending on the setting of bit 80. The band-
width of the field strength detection in the AGC is smaller than by using the level
detector because of additional selectivity between the 2^nd mixer and the 2^nd IF amplifier
particularly in AM and WB, but the field strength detection in the AGC is limited to the IF
AGC range. Usually the field strength from the level detector is used in FM/WB mode
and the AGC field strength is used in AM mode.
The field strength output at pin 9 can be adjusted by the bits 18 to 21 in 0.5 dB steps.
This offset also has an influence on the soft mute levels.
Table 25. Field Strength Offset
Offset Field Strength
0.0 dB
0.5 dB
1.0 dB
...
7.5 dB
Bit 21
0
0
0
...
1
Bit 20
0
0
0
...
1
Bit 19
0
0
1
...
1
Bit 18
0
1
0
...
1
Field Strength Selection Bit 80 and bit 13 allows the switches between narrow-band field strength and wide-band
field strength information.
Search Stop Detector
Table 26. Field Strength (Combined)
Field Strength (Pin 9) Narrow-band/Wide-band
FM field strength (wide band)
Multipath field strength and FM field strength (wide band)
AM AGC field strength (narrow band)
Multipath field strength and AM AGC field strength (narrow band)
Bit 80
0
0
1
1
Bit 13
0
1
0
1
A search stop detector is available in AM and FM/WB mode. A STOP condition is
signaled (with a low level at pin 21) if the frequency of the IF signal is within a window
around the center frequency of 450 kHz. The width of this search stop window can be
set by bit 85 to bit 87 in the range of 0.5 kHz to 80 kHz. The frequency of the IF signal is
measured by counting the number of periods of the IF signal during a measurement
time which is determined by bit 73 to bit 78. The inverted STOP signal is available at
pin 21 according to Table 29 on page 17. The frequency of 10.25 MHz at pin 22 is used
as a time reference.
Table 27. Search Stop Detector Measurement Time
Time Window for Stop Signal
1 × 3.1969 ms
.....
63 × 3.1969 ms
Bit 78 Bit 77 Bit 76
0
0
0
1
1
1
Bit 75
0
1
Bit 74
0
1
Bit 73
1
1
16 ATR4258
4838A–AUDR–10/04