ATR4255P-ILQY Datasheet ATR4255P-ILQY, ATR4255P-ILSY | P13-P15

4883B–AUDR–01/06

ATR4255P

Figure 3-3. FM Demodulator Automatic Fine Tuning

The center frequency of the FM demodulator can be adjusted by means of bits 56 to 59. At the

center frequency, the DC voltage at the MPX output pin 11 is equal to the MPX offset voltage

that can be measured at pin 11 while MPX mute is active (bit 7 = 1). This adjustment will affect

the center frequency of all integrated filters as mentioned before.

3.17 Soft Mute

The soft mute functionality is implemented to reduce the output level of the FM demodulator at

low input signal levels, and to limit the noise at MPX output at low input signal levels. If the input

level falls below an adjustable threshold, continuously the output of the FM demodulator is con-

tinuously muted with decreasing input level until a maximum mute value is reached. The

threshold for the start of soft mute and the maximum mute can be adjusted. The signal level for

3 dB mute can be set by means of bits 68 to 70 and the maximum value for soft mute can be

selected by bit 67. The steepness and the time constant of the soft mute can be adjusted by the

resistor and capacitor between pins 34 and 29.

The field strength signal available at pin 9 is used for soft mute. Therefore, the soft mute thresh-

old referred to the input of the FM mixer depends on the gain from FM mixer input to the field

strength sensor.

Table 3-16. Demodulator Gain in Weather Band Mode

Demodulator Gain in Weather Band Mode Relative to FM Mode Bit 72 Bit 71

12 dB 0 0

15 dB 0 1

18 dB 1 0

21 dB 1 1

Table 3-17. Demodulator Fine Adjustment

Demodulator Fine Adjustment Bit 53

Fine tuning ON 0

Fine tuning OFF 1

FM demod.

(10 nF)

AGC

B92

Center freq.

Bit 53

Automatic

frequency

adjustment

MPX

IFAGCH

4883B–AUDR–01/06

ATR4255P

Figure 3-4. Soft Mute

3.18 MPX Output

The output of the AM demodulator (AM mode) or the output of the FM demodulator (FM/WB

mode) are available at the MPX output (pin 11).

The MPX output signal can be muted by setting bit 7 to “1” (Table 3-20).

The bandwidth of the low-pass filter at the MPX output can be set to 90 kHz or 180 kHz using

bit 79 (

Table 3-21 on page 15).

Table 3-18. Soft Mute Threshold

Relative Soft Mute Threshold Bit 70 Bit 69 Bit 68

Soft mute OFF 0 0 0

–18 dB 0 0 1

–15 dB 0 1 0

–12 dB 0 1 1

–9 dB 1 0 0

–6 dB 1 0 1

–3 dB 1 1 0

0 dB 1 1 1

Table 3-19. Maximum Soft Mute

Maximum Value of Soft Mute Bit 67

28 dB 0

24 dB 1

Pin 34

Pin 29

Bit 67FS (Pin 9)

Bits 68 to 70

Gain FM demodulator

Table 3-20. MPX Output Mute

MPX Output Bit 7

MPX out, pin 11 normal operation 0

Mute ON 1

4883B–AUDR–01/06

ATR4255P

3.19 Receiving Condition Analyzer

ATR4255P implements several sensors that provide information about the receiving condition of

the selected station.

3.20 Field Strength Sensor

The field strength sensor provides a DC voltage at pin 9 which represents the logarithmic 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

mixer (pin 26) or from the IF-AGC depending on the setting of bit 80. The bandwidth of the field

strength detection in the AGC is smaller than when using the level detector because of addi-

tional selectivity between the 2

mixer and the 2

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 mode and the AGC field strength is used in AM mode.

3.21 Search Stop Detector

A search stop detector is available in AM and FM/WB mode. A STOP condition is signaled if the

frequency of the ZF signal is within a window around the center frequency of 450 kHz. The width

of this search stop window can be set in the range of 0.5 kHz to 80 kHz using bits 85 to 87. The

frequency of the ZF signal is measured by counting the number of periods of the ZF signal dur-

ing a measurement time which is determined by the value of bits 73 to 78. The inverted STOP

signal is available at pin 21 as shown in

Table 3-25 on page 16. The frequency of 10.25 MHz at

pin 22 is used as time reference.

Table 3-21. MPX Output Bandwidth

Bandwidth MPX Low-pass Filter Bit 79

90 kHz 0

180 kHz 1

Table 3-22. Field Strength Selection

Field Strength Narrow Band/Wide Band Bit 80

Field strength at pin 26 (wide band) 0

Field strength from IF-AGC (narrow band) 1

Table 3-23. Search Stop Detector Measurement Time

Time Window for Stop Signal Bit 78 Bit 77 Bit 76 Bit 75 Bit 74 Bit 73

1 × 3.1969 ms 000001

... ... ... ... ... ... ...

63 × 3.1969 ms 1 1 1 1 1 1

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P33 P34-P34

ATR4255P-ILQY

Mfr. #:

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Manufacturer:

Microchip Technology / Atmel

Description:

RF Receiver COM.RADIO - AM/FM RECEIVER

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