SI4721 datasheet(45/68 Pages) SILABS

45 / 68 page

AN388

Rev. 0.2

4.1.5. Audio Band Limits/Audio Frequency Response

The audio frequency response of a tuner is influenced by the quality of the IF section, detector, and de-emphasis

circuit. The tuner has a de-emphasis filter with a 6 dB per octave curve over the audio frequency range of 300 Hz

to 3 kHz. The SML generator only has a pre-emphasis filter for FM band with 75 µs or 50 µs. Since those cannot

be used for the weather band pre-emphasis, a correction will need to be applied to the frequency response results

to account for the lack of pre-emphasis.

1. Connect test equipment as shown in Figure 2.

2. Configure the tuner:

a. Set frequency = 162.4 MHz.

b. Set VOLUME = 63.

3. Configure the audio analyzer:

a. Select Analyze.

b. Set Function = RMS Select.

Set bandwidth = BP 3%.

ii. Set units = dBV.

iii. Set frequency mode = FIX: 1 kHz.

4. Configure generator #1:

a. Set carrier frequency = 162.4 MHz.

b. Set RF level = 60 dBµV.

c. Select FM Modulation.

Set FM Deviation = 3 kHz.

ii. Set source = LFGEN.

iii. Set LFGEN frequency = 1 kHz.

d. Enable modulation.

e. Enable carrier.

5. Audio 1 kHz level (dBV) = V1kHz.

6. Configure audio analyzer frequency mode = 300 Hz.

7. Configure generator #1 LFGEN frequency = 300 Hz.

8. Audio 300 Hz level (dBV) = V300Hz.

9. Configure audio analyzer frequency mode = 3 kHz.

10.Configure generator #1 LFGEN frequency = 3 kHz.

11. Audio 3 kHz level (dBV )= V3kHz.

12.Audio frequency response (dB) equals the greater magnitude of (V30Hz – V1kHz – 7.82) and (V15kHz – V1kHz

+ 9.21).

*Note: 7.82 and 9.21 are the correction factors applied to account for the lack of a pre-emphasis filter.

SI4721 Datasheet(PDF) 45 Page - Silicon Laboratories