SI4721 datasheet(21/68 Pages) SILABS

21 / 68 page

AN388

Rev. 0.2

2.3.9. Adjacent / Alternate Channel Selectivity

Selectivity is a measure of the performance of a radio receiver to respond only to the tuned transmission (such as

a radio station) and reject other signals nearby, such as another broadcast on an adjacent channel. Selectivity is

usually measured as a ratio in decibels, comparing the signal strength received against that of a similar signal on

another frequency. Selectivity measures the ratio of the unwanted to wanted RF input levels at which the audio-

frequency, signal-to-interference ratio (S/I ratio) is 30 dB. The audio output produced by the wanted RF signal is

the reference level (VAUDIO0). First the audio reference level is measured at high RF input level. Second, the

wanted RF level is un-modulated and the unwanted RF level is adjusted until the S/I ratio is 30 dB.

1. Connect test equipment as shown in Figures 1, 2, or 3.

2. Configure the tuner in standard configuration.

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 = 98.1 MHz.

b. Set RF level VRF0 =40 dBµV.

c. Select FM Modulation.

Set FM Deviation = 22.5 kHz.

ii. Set source = LFGEN.

iii. Set LFGEN frequency = 1 kHz.

d. Enable modulation.

e. Enable carrier.

5. Record the audio level, VAUDIO0.

6. Disable generator #1 modulation.

7. Configure generator #2:

a. Set carrier frequency = 97.9 MHz (adjacent channel) or

b. Set carrier frequency = 97.7 MHz (alternate channel).

c. Set RF level = 60 dBµV.

d. Select FM Modulation.

Set FM Deviation = 22.5 kHz.

ii. Set source = LFGEN.

iii. Set LFGEN frequency = 1 kHz.

e. Enable modulation.

f. Enable carrier.

8. Adjust generator #2 RF level*, VRF1, until the audio level, VAUDIO1 = VAUDIO0 –30dB.

9. Selectivity (dB) = VRF1 –VRF0.

*Note: Generator #1 RF level is set to 40 dBµV to be able to get selectivity results greater than 60 dB. This is not a limitation of

the FM tuner or the evaluation board. Maximum RF level setting for the R&S SML01 signal generator is 120 dBµV. You

cannot increase the RF level beyond this value, so if VRF0 = 60 dBµV, selectivity numbers would be limited to 60 dB.

SI4721 Datasheet(PDF) 21 Page - Silicon Laboratories

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