SI4721 datasheet(44/68 Pages) SILABS

44 / 68 page

AN388

Rev. 0.2

4.1.4. Adjacent/Alternate Channel Selectivity

Selectivity measurement in weather band is slightly different than the selectivity measurement in FM band. In

weather band, adjacent channel selectivity is a measure of the capability of the receiver to receive a wanted

modulated signal without exceeding a given degradation due to the presence of an unwanted signal that differs in

frequency from the wanted signal by an amount equal to the adjacent channel separation for which the equipment

is intended. Two generators are setup with the same FM deviation but with two separate modulation frequencies. A

sensitivity measurement is done with the wanted signal only. The unwanted signal is then turned on and its level is

increased until SINAD =12 dB.

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 generator #1:

a. Set carrier frequency = 162.4 MHz.

b. Select FM Modulation.

Set FM Deviation = 3 kHz.

ii. Set Source = LFGEN.

iii. Set LFGEN frequency = 1 kHz.

c. Enable modulation.

d. Enable carrier.

4. Make a sensitivity measurement.

5. Record the generator # 1 RF level at sensitivity as VRF0.

6. Increase generator #1 RF level by 3 dB.

7. Configure generator #2:

a. Set carrier frequency = 162.425 MHz (adjacent channel)

b. Select FM Modulation.

Set FM Deviation = 3 kHz.

ii. Set Source = LFGEN.

iii. Set LFGEN frequency = 400 Hz.

c. Set RF level = 10 dBµV.

d. Enable modulation.

e. Enable carrier.

8. Adjust generator #2 RF level*, VRF1, until the SINAD is 12 dB.

9. Selectivity (dB) = VRF1 – VRF0.

SI4721 Datasheet(PDF) 44 Page - Silicon Laboratories