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I used an RF generator transmitting FSK packets with a datarate of 50 kbps. The deviation was 12.5 kHz. With this combination of datarate and deviation the modulation index is 0.5. I found that my device undertest had an Rx sensitivity of ~102 dB. The IF BW is approximately 100 kHz.

I left DUT settings untouched. I changed the deviation of the signal sent by the generator from 12.5 kHz to 25 kHz. Datarate remained 50 kbps. This equals to modulation index = 1.

I found out that the sensitivity was a few dB's better. Someone told me that it is because the symbols are further apart from each other in the freq plane, and thus it is easier for the radio to demodulate it. My question is, why the wider deviation is easier for the radio?

The radio uses low IF receiver. RF is fed to an I/Q- down conversion mixer. The I/Q signals are sampled by IF ADC.

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  • \$\begingroup\$ What measurement produced the -102dBm input power level? was the BER 1 in 1 Million? 1 in 1 Billion? or some other measurement? Classic FM begins to have horrid (sounding) click errors (phase flipping) near +18dB SNR. If we use that 18, and add to 50dB for the 100KHz BW, the sum is +68dB. Add that to -174, and you get -106 dBm (meaning losses and front end amplifier noise summed are another 4dB impairment. \$\endgroup\$ – analogsystemsrf Sep 13 '17 at 12:39
  • \$\begingroup\$ 10% PER was used for the sensitivity. \$\endgroup\$ – user94729 Sep 14 '17 at 5:37
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The RF sensitivity is basically the point where the receiver starts to produce a bit error rate that becomes significant or problematic. An FSK receiver (or an FM receiver) converts a frequency deviation (peak to peak) to an analogue voltage (peak to peak). If the basic noise in the receiver is such that it is approaching a similar order to the demodulated analogue peak to peak voltage then you have reached the limit of acceptability for the receiver.

However, if you increase the deviation, the analogue signal increases and the noise is relatively smaller and less problematic. It's the same argument about using 5 volt logic levels - there is more noise immunity when using 5 volts than 3.3 volts.

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