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Rfm69- what is Frequency_deviation?
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Rfm69- what is Frequency_deviation?

by Rcayot on Fri Apr 30, 2021 8:14 am

I am beginning to use an rfm69 breakout board. One thing I am not clear on is the meaning of the "frequency_deviation" parameter.

Is it a measured value? What is the signifigance?

Using the simple test, my board reports that frequency_deviation is 22,000.

Is that good, bad, or indifferent?

Inquiring minds want to know.

Roger

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Re: Rfm69- what is Frequency_deviation?

by adafruit_support_mike on Sat May 01, 2021 10:31 pm

Frequency deviation is how much the frequency of an FM radio signal changes.

In the simplest form of FM, called Frequency Shift Keying (FSK), the radio operates at one frequency when it wants to send a 0, and another frequency when it wants to send a 1. In this case, the difference between those two frequencies is 22kHz.

Large frequency deviations are easier to detect. but reduces the number of different radio bands you can have within a given spectrum. If you have 1MHz of spectrum to play with and the frequency deviation is 100kHz, you can only have 5 different channels (100kHz for each channel, and 100kHz between channels). If the frequency deviation is only 10kHz, you can fit 50 channels in the same 1MHz range. The 100kHz channels will be much easier for the receiver to recognize, but the 10kHz channels will allow much more traffic.

Frequency drift also interacts with things like relative motion between the radios. Any relative motion between the radios creates a Doppler shift in the frequency the receiver sees, so if the nominal frequency deviation is small, the radios pretty much have to stand still. You also get things like frequency shift relative to the index of refraction of air.. light moves at different speeds through different materials, which looks like a change in frequency. Channels with larger frequency deviations are less sensitive to those errors.

For your purposes, it's just an operating parameter you won't want to change.

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