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After consulting with the radio manufacturer about a setup for my FSK radio module (HM-TRP), they have suggested I program it with the following values when setting up the module for 115kbps:

Receive bandwidth: about 120Khz
Frequency Deviation: about 57.6Khz
Modulation index is 1

Looking at this: https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/02/04/calculation_of_them-Vx5f suggests that the manufacturer took this equation into account:

(2 x Deviation) / data rate

I read post #4 from here https://www.physicsforums.com/threads/modulation-index.684409/ that I should aim for a higher modulation index for improved SNR (signal to noise ratio?).

What should my maximum modulation index value be if I want the radios to work at a range of about 200 meters?

And do I need to adjust the modulation index if I want to test the radios when they are only one meter apart?

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  • \$\begingroup\$ There is a CNR to SNR log improvement based on DR which is critical for BER improvement at fringe distances \$\endgroup\$
    – D.A.S.
    Commented Sep 10, 2019 at 1:33

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If the regulations allow you, as wide as your transceiver supports, as long as it doesn't harm the sensitivity. For example, under FCC regulations in the 915MHz band, a transceiver can use ~500KHz for FSK deviation to comply with wide band transceiver regulations. But the same deviation under ETSI in the 433MHz ISM band is not allowed, because its out of band noise will be too high.

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  • \$\begingroup\$ so basically I should make my modulation index value as high as possible or could I get away with maxing out both the deviation frequency and bandwidth values? The latter two are adjustable through software. The carrier is at 915Mhz and it will run in Canada. \$\endgroup\$
    – Mike -- No longer here
    Commented Sep 10, 2019 at 2:42
  • \$\begingroup\$ There is a sensitivity trade off here. Higher RX bandwidth (required to accommodate the higher deviation) will raise the noise floor by proportional amount, that is, 2x wider filter, and you will loose ~3dB sensitivity. Usually, the RX filter setting has only few discrete steps, so you will have to optimize your settings. \$\endgroup\$
    – Lior Bilia
    Commented Sep 10, 2019 at 10:41
  • \$\begingroup\$ the datasheet to my device allows me to choose from a large range. for bandwidth I think I can choose any number from 10 to 620Khz and for deviation I think any number from 20 to 160Khz \$\endgroup\$
    – Mike -- No longer here
    Commented Sep 10, 2019 at 22:35

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