I recently ordered two RFM23BP radio modules, which transmit at 434MHz. They have a built in amplifier that can produce a 0dBW (1000mW) signal, so I'm hoping that they can reach a few km or so.

Because I live in New Zealand, I have to conform to the radio standards, and thus I decided that it would be better to operate on the 458MHz band where I'm allowed a maximum of -3dBW (500mW).

The RFM23 module supports three modulation modes: OOK (On-Off Keying), FSK (Frequency Shift Keying), and GFSK (Gaussian Frequency Shift Keying). I wanted to know what these modulation modes looked like, so I pulled out my handy RTL-SDR compatible dongle:

458MHz Spectrum458MHz Spectrum

This is the total viewable bandwidth around the 458 MHz band. The red line is centered on that band, and the maximum allowable bandwidth around the band is 70kHz (Contrasted with a total viewable bandwidth of approx 2MHz)

You can also see a strong signal on the left, which appears to be some sort of coded signal.  The faint bands on each side are just artefacts.


OOK ModulationOOK Modulation

The first mode I tried is OOK, where the signal is simply turned on and off to code the data. You can see faint bands on either side of the signal, this is spectral splatter from the sharp signal.

Unlike the next two modes, OOK operates at only one frequency, at 458.58MHz.


FSK ModulationFSK Modulation

The next mode I tried was FSK, where the coded data switches between two frequencies on either side of the center frequency. As with the OOK modulation, there is unwanted spectral splatter on each frequency band, though not as visible here.

Incidentally, the bandwidth of 50kHz is just within the allowable frequency band for 458MHz.


GFSK ModulationGFSK Modulation

GFSK is the module's most efficient mode, as it reduces spectral splatter.  As you can see, it spreads the frequencies out using some sort of gaussian function, which reduces the harshness of the signal.


The above screenshots were captured without an antenna on the RFM23 module, with the power level set to only 1.3mW! So obviously it will have very good range once I add a proper antenna.

My next task will be to test the maximum range of this module, and hopefully it will extend far enough for my needs!

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