The ADF7021 is used by a vast majority, if not all, of amateur hotspots (RaspberryPi shields) out there. Back in the days when this kind of hardware was a novelty, this chip seemed to be one of the best options to choose. It supports 4FSK and raised-cosine (RC) baseband filtering out-of-the-box, the user only has to sustain a transfer of the symbol stream. Sounds good so far, but the problem is that the RC filter in the hotspot’s RF chip, when combined with a square-root-raised-cosine (RRC) filter at the user’s radio side, gives significant intersymbol interference. This is what renders the signal from hotspots unusable at longer distances – the eye plot is not opened wide enough, even at excellent signal-to-noise ratios. The signal transmitted with ADF7021 is just bad, right at the antenna.
Let’s take a closer look at my measurements taken a while back. The first comparison shows eye diagrams, as received with an SVA1032X with a reference RRC filter. The selected frequency, power, and SNR were roughly equal in both tests. A pseudorandom symbol stream was used.
Now, let’s take a look at adjacent channel power ratio (ACPR). It tells us how much power “leaks out” into adjacent channels, potentially interfering with the transmissions happening there. I performed two measurements: one for the channel immediately adjacent to the used radio channel (+/-12.5 kHz), the other for +/-25 kHz offsets.
It’s immediately obvious that the CC1200 gives much better ACPR. Last, but not least, let’s have a look at harmonics and other distortion.
With the measurement’s settings used, the CC1200’s harmonic content couldn’t even be detected.
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