My AFE822x coherent RFI filtering goal has been accomplished. Every SDR should be capable of spatial interference filtering techniques.
I will wrap up this series covering my early experience with the AFE822x with a preliminary report on using a dual channel SDR to eliminate or reduce RFI. I purchased this receiver mainly to experiment with spatial interference filtering techniques. In short, it works great! Phasing the IQ data to reverse local RFI performs on par with the outboard ANC4 or MFJ1026. Perhaps even better, as there is no insertion loss.
Later this summer I will produce some video demonstrations of using SIFT with the Afedri receiver. In the meantime, here is a short narrative description.
My worst source of local RFI comes from poorly wired halogen lighting in a neighbor’s kitchen across the street. After a few weeks of peace and quiet, they turned these lights on again. Since this interference source is band specific, I saw the noise floor on the 25 meter SWBC band jump to -95 dBm or around S6. This interfering signal spreads across several hundred kilohertz and is rich with 120 Hz harmonics. A real buzz saw.
By turning diversity reception on, I obtained a 24 dB noise floor reduction. The 120 Hz harmonic content disappeared. You can see the effect in the waterfall display above marked “On”. All that was required was to match the level of the noise in both receivers, and then phase or subtract the signal from the noise antenna from the signal from the main antenna. It’s easy to do this with the AFE822x and PowerSDR software.
What is the difference between this digital approach versus the analog reduction in an ANC4 noise canceller? Two things. First, with the digital phasing approach, you have a bit more control over level and phase adjustments. They are sharper and more easily repeatable. Second, you no longer experience the 6 dB insertion loss when you switch in the ANC4.
AFE822x Coherent RFI Filtering – Brushing Up on Trigonometry
I have two different, switchable noise probe antennas. Each is around 70′ from my main antenna and 120′ from each other. The halogen lighting RFI is about 300 feet from my main antenna and is strong enough to be picked up by both noise probes.
As I did the AFE822x coherent RFI filtering, I could accomplish reduction with either noise probe. But I found that the required amplitude and phase settings were quite different in each case.
This makes sense because the antenna spacing and feed line lengths all create time of arrival differences thar require compensation in the dual channel receiver. One of the things I need to do is calculate these timing deltas and then see if the selection of phasing angles is consistent with the math. But, that is for another day.
To sum up my early Afedri dual channel experience: great SDR receiver and its coherent/diversity features work as expected. Good stuff.