Diversity- Signal or Noise Canceling

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Diversity- Signal or Noise Canceling

Post by ve3hls » Thu Jul 11, 2019 9:59 am

Very happy to see the diversity capability is now available!

Unfortunately, it seems to be set up for combining signals to improve reception. Actually, I'm not sure about that because the instructions in the manual aren't clear to me and the reference Wikipedia article has left my head spinning. (Please don't tell me I have to be a mathematician or a programmer to use this).

I have been using antenna phasing equipment successfully for years and had heard that phasing two coherent receivers was a better way to go. I've been anxiously awaiting the day when I could try this.

With antenna phasing you adjust the signal amplifiers (one for each antenna) in the phasing unit until the signal strength from both antennas is equal, then adjust the phasing of the signals until they cancel each other out. I'm looking for a way to do this with the RSP Duo and I think I'm getting there. However, it requires a lot of switching back and forth between receivers and every time you do this the Spectrum and Waterfall displays zoom back out as far as they can, which really slows the process down. Surely there must be a way to switch between receivers without having to zoom the display back in every time...or is there a way to do phase cancellation that doesn't require switching between receiver displays. (Note: I only have one receiver on screen at a time.

I'm trying to understand the descriptions in the User manual, Page 78:
1. "This window shows the current phase and amplitude being applied and the automatic values that are being continuously calculated and applied" These automatic values are what? The phase angle and amplitude that give you a "best signal"?

2. "select a phase (indicated by the angle in the circle) and an amplitude (indicated by the length of the line to be applied to the incoming IQ streams" I think I understand the idea here. In my way of thinking I would adjust the phase angle to show 0 (zero) degrees (or 180 degrees, which is where the signals ought to be perfectly out of phase for proper cancellation. However, what am I adjusting with the amplitude control; the amplitude of one of the two signals. If not then I don't know how one would ever be able to cancel (phase out) a strong signal.

Many thanks for any help!

Ken Alexander
So Phisai, Thailand

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Re: Diversity- Signal or Noise Canceling

Post by DanubeBCL » Thu Jul 11, 2019 12:07 pm

The idea behind noise cancelling by diversity is that the noise originating from your neighbourhood (switched power supplies, lawn mowing robots, flatscreens, powerline modems and whatever delights us nowadays) is always coming into your two antennas with constant phase angles and amplitudes. Thus you can adjust phase and amplitude of the signal in one antenna in a way that this noise is nulled (as far as possible). The "payload" signal (the radio station) is normally not nulled because its phase and amplitude always varies during propagation (especially on shortwaves).

Noise cancelling by diversity can substantially improve SNR without affecting the "main" signal in many cases. Provided you only have one noise source in your neighbourhood. When you are surrounded by multiple noise sources (e. g. powerline modems in every house in your street) it fails.

I am using noise cancelling by diversity for years with my Anan Angelia board and PowerSDR mRX. Without this option I would have given up radio listening already years ago. It brings me back at least some of the joy whe had listening to radio some 20 years ago when the RFI level was almost null. But times have changed.

I welcome that with the RSPDuo and SDRuno 1.32 the noise cancelling feature is now available at an affordable price. (Although I did not yet have time to test it.) For the naked Anan Angelia board alone I had to pay appr. 1500 $.

73, Heinrich

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Re: Diversity- Signal or Noise Canceling

Post by jon » Wed Jul 17, 2019 7:30 pm

We are hoping to provide more practical help by way of documentation/video guides on diversity reception in the near future.

Meanwhile some more thoughts are here: https://www.sdrplay.com/more-about-dive ... he-rspduo/

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Joined: Thu Mar 02, 2017 9:56 am
Location: Tasmania

Re: Diversity- Signal or Noise Canceling

Post by vk7jj » Fri Jul 19, 2019 12:33 am

You're all making me very envious. Some time ago living by the sea for quite a few years, except during periods of rain there was continuous HF and even VHF noise generated by electrical discharges across all the power line insulators.

I built several unsuccessful noise reducing devices that combined the signals from my main antenna and a noise antenna and varied phase and amplitude. Finally bought a "real one", an MFJ-1025 (which easily convert into a 1026).

After a lot of experimenting it worked very well but the key to success was building a special noise antenna, ie. one that picked up the highest ratio of the actual noise to wanted signal possible which ended up being an old 27MHz Ringo repurposed minus the ring as a simple untuned vertical, placed behind the house and close to the power pole that carried the feeder to our house.

It required really patient and fine adjustment to achieve a noise null but the result was excellent, a real revelation, on the local 80m net. Unfortuntely the whole process was too laborious by far to be able to browse around the bands.

Of course the MFJ could not assist at all with distant noise, seems a few negative reviewers failed to understand why.

Since moving to my current rural location I've tried using the MFJ as a combiner to enhance the signals from two very similar HF loops but the signal phase changes happen too quickly and even at it's best it's not a lot of benefit, probably again it needs a differently polarised antenna but that would introduce more noise as well.

Obviously I'm really fascinated by what's happening with the Duo and watching with very keen interest.


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