Diversity Operations

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EA1DDO
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Re: Diversity Operations

Postby EA1DDO » Fri Jan 26, 2018 5:54 pm

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w-u-2-o
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Re: Diversity Operations

Postby w-u-2-o » Fri Jan 26, 2018 9:20 pm

Maximo,

To be coherent, or phase and sample synchronized, you have to be much, much better than that.

Let's take a 20M frequency for example, say 14.2MHz. At that frequency, a 10 degree phase shift is equivalent to approx. 2nS.

That is why both ADCs need to be clocked from the same sample clock, and clocked at the same time, i.e. there should be no differential delay between samples.

Now our radios do allow for an external frequency reference, so you could feed each one from the same reference using a set of matched length cables. This would at least get everything on the same timebase. BUT...the radios have no mechanism for synchronizing their sample times.

That said, if you had two radios, both locked the same 10MHz reference, and two data streams, theoretically they would be phase locked at some random phase. You could then simply guess at the phase difference you needed to steer a beam by trail and error until you got what you wanted. With only two radios and two antennas this would not be too difficult. Indeed, that's what we currently do in our current dual ADC radios. Even though they are phase and sample locked, we just guess. However in your picture you have three antennas. That would be darn near impossible to guess at.

73,

Scott
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Re: Diversity Operations

Postby EA1DDO » Fri Jan 26, 2018 10:15 pm

That is why both ADCs need to be clocked from the same sample clock, and clocked at the same time, i.e. there should be no differential delay between samples.


So, there isn´t any way to clock two separated SDR ?

Perhaps.... a NCO ?

https://en.wikipedia.org/wiki/Numerically_controlled_oscillator
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Re: Diversity Operations

Postby w-u-2-o » Sat Jan 27, 2018 1:49 am

Certainly there are ways to do what you want, just not with the hardware and firmware that exists in any openHPSDR architecture radio, including Apache Labs radios.

There would need to be a sync signal in addition to the 10MHz, and it would need to be accurate to around +/-1nS, and/or there would need to be a way to calibrate the various receivers using a calibration signal to measure what the differentials are.

73,

Scott
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Re: Diversity Operations

Postby EA1DDO » Sun Jan 28, 2018 3:36 pm

Then, an easier way; 2x SDR boards (Hermes, Orion, etc.), sharing clock, in same enclosure, let´s say back-to-back.
This way you get two coherent boards, with two coherent receivers and two coherent transmitters, starting point for beamforming.

GPS is not mandatory.

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Re: Diversity Operations

Postby w-u-2-o » Sun Jan 28, 2018 5:08 pm

Yes, you'd have to modify the boards to allow a single board to supply the same ADC clock to all ADCs. I have not studied the problem. Surely not a difficult technical challenge, however you may have to devise a buffer/driver circuit to ensure signal integrity.

Then there is the problem of writing firmware and software to exploit this hardware. There would need to be sufficient time tags embedded in the IF data streams to be able to match up the streams on a sample basis coming from multiple radios. Again, not a difficult technical challenge, there are thousands of DF systems around the world that do exactly that .

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