Re: [Discuss-gnuradio] Beamforming with GNU Radio and USRPs

[jmfriedt]

I am not aware of the details of the GPSDO implementation but your sources must
be phase locked, not only synchronized through the GPS signal. The GPS control 
loop
on the LO only has a finite bandwidth above which each oscillator is free to 
drift
at will. Under such conditions, the interference pattern will be fluctuating 
because
the PLL bandwidth is necessarily much lower than the carrier frequency.

JM

> I am currently trying to get 2 N210 to transmit coherently to a third N210.
> 
> All devices have GPSDO kits and the "gps_locked" status is on "locked",
> there is no MIMO cable in the setup.
> 
> I am currently doing some preliminary tests where I feed both sinks with
> the same signal i.e a complex sine wave using the signal source from GNU
> Radio and I observe the signal received by the third USRP. I made sure that
> both transmitted signals are received with the same amplitude when taken
> independently.
> 
> My understanding is that if both transmitters are locked to the GPS there
> should be no frequency  offset between them and there should only be a
> constant phase offset between them which would give me a somewhat constant
> amplitude signal at the receiver.
> 
> On the receiver, I observe the signal with both a time sink and a frequency
> sink.
> My problem is that the received signal seems to be varying a lot in
> amplitude, it looks like a slow AM modulation which is something I would
> get if both transmitters weren't perfectly aligned in frequency.
> 
> Changing the frequency of one of the transmitters makes the problem worse.
> 
> Does anyone have prior experience with this kind of setup and experienced
> the same issues?
> 
> Thanks in advance,
> Jawad


-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 
Besancon, France