[USRP-users] XCVR2450 measurements

Jorge Miguel jmiggal at gmail.com
Wed Nov 17 11:15:32 EST 2010


Mike,

Yes, at this moment I have no problems to work in real time. I think I will
test their godness when I increase the signal processing in Simulink (I need
FFTs). Now I am only testing the XCVR2450 TX, RX and how to switch between
them. My questions about the spectrum shape of the output signals are while
working in real time. (It is a sine generator conected to the USRP TX, so no
signal processing at all). Questions bellow.

Regards,
Jorge

On 17 November 2010 17:06, Mike McLernon <Mike.McLernon at mathworks.com>wrote:

>  Hi Jorge,
>
>
>
> Have you tried all the tips in my previous e-mail to get Simulink to run
> real-time?
>
>
>
> Best,
>
> Mike
>
>
>
>
>
> *From:* usrp-users-bounces at lists.ettus.com [mailto:
> usrp-users-bounces at lists.ettus.com] *On Behalf Of *Jorge Miguel
> *Sent:* Wednesday, November 17, 2010 10:26 AM
> *To:* Steven Clark
> *Cc:* usrp-users at lists.ettus.com
> *Subject:* Re: [USRP-users] XCVR2450 measurements
>
>
>
> Steven,
>
>
>
> Thanks for your comments. Some answers bellow.
>
>   I tested both transmitter and receiver path of XCVR2450 on a USRP2 under
> Matlab r2010b. Here some comments about details I found and I don´t
> understand.
>
>
>
> TX path: (I transmit a 100KHz single tone modulated at 5.5GHz) to a
> spectrum analyzer. The expected spectrum is two components at 5.4999GHz and
> 5.5001GHz
>
>
>
> 1) Besides the expected spectrum, I can see a powerful (20dB less that the
> modulated signal) component at 5.5GHz (Leakage from the MAXIM Local
> oscillator?)
>
>
>
>  What is the scale of the samples you are sending to the USRP sink? If you
> increase the scale, does the delta in power between your expected stuff and
> the LO also increase? It may be you aren't exercising enough of the DAC
> bits.
>
>
>
>
>
>
>
> The scale is one (1). When I set a lower value (0.5/0.1/0.01) the desired
> components start to decrease while the centre component (5.5GHz) remains at
> -30dB (i.e. the DAC doesn't use all its bits). If I set a bigger value,
> neither the desired components nor undesired one change their power value.
> However my spectrum becomes awfull with a lot of poweful harmonics at
> (centre frequency+ N*2*Fsingle tone) where N=1, 2,3....
>
>
>
> Different sample frequencies don't change this behaviour.
>
>
>
> I assume 1 is the optimum value. However I get a -30dBm undesired component
> at the carrier frequency. (While the desided ones are -6dBm with 0 XCVR2450
> gain)
>
>
>
> 2) I see how after 20 or 30 seconds the signal spectrum suddenly (a fast
> change) becomes very degraded (a lot of noise in my whole band of interest
> appears and the signal is very degraded. The only good news is that the
> component at 5.5GHz becomes smaller)
>
>
>
>
>
>  Does the behavior here have any correlation with your sample rate /
> interpolation? What is your sample rate?
>
>
>
>
>
> YES. You are right. I generate signal in simulink at (1e8/512) Fs and the
> set an interpolation of 512 to reach the 100MS/s expected by the ADC. After
> about 2 minutes my signal (still a modulated single tone) becomes awfull
> (seen with a spectrum analyzer. This means that noise raises up to the level
> of the undesired carrier frequency (-30 dBm).
>
>
>
> This behaviour is not affected by the amplitude of the digital signal.
> However it is dependent on the Fs/interpolation of my USRP.
>
> At 1e8/512 Fs and 512 interpolation the signal spectrum degrades and never
> comes back (improve)
>
> At 1e8/128Fs and 128 interpolation the signal spectrum degrades and from
> time to time there is a sweep in which the goos spectrum comes back!
>
> At 1e8/92Fs and 92 interpolation the signal spectrum degrades and comes
> back every other sweep of the sprectrum analyzer.
>
>
>
> However Simulink real time operation is needed to work with the USRP and as
> long as we do more Simulink process we cannot achieve real time !! Why I
> cannot transmit a good modulated tone at 512 interpolation??
>
>
>
>
>
>
>
> 3) From the setting I specify on my USRP TX software I can see about 8KHz
> frequency offset in the spectrum analyzer received signal. I connected a
> signal generator (a good one) to the same spectrum analyzer and the offset
> was 0.2 KHz at the same frequency (5.5GHz). Furthermore there is an
> instability over time. (The signal is moving over time around 0.2KHz)
>
>
>
>
>
>  I think this is expected. Oscillator only good to ~20 ppm or so?
>
>
>
> Yes, absolutely.
>
>
>
> 4) Last comment. When setting a gain value (10dB )in the XCVR2450 (still in
> TX mode) I can see in my spectrum analyzer the same harmonics than see the
> same harmonics that I see when increasin the level of my digital signal in
> simulink. My feeling is that you make use of the whole range of the DAC you
> canot set gain in the XCVR2450. Otherwise the daughterboard is saturated. Am
> I right?
>
>
>
> Many thanks,
>
> Jorge.
>
>
>
>
>
> Can anybody explain some of these issues?
>
>
>
> Many thanks,
>
> Jorge.
>
>
>
>
>
>
>
>
>
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>
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