Re: [Discuss-gnuradio] benchmark_xx.py Transmits Images at +/- 0.5MHz When Center Freq Is 2.4GHz [USRP N200 and XCVR2450]

[Jason Tran]

Hi All,

Here are images of what I'm seeing in my setup.

Pictures of Images/Spikes:

center freq set to 2.4GHz. random image (oscillioscope)

http://img585.imageshack.us/img585/5871/img20120503124151.jpg

center freq set to 2.4GHz. images +/- 0.5MHz (uhd_fft.py)

http://img801.imageshack.us/img801/3015/imagesonuhdfft.png

Pictures of Clean Signal:

center freq set to 0; Tx "defaults" to 4.8GHz but is actually 2.4GHz (uhd_fft.py):

http://imageshack.us/photo/my-images/213/noimagesuhdfft.png/

(oscilloscope version)

http://img801.imageshack.us/img801/4141/img20120503124257.jpg

-Jason

---

From: Jason Tran <address@hidden>
To: "address@hidden" <address@hidden>; "address@hidden" <address@hidden>
Sent: Wednesday, May 2, 2012 10:28 PM
Subject: Re: [Discuss-gnuradio] benchmark_xx.py Transmits Images at +/- 0.5MHz When Center Freq Is 2.4GHz [USRP N200 and XCVR2450]

Hi Josh,

Thanks for the response!

>Do you have a screencap of the problem spectrum? Maybe from the
>perspective of the receiver. 

I'll generate a screencap tomorrow morning when I get back to the equipment.

>Well XCVR2450 has a dual band. I think you are seeing some weirdness
>with the code either trying to auto select the middle frequency, and the
>actual frequency being coerced.

This is exactly what I'm thinking as well. I've been trying to find the trail of relevant pieces of code with no avail.

But the bigger question I'm trying to figure out is why the images occur when the frequency is set at 2.4GHz

as opposed to having no images when the code is setting the frequency to 2.4GHz on its own.

--start code snippet---

uhd::tune_result_t set_center_freq(double freq, size_t chan = 0){

        return set_center_freq(uhd::tune_request_t(freq), chan);

    }

--end code snippet--

-Jason

 

---

From: Josh Blum <address@hidden>
To: address@hidden
Sent: Wednesday, May 2, 2012 9:14 PM
Subject: Re: [Discuss-gnuradio] benchmark_xx.py Transmits Images at +/- 0.5MHz When Center Freq Is 2.4GHz [USRP N200 and XCVR2450]

On 05/02/2012 07:42 PM, jasonatran wrote:
>
> Hi All,
>
> I have a pair of USRP N200s with XCVR2450 daughterboards running the latest
> GNU Radio version with UHD. This is an odd problem, but I'm wondering if
> anyone has seen this before.
>
> When I run the benchmark_rx.py/benchmark_tx.py specified at 2.4GHz, the
> transmitter is transmitting images of lower magnitude at +/- 0.5MHz, and the
> receiver isn't able to decode any radio packets. Some frames are received,
> but they are all "ok=FALSE" with the lengths lower than the length of the
> actual packet sent. These images persist when I set the XCVR2450's to tx/rx
> within its specified frequency range.
>

Do you have a screencap of the problem spectrum? Maybe from the
perspective of the receiver.

> note: the benchmark scripts are from
> ~/gnuradio/gr-digital/examples/narrowband/
>
> However, when I run the benchmark scripts at "--freq 0" on both sides, the
> transmitter says it is automatically set to "4800MHz" (although this is
> outside of the XCVR2450's range), and the receiver says it is automatically
> set to "2400MHz." Somehow, the transmitter is transmitting at 2.4GHz and the
> receiver is tuned to 2.4GHz. The signal is clean, and there are no images in
> sight. I used both uhd_fft.py and a spectrum analyzer to check.
>

Well XCVR2450 has a dual band. I think you are seeing some weirdness
with the code either trying to auto select the middle frequency, and the
actual frequency being coerced.

-Josh

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