Re: [Discuss-gnuradio] How can we measure the frequency offsets between two USRPs?

[Nick Foster]

On Mon, 2011-05-16 at 12:23 -0500, John Andrews wrote:
> I am using GRC. I used a signal source block generating a complex sine
> at 100kHz. The USRP interpolation is 128 and the sampling rate of the
> sine generator is 1MHz. The USRP connected to another computer has
> USRP source configured at 64 decimation and is connected to an FFT
> block. I don't see any peak at the expected frequency or anywhere in
> the plot. Its just a flat plot. I checked the USRP settings on both
> and they are configured right. I even have transmit gain and receive
> gain as 10dB on both sides.
> 
> What can be wrong here?

What daughterboards are you using? What frequency are you using on the
source/sink blocks? The BasicRX/TX should be used with >1MHz signals
(configure the USRP source/sink center freq to 1MHz or above), since the
transformers won't pass lower frequencies than this.

--n

> 
> Thanks
> 
> On Mon, May 16, 2011 at 12:09 PM, Marcus D. Leech <address@hidden>
> wrote:
>         On 16/05/2011 1:03 PM, John Andrews wrote: 
>         > Shouldn't I use some kind of modulation scheme to do this,
>         > like FM or AM, to transmit a tone?
>         No, you can just transmit a narrow, single-frequency tone, and
>         use the receivers FFT to determine how far off it is from
>           where you expect it.
>         
>         use a signal-source producing a SIN wave at, let's say, 1KHz,
>         feed that into a UHD/USRPx sink tuned to whatever your
>         frequency is.
>           The tone will appear at TUNED-FREQUENCY+1KHz.
>         
>         
>         
>         
>         > 
>         > On Mon, May 16, 2011 at 10:02 AM, Marcus D. Leech
>         > <address@hidden> wrote:
>         >         On 16/05/2011 10:26 AM, Alexander Chemeris wrote:
>         >                 You may also look into this code:
>         >                 http://thre.at/kalibrate/
>         >                 It estimates offset of an USRP with regards
>         >                 to a GSM base station, but
>         >                 it can be easily modified to measure offset
>         >                 from any clean tone, e.g.
>         >                 transmitted by a second USRP.
>         >                 
>         >         Keep in mind that the offset measured must
>         >         necessarily be the total offset--that is, both Rx
>         >         and Tx can be "off" in frequency.
>         >         
>         >         The practical consequence should be NIL, because
>         >         frequency correction should normally only be done on
>         >         the Rx-side, and it should
>         >          simply adapt to whatever it sees, regardless of the
>         >         Tx and Rx components of the offset.
>         >         
>         >         
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>         
>         
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