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

[Nick Foster]

On Mon, 2011-05-16 at 12:32 -0500, John Andrews wrote:
> I am using the 1st generation USRP with RFX2400 daughterboards each
> connected to the TX/RX interface.
> 
> In the sine source block I am using a frequency of 100kHz. As the
> interpolation of USRP sink is 128 I am using a sampling frequency of
> 1Msps.

And what center frequency are you running at?

--n

> 
> On Mon, May 16, 2011 at 12:27 PM, Nick Foster <address@hidden> wrote:
>         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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