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

[Nick Foster]

On Mon, 2011-05-16 at 15:34 -0500, John Andrews wrote:
> I am running it at 2.5GHz.

What magnitude are the samples you're feeding into the USRP sink?

--n

> 
> On Mon, May 16, 2011 at 12:39 PM, Nick Foster <address@hidden> wrote:
>         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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