[USRP-users] How to correct for the drift in an (FMCW) Rx signal?

Marcus D. Leech mleech at ripnet.com
Tue Dec 2 10:11:19 EST 2014


On 12/02/2014 10:06 AM, khalid.el-darymli wrote:
> Hi Marcus,
>
> Is there a temperature sensor on-board the N200 unit? If not, does it 
> support installing any such sensor?
>
> Thanks.
No, and no.

But there are a tonne of USB-based temperature sensors out there.  
Google is your friend, etc.



>
> Best regards,
> Khalid
>
>
> On Fri, Nov 28, 2014 at 5:44 PM, Marcus D. Leech <mleech at ripnet.com 
> <mailto:mleech at ripnet.com>> wrote:
>
>     On 11/28/2014 03:41 PM, khalid.el-darymli wrote:
>>
>>
>>     Back to my original question, what should I do to correct for this?
>>
>>     Thanks in advance.
>>
>>     Best,
>>     Khalid
>>
>>
>     Khalid:
>
>     Thanks very much for the very-extensive data.  My main concern, as
>     one of the Ettus support team, was that there was something wrong with
>       the hardware, but the magnitude of both the apparent phase and
>     magnitude drift is entirely consistent with analog-hardware
>     temperature
>       effects, unrelated to clock stability, etc.
>
>     Coax cables, for example, will change their loss characteristics
>     and *effective length* with temperature, so with precise hardware
>     like USRPs, it's
>       easy to see these effects.
>
>     FMCW radar isn't my area of expertise, so hopefully others can
>     comment on RX-processing strategies to deal with this, as it
>     *must* also be a problem
>       with non-SDR FMCW radar implementations.
>
>
>>
>>
>>
>>
>>     On Fri, Nov 28, 2014 at 12:08 PM, <mleech at ripnet.com
>>     <mailto:mleech at ripnet.com>> wrote:
>>
>>         What is the magnitude of the frequency drift?
>>
>>         What is the magnitude of the gain drift?
>>
>>         What are you measuring backscatter *from*?
>>
>>         On 2014-11-28 10:14, khalid.el-darymli via USRP-users wrote:
>>
>>>         Hi,
>>>
>>>         Given a set of synced /(i.e., using external GPS REF/PPS)/,
>>>         time-commanded and calibrated /(i.e., through compensating
>>>         for the phase/mag offset between digital Tx chirp prior to
>>>         transmission and ADC'ed Rx signals) /N200 devices with
>>>         LFTX/LFRX daughterboards, that work with coherent LFMCW
>>>         chirps, I am still seeing a tiny drift (both in the
>>>         magnitude and frequency) of the calibrated  back-scatter Rx
>>>         chirp received at time t1 when compared to an Rx chirp
>>>         received at an earlier time t0.
>>>
>>>         The more the N200 device runs (e.g., 5 hours), the greater
>>>         the drift is. Obviously, this drift is pertinent to both the
>>>         DAC and ADCs and the GPS referenced clocks of the N200 devices.
>>>
>>>         My questions are:
>>>
>>>         1- Why I still see such drift although my devices are synced
>>>         with an external GPS? and how do I correct for it?
>>>
>>>         2- Can the *PLL Carrier Tracking *block in GRC be used to
>>>         track and correct for such a drift? If so, how do I set the
>>>         max/min freq inputs for this block?
>>>
>>>         3- Can *AGC2* or *AGC3 *block be useful in this regard? If
>>>         so, are there any examples to explain how the input
>>>         parameters of these blocks can be set up?
>>>
>>>
>>>         Thanks.
>>>
>>>         Best regards,
>>>         Khalid
>>>
>>>
>>>         _______________________________________________
>>>         USRP-users mailing list
>>>         USRP-users at lists.ettus.com  <mailto:USRP-users at lists.ettus.com>
>>>         http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>>
>
>
>     -- 
>     Marcus Leech
>     Principal Investigator
>     Shirleys Bay Radio Astronomy Consortium
>     http://www.sbrac.org
>
>
>


-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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