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

khalid.el-darymli khalid.el-darymli at mun.ca
Tue Dec 2 10:06:48 EST 2014


Hi Marcus,

Is there a temperature sensor on-board the N200 unit? If not, does it
support installing any such sensor?

Thanks.

Best regards,
Khalid


On Fri, Nov 28, 2014 at 5:44 PM, Marcus D. Leech <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> 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 listUSRP-users at lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
>
>
>
>
>
> --
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortiumhttp://www.sbrac.org
>
>
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