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

Marcus D. Leech mleech at ripnet.com
Tue Dec 2 11:13:47 EST 2014


On 12/02/2014 11:02 AM, khalid.el-darymli wrote:
> I am not sure if we are talking about the same thing. Since I need to 
> track the internal electronic circuit variation in temperature, which 
> pins on the USRP board/daughterboard are suitable for extracting 
> information that correlates with the temperature variation?
>
> Thanks,
> Khalid
>
With the exception of the TVRRX2, there are no internal temperature 
sensors in USRP hardware.  That's why I suggested a separate device, placed
   inside the N210 cabinet to be used as a proxy.  There are very few 
electronic components that provide an "internal temperature" output as
   a kind of monitoring thing.  Particularly in the analog RF space.  
Some computer motherboards have this, but it's not an inherent property of
   electronics to provide a temperature output as a side effect or anything.


>
> On Tue, Dec 2, 2014 at 11:53 AM, Marcus D. Leech <mleech at ripnet.com 
> <mailto:mleech at ripnet.com>> wrote:
>
>     On 12/02/2014 10:21 AM, khalid.el-darymli wrote:
>>     Thanks for the prompt reply. I need to monitor the*internal
>>     temperature *(i.e., not the ambient temperature) of the N200 USRP
>>     board/ chip/etc., to compensate for the temperature-based
>>     variations in the Rx FMCW chirp.
>>
>>     How the USB based sensor can do that?
>>
>>     Khalid
>>
>     Plenty of room inside the enclosure, you'd have to use some
>     mechanical creativity.
>
>
>
>>
>>     On Tue, Dec 2, 2014 at 11:41 AM, Marcus D. Leech
>>     <mleech at ripnet.com <mailto:mleech at ripnet.com>> wrote:
>>
>>         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
>>
>>
>>
>>
>>     -- 
>
>
>     -- 
>     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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