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

[Marcus Müller]

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Hello Khalid,

> rather than having a standalone USB connection to the computer, is
> there anyway to piggyback the temperature data from the temp sensor
> onto the digital interface to the FPGA board of the N200?

That's what you would do if you attached a sensor to the FPGA's debug
header, modified the FPGA image and the host driver and got your data
out of that.
Definitely possible, definitely a huge effort for a few bytes of data.

I'd either go with the GPIO interface[1], or use the fact that the
N2x0 works on standard Gigabit Ethernet / IP, and you can simply
attach any networked hardware to a Gigabit Ethernet switch. Raspberry
Pi comes to mind, if you want something rather capable and running a
stock linux, for which many kernel drivers already allow you to talk
to hardware attached to things like I2C or SPI.

Currently I'm playing around with a stamp-sized openWRT-running board
that would potentially even fit inside the casing of the N200, but I
don't think that would be a good idea (it being a least cost, and
definitely not EMI-tested, one-man project).

Greetings,
Marcus

[1] which many daughterboards expose; what daughterboard are you using?
The information about the X3x0's front panel GPIO applies to the N2x0
GPIO, too, mostly: http://files.ettus.com/manual/page_gpio_api.html

On 12/03/2014 01:51 PM, khalid.el-darymli wrote:
> Thank you Ralph and Marcus for this information. That is so
> helpful. One final question on this, rather than having a
> standalone USB connection to the computer, is there anyway to
> piggyback the temperature data from the temp sensor onto the
> digital interface to the FPGA board of the N200?
> 
> Thank you.
> 
> Best regards, Khalid
> 
> 
> On Wed, Dec 3, 2014 at 3:36 AM, Ralph A. Schmid, dk5ras
> <address@hidden> wrote:
> 
>> In my job we have learned that lesson and fitted on all our DSP
>> systems (you could call them direct digitizing SDRs, although we
>> do not radio stuff) a cheap I2C temperature sensor. No big thing
>> by means of software, costs and PCB space, but already proved
>> being very useful. Could be considered for future products…
>> 
>> 
>> 
>> Ralph.
>> 
>> 
>> 
>> *From:* USRP-users [mailto:address@hidden
>> *On Behalf Of *Marcus D. Leech via USRP-users *Sent:* Tuesday,
>> December 2, 2014 4:11 PM *To:* khalid.el-darymli *Cc:*
>> address@hidden; address@hidden *Subject:*
>> Re: [USRP-users] How to correct for the drift in an (FMCW) Rx 
>> signal?
>> 
>> 
>> 
>> 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
>> <address@hidden> 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, <address@hidden> 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
>> 
>> address@hidden
>> 
>> 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
>> 
>> 
> 
> 
> 
> _______________________________________________ Discuss-gnuradio
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