[USRP-users] Phase Noise for USRP N200 with LFTX/LFRX

Marcus D. Leech mleech at ripnet.com
Thu Dec 11 09:55:08 EST 2014

On 12/11/2014 08:04 AM, khalid.el-darymli wrote:
> I would like to thank you all for this information. You are always a 
> great help!
> - Now, given that my FMCW chirp is digitally synthesized (i.e., using 
> the digital DDS architecture of N200) before it is up-converted to an 
> analog form, is it correct to say that the temperature-based drifts I 
> am seeing (i.e., in the Tx FMCW chirp directly loop-backed to the Rx) 
> are not due to the Tx chain but rather they are solely from the Rx 
> chain (with the exception of the DAC from the Tx side)? Or they are 
> (roughly *EQUAL*) mixture of temperature drifts in both Tx (i.e., DAC) 
> and Rx (i.e., ADC) chains? In other words, what are the major 
> contributors of these temperature drifts? and are they 'roughly' equal 
> in both the Tx and the Rx sides?
It's hard to say, piece by piece, what contributions to very-small 
analog gain drifts can be ascribed to which piece.  The LFRX/LFTX are 
very similar,
   electronically, so I would any temperature coefficients to apply 
roughly equally to both sides.  Keep in mind that in a production 
system, your
   coax cables will also be contributing to both gain and phase drift, 
as they change temperature.

> - On another note, given that the system is driven by a 100 MHz 
> reference clock, images will be produced at some multiples of the 
> clock frequency. For example, if the requested center frequency was 20 
> MHz, then:
> 1st +image @: 100 MHz-20=80 MHz
> 2nd +image @ 100 MHz+20=120 MHz
> 3rd +image @ 200 MHz-20=200MHz-20=180 MHz
> and so on.
Images should be well-suppressed, as far as I know--the upconversion is 
done digitally, so it's like a "perfect" image-reject mixer.

> Given that the LFTX daughter-board has an in-built 30 MHz low-pass 
> filter, is it correct to say that all such images will be 
> filtered-out, and it is 'guaranteed' that the generated FMCW chirp 
> (e.g., with a BW of 200 KHz) up-converted to some specific center 
> frequency (e.g., 20 MHz), is guaranteed to 'ONLY' be within that BW 
> around the center frequency? In other words, do I need not to worry 
> about generating an out-of-band noise/interference (for whatever 
> reason such as images, phase truncation spurs, harmonic spurs due to 
> DAC nonlinearity, clock feed-through, etc.) that entail installing an 
> additional external analog band-pass filter?
At the end of the day, you'll have to measure in the laboratory what 
your undesired output signals are.  It is nearly always the case that 
   filtering needs to be applied by the end-user in an SDR 
implementation, since the platform is designed not for any specific 
application.  Assuming that
   in end use, you use an RF amplifier, you'll have to revisit your 
filtering *after* said amplifier, because they nearly always introduce 
some small
   undesired products as well.

> - Finally, what effect does the external REF clock (used for syncing 
> multiple N200 devices) has on the internal clock (CVHD-950) of the 
> N200? In other words, is the common external clock meant to OVERRIDE 
> CVHD-950? If so, if the phase noise of my external clock is worse than 
> that of the CVHD-950, will that affect the phase noise of the signal 
> generated/received in USRP N200?
When you're using an external REF clock, your phase noise is utterly 
dominated by the phase-noise of your external reference.  So, if your
   external reference has worse phase noise, then your signals will have 
worse phase-noise.  But, I want to ask, why use an external reference
   with worse phase noise?

> Thanks.
> Best regards,
> Khalid
> On Tue, Dec 9, 2014 at 11:59 AM, Marcus D. Leech <mleech at ripnet.com 
> <mailto:mleech at ripnet.com>> wrote:
>     On 12/09/2014 07:53 AM, khalid.el-darymli wrote:
>>     Thanks very much for this information. According to the datasheet
>>     you mentioned in the link below [1], the supported oscillating
>>     frequency by CVHD-950 is 50 MHz to 130 MHz. The PN specs are
>>     provided for some center frequencies in this range.
>>     However, I am interested in the HF band for which the LFTX/LFRX
>>     support center frequencies up to 30 MHz. For requested center
>>     frequencies in (DC, 30 MHz], do you digitally down-sample the
>>     center frequency of CVHD-950? Is there any description on how
>>     exactly you do that? and what effect does this may have on the
>>     phase noise of CVHD-950?
>     No, that is the reference clock for the entire platform--driving
>     all timing in the platform, including the 100MHz sample clock for
>     the ADCs. The schematics
>       are here:
>     http://files.ettus.com/schematics/n200
>>     I am working on an ocean-based HF radar. Basically, the system
>>     measures the Doppler frequency of ocean waves for different sea
>>     states. It is important that I characterize my system to know its
>>     limits and capabilities.
>>     Thanks for your help.
>>     Best,
>>     Khalid
>>     [1]
>>     http://www.crystek.com/home/oscillator/vcxodetail.aspx?pn=CVHD-950
>>     On Mon, Dec 8, 2014 at 7:35 PM, Matt Ettus <matt at ettus.com
>>     <mailto:matt at ettus.com>> wrote:
>>         With the LFRX and LFTX, there is no mixer, so all phase noise
>>         is from the ADC clock.  The oscillator used is a CVHD-950
>>         from Crystek and you can find the phase noise spec for it on
>>         their web site.
>>         Matt
>>         On Mon, Dec 8, 2014 at 2:11 PM, khalid.el-darymli via
>>         USRP-users <usrp-users at lists.ettus.com
>>         <mailto:usrp-users at lists.ettus.com>> wrote:
>>             Hi,
>>             According to the specs provided in [1], the RF
>>             performance of USRP N200/N201 is characterized for the
>>             WBX daughter-board. Particularly, phase noise
>>             measurements are given for three offsets from a center
>>             frequency of 1.8 GHz.
>>             My question is, are there any such measurements for the
>>             LFTX/LFRX daughter-board?
>>             If not, is there a recommended procedure for doing these
>>             measurements?
>>             Among others, I am particularly concerned with the phase
>>             noise and its short-term effect on my Doppler measurements.
>>             [1]
>>             https://www.ettus.com/content/files/07495_Ettus_N200-210_DS_Flyer_HR_1.pdf
>>             Thanks,
>>             Khalid
>>             _______________________________________________
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>     -- 
>     Marcus Leech
>     Principal Investigator
>     Shirleys Bay Radio Astronomy Consortium
>     http://www.sbrac.org

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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