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

Marcus D. Leech mleech at ripnet.com
Thu Dec 11 11:05:27 EST 2014

On 12/11/2014 10:23 AM, khalid.el-darymli wrote:
> On another note, since the reference clock for the whole system will 
> be the external 10 MHz signal, the digital images would be situated at 
> multiples of this 10 MHz frequency (i.e., m*10 MHz - [e.g., carrier 
> frequency] 15 MHz; m=-/+1, -+2, ...). Hence, for this case, the first 
> few images will fall within the bandwidth of the built-in LFTX/LFRX 
> LPF filter. However, based on your earlier explanation,these are 
> totally taken care of 'digitally' and I don't have to worry about any 
> of that, am I right?
NO.  The 10MHz external clock is used as a *reference*.  The samples are 
still clocked-out to the DAC at 100MHz, the reference clocks are used to
   synthesize all the internal clocks, which run at a fixed rate, and 
the DAC/ADC are clocked at 100MHz, regardless of whether you are using the
   internal reference, or the external reference.

> Best,
> Khalid
> On Thu, Dec 11, 2014 at 11:25 AM, Marcus D. Leech <mleech at ripnet.com 
> <mailto:mleech at ripnet.com>> wrote:
>     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 app-specific
>       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
>>>                 _______________________________________________
>>>                 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

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