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

khalid.el-darymli khalid.el-darymli at mun.ca
Thu Dec 11 08:04:04 EST 2014


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?

- 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.

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?

- 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?


Thanks.

Best regards,
Khalid



On Tue, Dec 9, 2014 at 11:59 AM, Marcus D. Leech <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> 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> 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
>>> http://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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