[time-nuts] Commercial software defined radio for clock metrology

jimlux jimlux at earthlink.net
Sat May 28 09:11:47 EDT 2016


On 5/27/16 5:17 PM, Bruce Griffiths wrote:
> On Thursday, May 26, 2016 06:40:26 PM Bob Camp wrote:
>> Hi
>>
>> Very interesting paper, thanks for sharing !!
>>
>> One question:
>>
>> In many DMTD (and single mixer) systems, a lowpass and high pass filter are
>> applied to the signal coming out of the mixer. This is done to improve the
>> zero crossing detection. It also effectively reduces the “pre detection”
>> bandwidth. My understanding of the setup in your paper does not do this
>> sort of filtering. It simply operated directly on the downconverter signal.
>>  Is this correct? I may have missed something really obvious in a quick
>> read of the paper…..
>>
>> Thanks!
>>
>> Bob
>
> All the filtering and down mixing is done in the digital domain.
> Anitialiasing filters in front of the ADCs are also be required.
>
> A 2  (or more) receive channel SDR board would be a nice tool to use for this
> provided the FPGA is large enough.

Most of the off the shelf SDR units (like the USRP) have more than 
enough FPGA. The "standard" software in the USRP  is a digital down 
converter with an "IF" filter used as a front end for gnuradio. It 
streams the downconverted and bandlimited samples to a back-end PC via 
USB or Ethernet.

If you pick the right USRP models, you can lock the sampling clocks 
together or distribute the clock.  I don't know if that distribution is 
sufficiently high quality for time-nuts kinds of applications.  My 
experience with the USRP has been that the detailed documentation on 
this sort of thing tends to be kind of light, if not non-existent.  You 
might be digging into the source code to figure out what the interface 
is and how it's implemented.


The other thing is that the USRP is made as a "radio" and more 
particularly, one target market is people like grad students developing 
wireless comm software, things like MIMO algorithms, etc.  In general, 
the oscillator and sampler quality isn't at "gnat's eyelash" kind of 
performance.  If your primary market is developing multimegabit/second 
phy layer wireless comms, you tend not to be worried about phase noise 
at 10 Hz from the carrier.


The NIST paper discusses this aspect.


>
> Bruce
>
>>
>>> On May 25, 2016, at 12:01 PM, Sherman, Jeffrey A. (Fed)
>>> <jeff.sherman at nist.gov> wrote:
>>>
>>> Hello,
>>>
>>> A recently published paper might be of interest to the time-nuts
>>> community. We studied how well an unmodified commercial software defined
>>> radio (SDR) device/firmware could serve in comparing high-performance
>>> oscillators and atomic clocks. Though we chose to study the USRP
>>> platform, the discussion easily generalizes to many other SDRs.
>>>
>>> I understand that for one month, the journal allows for free electronic
>>> downloads of the manuscript at:
>>> http://scitation.aip.org/content/aip/journal/rsi/87/5/10.1063/1.4950898
>>> (Review of Scientific Instruments 87, 054711 (2016))
>>>

>>> Perhaps the biggest worry about the SDR approach is that fast ADCs are in
>>> general much noisier than the analog processing components in DMTD.
>>> However, quantization noise is at least amenable to averaging. As you all
>>> likely appreciate, what really limits high precision clock comparison is
>>> instrument stability. In this regard, the SDR's digital signal processing
>>> steps (frequency translation, sample rate decimation, and low-pass
>>> filtering) are at least perfectly stable and can be made sufficiently
>>> accurate.



More information about the time-nuts mailing list