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

[paul swed]

Jeff thanks for sharing the document with time-nuts. Others with more
knowledge will comment I suspect over the next 24 hours. I did read through
the document with interest and there are some alternates to a $1K sdr as
you suggest.
However since I have had an opportunity several times to build a analog
DMDT and haven't I suspect this is even further down the road.
But on time-nuts there are others that do need a DMDT solution and I look
forward to reading their comments and learning.
Regards
Paul
WB8TSL

On Wed, 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))
>
> Afterwards, a preprint will remain available at:
> http://arxiv.org/abs/1605.03505
>
> There are commercial instruments available with SDR architecture
> under-the-hood, but they often cost many thousands of dollars per
> measurement channel. In contrast, commercial general-purpose SDRs scale
> horizontally and can cost <= $1k per channel. Unlike the classic dual-mixer
> time-difference (DMTD) approach, SDRs are frequency agile. The
> carrier-acceptance range is limited not by the sample clock rate but by the
> ADC's input bandwidth (assuming one allows for aliasing), which can be many
> times greater. This property is an important feature in considering the
> future measurement of optical clocks, often accomplished through a
> heterodyne beatnote (often at "practically any" frequency between ~1 MHz to
> 500 MHz) with a femtosecond laser frequency comb. At typical microwave
> clock frequencies (5 MHz, 10 MHz), we show that a stock SDR outperforms a
> purpose-built DMTD instrument.
>
> 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.
>
> We found that in the studied units the limiting non-stationary noise
> source was likely the aperture jitter of the ADC (the instability of the
> delay between an idealized sample trigger and actuation of the sample/hold
> circuitry). However, the ADC's aperture jitter appears highly common-mode
> in chips with a second "simultaneously-sampled" input channel, allowing for
> an order-of-magnitue improvement after channel-to-channel subtraction. For
> example, at 5 MHz, the SDR showed a time deviation floor of ~20 fs after
> just 10 ms of averaging; the aperture jitter specification was 150 fs. We
> also describe tests with maser signals lasting several days.
>
> Best wishes,
> Jeff Sherman, Ph.D.
> --------------------------------------------------------------------
> National Institute of Standards & Technology
> Time and Frequency Division (688)
> 325 Broadway / Boulder, CO 80305 / 303-497-3511
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