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

[Bruce Griffiths]

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.

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))
> > 
> > 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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