Thx!!! André Esteves 2016-05-25 17:01 GMT+01:00 Sherman, Jeffrey A. (Fed) <jeff.sher...@nist.gov> :
> 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 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
