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

[Sherman, Jeffrey A. (Fed)]

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.
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