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

Bob Camp kb8tq at n1k.org
Fri May 27 21:15:16 EDT 2016


Hi


> On May 27, 2016, at 8:17 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> 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.

So … is there an explicit high pass in the FPGA other than the dc offset elimination high pass? 
There is obviously a lowpass function in the decimating FIR’s and the CIC. That appears to be 
optimized simply for sample rate rather than for noise. Thus the same question applies to 
low pass as well.  

Bob


> 
> 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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>> 
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