On 5/27/16 5:17 PM, Bruce Griffiths 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.
Most of the off the shelf SDR units (like the USRP) have more than
enough FPGA. The "standard" software in the USRP is a digital down
converter with an "IF" filter used as a front end for gnuradio. It
streams the downconverted and bandlimited samples to a back-end PC via
USB or Ethernet.
If you pick the right USRP models, you can lock the sampling clocks
together or distribute the clock. I don't know if that distribution is
sufficiently high quality for time-nuts kinds of applications. My
experience with the USRP has been that the detailed documentation on
this sort of thing tends to be kind of light, if not non-existent. You
might be digging into the source code to figure out what the interface
is and how it's implemented.
The other thing is that the USRP is made as a "radio" and more
particularly, one target market is people like grad students developing
wireless comm software, things like MIMO algorithms, etc. In general,
the oscillator and sampler quality isn't at "gnat's eyelash" kind of
performance. If your primary market is developing multimegabit/second
phy layer wireless comms, you tend not to be worried about phase noise
at 10 Hz from the carrier.
The NIST paper discusses this aspect.
Bruce
On May 25, 2016, at 12:01 PM, Sherman, Jeffrey A. (Fed)
<jeff.sher...@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))
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.
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