[time-nuts] DMTD - analog multiplier vs. diode mixer ?
kb8tq at n1k.org
Wed Jan 6 01:35:46 UTC 2016
Ok, so what needs to be done with the output of the mixer (no matter how you do it)?
Assume you start from 10 MHz and head down to 10 Hz.
Assume you are mad at your 5370 and want significantly better performance.
Where does that get you?
The 5370 already is in the ~ 20 ps range. A lot depends on your definitions and
how good your sample is running. Let’s call that 2x10^-11 at tau = 1 second. You
could indeed call it a couple of other things as well.
Simply moving up a decade with a whole bunch of gear and it’s limitations seems like
a waste. To me you want to go for 1 to 2x10^-13 as your target. It is an achievable target
and there are a number of papers that validate it as a reasonable DMTD target.
You get a 1x10^6 “amplification due to your down mix from 10 MHz to 10 Hz. You then
need another 1x10^7 to get you to your target. All errors from everything included, you need to
work out the location of the zero crossings to within 100 ns.
The practical examples of doing it include some fairly tight lowpass filtering as well as high
pass filtering ahead of the detection process. I have never seen it done without this filtering
as part of the setup. There is just to much noise at the detector otherwise. Most systems
have something like a 15 Hz lowpass and a 5 Hz high pass for a 10 Hz note.
With fairly good diode ring phase detectors and a less than perfect (not 25 stage Collins style)
analog limiter, you can indeed get to the target.
Doing it digitally assumes you have a pretty good clock and sampler. If you look at it as a
3V p-p triangle waveform at 10 Hz, you have a 60V / second slew rate. (a 1 V p-p sine wave
is pretty close to the same number). You need to filter that at 15 Hz and then resolve it to about
6 uV at the zero crossing. You can either keep a high sample rate and make your filter a
major nightmare or you can decimate ahead of the filter and turn the resolver into a headache.
Either way, there is some work to be done.
A couple of op-amp packages is about all it takes to do the limiter with the analog approach ….
> On Jan 5, 2016, at 6:58 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> Hi Poul-Henning,
> On 01/06/2016 12:28 AM, Poul-Henning Kamp wrote:
>> In message <568C46B9.4020303 at rubidium.dyndns.org>, Magnus Danielson writes:
>>> The white noise will be particularly annoying as it then converts to
>>> jitter through the slew-rate limitation as you go into the
>> Digitize the LPF output and do a curve-fit to find the zero-crossings ?
> That would work. You could least-square fit it with very cheap processing. The LPF would mainly need to reject the sum frequencies to act as anti-aliasing filter, and the noise would be filtered out by the least-square processing.
> Estimating the phase and slew-rate, and then use those to calculate the actual through-zero phase would not be too hard. As a consequence you get a slew-rate monitor, which act as an observation of signal level.
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