Re: [time-nuts] 100 MHz decade divider advice needed

[ed breya]

I'd recommend good old ECL and related families for these sorts of 
things. It's a lot more RF-friendly. 116/216 type receivers and small RF 
transformers can work for I/O, the signals are reasonably-sized (<1 V 
pp), can be all differential, can operate over a wide impedance range, 
and is quiet, power supply-wise (no supply or ground glitches on 
transition). The 10H series should be nice for 100 MHz, and there are 
newer series like "EL" that can go much higher, and more functions are 
available.


The downside is powering it up - whether it's regular ECL or PECL for +5 
V, it's pretty power hungry, but can be well worth it. Since it's 
low-gain, non-saturating logic, you can have additional noise issues, 
but operating as ECL with very clean and proper -5.2 V should minimize it.


Ed

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Re: [time-nuts] A simple sampling DMTD

[Joseph Gwinn]

Re: time-nuts Digest, Vol 184, Issue 40
On Fri, 29 Nov 2019 20:37:02 -0500, time-nuts-requ...@lists.febo.com 
wrote:

[snip]
> Message: 6
> Date: Fri, 29 Nov 2019 20:37:16 +0100
> From: Gerhard Hoffmann 
> To: time-nuts@lists.febo.com
> Subject: Re: [time-nuts] A simple sampling DMTD
> Message-ID: 
> Content-Type: text/plain; charset=utf-8; format=flowed
> 
> Am 29.11.19 um 11:45 schrieb Jan-Derk Bakker:
>> In general: as much as I like having it in my toolbox, I don't see how
>> using an FFT would be the best tool for the job in a zero-crossing detector
>> for a DMTD, let alone this particular sampling DMTD. For one, this 8-bit
>> processor doesn't have the spare cycles to run FFTs on the 32-bit data I
>> get from my CIC^2 decimator; besides that, I would only be interested in a
>> single bin (the beat frequency), where it would be more efficient to simply
>> I/Q-demodulate the samples in software (O(N) vs O(N log N)). While I admit
>> that in the latter case windowing would help, at this point I/Q
>> demodulating (effectively calculating only a single bin of the DFT) does
>> not appear to have advantages over least squares fitting the arcsine of the
>> incoming samples. Am I missing something here?
> 
> I admit that I did not follow this thread closely, but the Goerzel filter 
> is the single output line DFT , with O(n).
> 
> 

> 
> https://en.wikipedia.org/wiki/Goertzel_algorithm ??? >
> 
> If you need to simulate floats, fractional integers are easiest.
> I/Q demodulation probably requires to recreate a clean carrier if you want
> absolute phase and not only relative jumps. That sounds more like FPGA
> than 8051, or whatever the 8 bit processor of the day may be.
> 
> regards, Gerhard
> 
> 
> 
> OK, in a previous life I did build a system for geophysics, where they fed
> dangerous amounts of AC into the soil and measured the potential at
> some 50 nodes. Rubber boots required.
> Each node had a 8951 to control some switches and communicate setups.
> They were most excited when I gave them the sources so they could
> implement FFT pre-processing locally on each node themselves. 
> That required willingness to suffer.
> 
   
   
I must say that the above all seems to me to be more complex than 
needed.  

The fundamental problem to be solved is to estimate the phases of two 
beatnotes, one per channel (ref, signal under test), the phase 
difference being converted into a relative time delay.  So, we are 
estimating phase twice, against an unknown but common internal 
reference, and the key question here is how best to measure those two 
phases.  Detection of zero crossings is one way, but there are others. 

We know the frequencies of the beatnotes quite accurately, and that the 
waveforms are sine waves (which we will have band-pass filtered in 
hardware before conversion to digital data).  Here, I will assume that 
the frequencies are the same.  The only free variables are thus 
amplitude and phase; these can be estimated using least squares applied 
to successive batches of I&Q samples.  Windowing is still useful to 
reduce end splice effects, as previously discussed.  Given that we are 
working in the numerical domain, it's probably adequate to apply the 
window function to the product of the proposed match and the actual 
data, and then sum the windowed products.

As a quality check, if the estimated amplitude is too small (or too 
large), reject the phase estimate and try again.  If this condition 
persists too long or becomes too common, something is broken.

I doubt that anything of the 8-bit class is practical for this, and 
certainly not for a small-volume product, because programming effort 
increases sharply if the chosen processor is too limited.  Ardinuo and 
maybe StrawberryPi seem more like it.

As for emulation of floating point, the least-squares algorithm defined 
above can certainly be implemented in fixed-point arithmetic, called 
fractional integers above.  


In the extreme, this kind of algorithm will work with data clipped to a 
few bits per sample.  The underwater sonar folk are masters of this, 
especially back in the days when signal processors were necessarily 
bespoke hardware.  The search term is "one-bit correlator" (without the 
quotes).

Joe

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Re: [time-nuts] John Fluke test equipment tutorial

[Attila Kinali]

On Wed, 27 Nov 2019 12:01:56 -0600
Steven Sommars  wrote:

> https://tycho.usno.navy.mil/ptti.html archived 2012 and earlier PTTI
> papers.  That site seems to be unavailable now, but the Internet Archive
> still holds copies.  

On Magnus' request, I put my copy online. You can find it at
http://time.kinali.ch/ptti/

> Circa 2013 PTTI and ION merged, new papers are paywalled.


And ION is the most *beep* publishing organisation I've ever seen.
Even if you are member, you get only 12 papers to download
per year. And apparently, they don't like giving universities
access either. So, publishing at PTTI has basically become
a black hole. As a scientist I have no incentive to publish
there, because nobody will be ever able to read my paper.


Attila Kinail

-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes

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Re: [time-nuts] Garmin 18x GPS WNRO, revisited

[Rich Wales]

My GPS has been powered down from time to time over the past 10 years.
Its recent WNRO issue was not associated with any power-cycling event.

I am using ntpsec's "SHM" driver (*not* the type-20 NMEA driver), with
the GPS data captured by gpsd.  Gpsd is sensing the PPS signal from the
GPS without difficulty.

I bought a new Garmin 18x LVC recently, btw, and it is running fine on
another computer at my home without any WNRO problems.  I assume the
firmware on the new unit is newer than on my other, 10-year-old GPS, and
that this is why the new GPS is giving good time without my needing to
fudge it in ntpsec.

Rich Wales
ri...@richw.org

On 2019-11-29 10:27, David J Taylor via time-nuts wrote:

Thanks for reporting that, Rich.  I haven't seen that issue with either
the GPS18 LVC, or the GPS18x LVC, but neither have been powered down. 
Were yours running continually?  Anyway, I love the fix!

I'm running reference NTP here, and noted that with a type 22 PPS
driver, the "prefer" must include the type 20 NMEA driver.  With the
problem pending, I had hoped that using an e.g. Internet source for
coarse time might have been enough, and had commented out the NMEA
source, but it appears not to be the case.  The PPS was never used (but
it appeared in the billboard).

Cheers,
David

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