[time-nuts] chrony vs ntpd

jimlux jimlux at earthlink.net
Sat Oct 28 18:20:34 UTC 2017

On 10/28/17 10:34 AM, John Ackermann N8UR wrote:
> Jim, I thought about using an RF-input sync pulse for alignment during 
> the Solar Eclipse measurement experiment, but ended up running out of 
> time to implement it.  But some very crude experiments indicated that 
> it's not hard to generate an edge out of a PPS that creates a comb well 
> past HF.  My idea was to do a divide-by-sixty to end up with 
> pulse-per-minute rather than PPS.  The lower rate would be less annoying 
> to filter out of the results.
> I'm interested to hear if you end up doing this, and if so how.

Yes, a nice narrow pulse makes a nice comb.  I've done it for a single 
shot wideband gain calibration across the band for my space HF receiver 
(in ground test).

The tricky parts, I have found, are:
1) the rise and fall time have a big effect on the relative heights of 
the comb vs freq - perfectly square gives you a nice sin(x)/x, but if it 
starts to be not-square, then it rolls off faster.  I've been thinking 
about how to do something that measures it

2) Amplitude of the pulse - that one seems pretty straightforward - a 
good switch from a regulated voltage.

3) The effects of the antenna and receiver impedances - well - to a 
certain extent, that's what I want to measure.   So the idea is that if 
you inject a pulse through a known resistance into the receiver/antenna 
combination (at the receiver input), and, I do this at two or three 
different impedances, I should be able to back out the impedance effects 
(with some TBD uncertainty).

So far, I've been experimenting with RF tone bursts from a 33622 
function generator - Easy to detect, but I've not found a good way to 
get a nice sharp marker - you can slide a matched filter along and get a 
sort of pulse, but it's not what I want.

I'm starting to think that some sort of PN code might be the way to go - 
It makes it easy to integrate over a longer time (e.g. many edges to 
look at).

> John
> ----
> On 10/28/2017 12:04 PM, jimlux wrote:
>> Now that I have successfully connected my GPS receiver to my beagle 
>> and I'm getting pps ticks into the driver, etc. (thanks to info from 
>> several folks on this list!) the question arises of whether to use 
>> ntpd or chrony.
>> For my particular application, I'm more interested in synchronizing 
>> time on the local machine, not necessarily being a NTP server - all of 
>> my beagles have a GPS on them.  Of course, there may be times when a 
>> GPS doesn't work, or something else comes up where it would be useful 
>> for one of the machines to "get time" from somewhere else.
>> What I am doing is using the Beagle to capture RF samples (RTL-SDR) in 
>> a distributed array, with wireless connections among the nodes.  The 
>> processing isn't necessarily real-time (maybe later..), for now, it's 
>> "trigger some seconds of capture at approximately the same time" and 
>> post process in matlab/octave.
>> There's all kinds of nondeterministic latency issues with the 
>> USB/RTL-SDR path, so I'm under no illusion that I can capture samples 
>> aligned to the 1pps.  However, what I *can* do is generate a "sync 
>> pulse" from the 1 pps and feed it into the RTL's RF input in some 
>> (TBD) way.
>> And the 1pps might give me a clever way to calibrate the frequency 
>> drift of the RTLSDR's clock.
>> Right now, I'm interested in HF signals (so the period is 30 ns at the 
>> top end, and 500 ns at the bottom end)
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