[time-nuts] chrony vs ntpd
jimlux at earthlink.net
Sat Oct 28 14:20:34 EDT 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
> 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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