[time-nuts] chrony vs ntpd
djl at montana.com
Sat Oct 28 17:25:20 EDT 2017
Would a step recovery diode be better?
On 2017-10-28 12:20, jimlux wrote:
> 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).
>> 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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Dr. Don Latham
PO Box 404, Frenchtown, MT, 59834
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