[ntp:questions] ntpd: time reset problem

David Lord snews at lordynet.org
Fri Sep 18 09:18:08 UTC 2009


David J Taylor wrote:
> 
> "David Lord" <snews at lordynet.org> wrote in message 
> news:7hg1jtF2s1lvqU1 at mid.individual.net...
>> David J Taylor wrote:
>>
>>> You'll have to get a GPS puck and wireless PC on the roof!  Safely
>>
>> That looked a possibility earlier this year but treeline front
>> and back has shot up maybe a couple of metres already and if
>> same next year I'd be needing a mast.
> 
> Don't worry about trees - a modern GPS such as the GPS 18x LVC is very 
> sensitive, and can work with a lot of obscuration.  Although my GPS 18 
> LVC is sitting on the roof, the GPS 18x LVC works just as well sitting 
> in a room on the top floor of the house.

It's very wet round here with 90 +/- 10% R/H being common in
various locations including out back of house which is often
95%. You could only hang washing out for it to get wet. This
means leaves are soaking wet and beginning of year when I
swapped from BR304 to GPS18-LVC I was getting continuous
reach 377 but this deteriorated as trees came into leaf.

> 
>> I'm signed up in protest at various network over mains systems
>> due to them splattering and killing QRP reception.
> 
> Me also.
> 
> []
>> I've not mastered the demodulation though and whilst on scope
>> it looks to be a clean and identifiable MSF timecode no
>> different than from Conrad module, a much closer look shows
>> noise spikes. I used an RC filter at output and suspect a
>> Schmitt as with Maplin circuit is needed.
>>
>> David
> 
> What sort of filtering is used these days?  I used to think a 60KHz 
> crystal was required, with a bandwidth of a few 10's or 100's of Hz 

Datasheets on some receiver chips seem to use just a crystal
but some seem to have some coupling back from output to input
but it's not clear if feedback or forward. Otherwise all trf
with agc from a peak detector but filtering time constants
aren't easy to work out as just giving a suggested capacitor
value.

I tried similar interstage crystal coupling which produced a
good oscillator until receiver module was shielded and moved
away from aerial. That also lost me some of timecode pulses
as not enough damping.

> (there used to be a fast-code transmission on the signal).  What's the 
> rise-time required through the filter?  20ms fast enough?  A Schmitt 
> trigger would be worth trying, because although there will be some 
> hysteresis on the PC input port, it will be more appropriate for RS-232 
> levels and not tailored to your signal.

Maplin circuit I have shows the fast timecode but more recent
documentation from NPL doesn't mention any fast code and the
slow code shown is different to that given with the Maplin
circuit.

I've seen widely different values used for filter of the
NE567 from about 1ms upwards. Maplin design seems to be
for loop filter of NE567 with bandwidth of about 8kHz
(depends on input level) and 1ms with option of extra
capacitor at input of Schmitt to give about 15ms to
remove fast pulses.

I'm suspecting I've something broken on my demodulator
pcb so will be rebuilding with added 4093 and couple of
pots to try different time constants. That's just a
challenge since Conrad module with 60kHz crystal is
tiny, low power and works ok.

The Conrad module doesn't give a 60kHz output though and
my aim was to use that as frequency reference same as
Andy Talbot (G4JNT) design.

David




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