PSN-L Email List Message

Subject: Computer Timing Problems / Solutions
From: ChrisAtUpw@.......
Date: Fri, 15 Apr 2005 21:21:13 EDT


Hi there,
 
    I have inspected six new computer motherboards.  They all appeared to be 
using the miniature cylindrical 32,768  Hz watch crystals. You can get AT cut 
crystals in these cases  but watch crystals are the commonest. 32 KHz Watch 
crystals have a  parabolic error plot with temperature, peaking at about 25 or 
30 C, +/-5 C. The  coefficient is ~ 0.04 x (Cdiff)^2 ppm. If it is a 30 C 
crystal and  the temperature falls to 5 C, you can expect to get 25 ppm drift - 
about 2  sec per day.     
    However, the time loss errors shown on my  newish computers are very 
considerably above this, so I suspect that this must  be due to changes / errors 
in the counted interrupt rate. On older  computers the disk drive R/Ws could 
override the regular interrupt timing. The  time update options given in Windows 
do NOT seem to "discipline" the rate loss  of the time system. Some other 
programs can do this.
 
    Most of the USA can receive the WWVB 60 KHz timing  signals. The coverage 
is shown on _http://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm_ 
(http://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm)  Galleon  
Systems at 
_http://www.ntp-time-server.com/_ (http://www.ntp-time-server.com/)    
produce a range of timing products, from complete time server units to separate  
module boards.
    The receivers use a 60 KHz tuned ferrite  aerial, costing $2.68.
    The EM2S receivers cost $10.89. They have a normal  operating range of 3 
to 12 V DC. They use a crystal filter to get a high  stability narrow band 
response and have an AVC circuit to optimise  reception for changes in signal 
strength. 
    The MCM-RS232 Microcontroller Module costs $15.89.  It operates off 3 V 
DC. When combined with the EM2 Receiver Module and the  Antenna, it provides 
time information in standard RS232 data format via a serial  interface. External 
buffering to full RS232 levels is required using a  proprietary RS232 level 
shifter IC. The advantage of the module compared to  direct host decoding of 
the receiver output is the continuous availability of  exact decoded time 
information with no host processing overhead. The  module has a real time clock that 
recalibrates itself against the Atomic  Radio Time Signal.
 
    I have used these modules 1,800 miles from the  transmitter and they 
worked reliably. At ranges over about 500 miles, the  ferrite aerial should be 
horizontal, mounted perpendicular to the direction of  the transmitter and it can 
be an advantage to mount the modules high up and  external to the building, 
facing in the direction of the transmitter. I  tried a variety of situations to 
try to make the system fail. Failures were all  due to the presence of strong 
interfering radio signals and not to signal  strength / internal noise 
problems. The decoder modules need about 4 mins  of clear signal to synchronise 
initially, but can update hourly in less than 90  sec. I connected a piezo 
earpiece to the receiver output and listened for  any interfering signals or changes 
in the output pulse rate. I also measured the  voltage on the AVC capacitor, 
min 0.7 V up to the normal range of 0.9 to 1 V, as  a logarithmic indication of 
the signal strength when selecting reception  sites.
 
    The receiver + aerial need to be >6 ft away from  CRT computer monitors 
and TV sets. It didn't work well very close to radio  and TV transmitters. 
Steel framed buildings and Al foil / corrugated iron  covered roof spaces showed 
low signal levels. Roof spaces can suffer from  very large temperature 
variations. The radio signal was still received OK, but  the presence of utility power 
wiring allowed the pickup of RFI. Local  lightning can prevent clear 
reception. Local electric arc welding may also  give problems.
 
    The EM2S receiver would work without problems in  poor signal locations 
where my LW AM radio was seriously effected by  internal noise. You can buy 
(borrow?) battery clocks which give  'Atomic Time' eg Oregon Scientific. These 
work off the WWVB signal  and those with LCD displays have a radiating aerial 
mast display with four  levels indicating the signal strength at the last 
update. My clock updates every  hour. I have no financial or other connection to 
Galleon Systems.
 
    If you look at 
_http://www.boulder.nist.gov/timefreq/stations/wwvbtimecode.htm_ (http://www.boulder.nist.gov/timefreq/stations/wwvbtimecode.htm)  you  
will see that the 59th and the 1st second of WWVB signal both start with 800 
mS  low pulses. It should be quite easy to set up a dual retriggerable 
monostable  multivibrator to detect this and to give high precision minute timing and 
/ or  second timing pulses.  
 
    Larry, I would like to suggest that you give  consideration to providing 
the 16-bit Serial Output A/D Board for WinSDR with an  option to receive and 
fully decode WWVB signals, or the ability to read the  output of a MCM 
microcontroller module. I have read your note, dated 1998, on  WWVB signal reception. 
    I note that the serial board can currently use WWV  minute tone decode 
signals, but you state that 'You will not get 24 hour  reception on any one 
channel, as long as you can get 4 to 6 hours per day will  be fine. At my location 
I get best reception on 5.0Mhz at night and during the  day 10.0Mhz or 
15.0Mhz.' 
    Does the timing for SDR originate on the board, or  is it dependant on 
the computer clock, please? My computers can NOT keep time  over a timing break 
of maybe 12 hours, to better than about 2 sec. This is  certainly NOT good 
enough for seismic work! Unlike WWV, WWVB has the  potential to provide accurate 
timing signals over the full 24 hrs for most  places in the USA. The extreme 
range daytime WWVB signal is certainly a lot  weaker than the nightime signal, 
but with the possible exception of Maine,  it should be satisfactory
    The ability to fully decode the WWVB signal could  cope with the 
situation of a power outage. It wasn't until I bought a 60 KHz  radio corrected 
digital clock that I realised just how bad my computer timing  systems were! Another 
reason for doing this is to provide a timing  system totally independant of 
the www. There are already  predictions of future communications problems on 
the web. 
 
    The total cost could well be about that of  just a GPS ANTENNA !
    
    Regards,
 
    Chris Chapman





Hi there,
 
    I have inspected six new computer motherboards.= =20 They all appeared to be using the miniature cylindrical 32,768=20 Hz watch crystals. You can get AT cut crystals in these cases=20 but watch crystals are the commonest. 32 KHz Watch crystals have a=20 parabolic error plot with temperature, peaking at about 25 or 30 C, +/-5 C.=20= The=20 coefficient is ~ 0.04 x (Cdiff)^2 ppm. If it is a 30 C crystal and=20 the temperature falls to 5 C, you can expect to get 25 ppm drift - abou= t 2=20 sec per day.     
    However, the time loss errors shown on my=20 newish computers are very considerably above this, so I suspect that this mu= st=20 be due to changes / errors in the counted interrupt rate. On older=20 computers the disk drive R/Ws could override the regular interrupt timing. T= he=20 time update options given in Windows do NOT seem to "discipline" the rate lo= ss=20 of the time system. Some other programs can do this.
 
    Most of the USA can receive the WWVB 60 KHz tim= ing=20 signals. The coverage is shown on http= ://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm Galleon=20 Systems at
http://www.ntp-time-server.com/=  =20 produce a range of timing products, from complete time server units to separ= ate=20 module boards.
    The receivers use a 60 KHz tuned ferrite=20 aerial, costing $2.68.
    The EM2S receivers cost $10.89. They have a nor= mal=20 operating range of 3 to 12 V DC. They use a crystal filter to get a high=20 stability narrow band response and have an AVC circuit to optimise=20 reception for changes in signal strength.
    The MCM-RS232 Microcontroller Module costs $15.= 89.=20 It operates off 3 V DC. When combined with the EM2 Receiver Module and the=20 Antenna, it provides time information in standard RS232 data format via a se= rial=20 interface. External buffering to full RS232 levels is required using a=20 proprietary RS232 level shifter IC. The advantage of the module compared to=20 direct host decoding of the receiver output is the continuous availability o= f=20 exact decoded time information with no host processing overhead. The=20 module has a real time clock that recalibrates itself against the Atomi= c=20 Radio Time Signal.
 
    I have used these modules 1,800 miles from= the=20 transmitter and they worked reliably. At ranges over about 500 miles, t= he=20 ferrite aerial should be horizontal, mounted perpendicular to the direction=20= of=20 the transmitter and it can be an advantage to mount the modules high up and=20 external to the building, facing in the direction of the transmitter. I= =20 tried a variety of situations to try to make the system fail. Failures were=20= all=20 due to the presence of strong interfering radio signals and not to signal=20 strength / internal noise problems. The decoder modules need about 4 mi= ns=20 of clear signal to synchronise initially, but can update hourly in less than= 90=20 sec. I connected a piezo earpiece to the receiver output and listened f= or=20 any interfering signals or changes in the output pulse rate. I also measured= the=20 voltage on the AVC capacitor, min 0.7 V up to the normal range of 0.9 to 1 V= , as=20 a logarithmic indication of the signal strength when selecting reception=20 sites.
 
    The receiver + aerial need to be >6 ft away=20= from=20 CRT computer monitors and TV sets. It didn't work well very close to ra= dio=20 and TV transmitters. Steel framed buildings and Al foil / corrugated ir= on=20 covered roof spaces showed low signal levels. Roof spaces can suffer fr= om=20 very large temperature variations. The radio signal was still received OK, b= ut=20 the presence of utility power wiring allowed the pickup of RFI. Local=20 lightning can prevent clear reception. Local electric arc welding may a= lso=20 give problems.
 
    The EM2S receiver would work without problems i= n=20 poor signal locations where my LW AM radio was seriously effected by=20 internal noise. You can buy (borrow?) battery clocks which give=20 'Atomic Time' eg Oregon Scientific. These work off the WWVB signal=20 and those with LCD displays have a radiating aerial mast display with f= our=20 levels indicating the signal strength at the last update. My clock updates e= very=20 hour. I have no financial or other connection to Galleon Systems.
 
    If you look at http= ://www.boulder.nist.gov/timefreq/stations/wwvbtimecode.htm you=20 will see that the 59th and the 1st second of WWVB signal both start with 800= mS=20 low pulses. It should be quite easy to set up a dual retriggerable monostabl= e=20 multivibrator to detect this and to give high precision minute timing and /=20= or=20 second timing pulses.  
 
    Larry, I would like to suggest that you give=20 consideration to providing the 16-bit Serial Output A/D Board for WinSDR wit= h an=20 option to receive and fully decode WWVB signals, or the ability to read the=20 output of a MCM microcontroller module. I have read your note, dated 1998, o= n=20 WWVB signal reception.
    I note that the serial board can currently use=20= WWV=20 minute tone decode signals, but you state that 'You will not get 24 hour=20 reception on any one channel, as long as you can get 4 to 6 hours per day wi= ll=20 be fine. At my location I get best reception on 5.0Mhz at night and during t= he=20 day 10.0Mhz or 15.0Mhz.'
    Does the timing for SDR originate on the board,= or=20 is it dependant on the computer clock, please? My computers can NOT keep tim= e=20 over a timing break of maybe 12 hours, to better than about 2 sec. This is=20 certainly NOT good enough for seismic work! Unlike WWV, WWVB has t= he=20 potential to provide accurate timing signals over the full 24 hrs for most=20 places in the USA. The extreme range daytime WWVB signal is certainly a= lot=20 weaker than the nightime signal, but with the possible exception of Mai= ne,=20 it should be satisfactory
    The ability to fully decode the WWVB signal cou= ld=20 cope with the situation of a power outage. It wasn't until I bought a 60 KHz= =20 radio corrected digital clock that I realised just how bad my computer timin= g=20 systems were! Another reason for doing this is to provide a timing= =20 system totally independant of the www. There are already=20 predictions of future communications problems on the web.
 
    The total cost could well be about that of=20 just a GPS ANTENNA !
    
    Regards,
 
    Chris Chapman

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