PSN-L Message - Subject: Computer Timing Problems / Solutions

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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