WWV Time Correction Option

With a short-wave receiver tuned to one of the WWV time signal stations 
(2.5Mhz, 5.0Mhz, 10.0Mhz and 15.0Mhz) it is possible to keep SDR's (and your 
event files) time to within +-20 milliseconds. This is done by taking the 
audio out of the receiver, usually from the external speaker or earphone jack,
and connecting to pins 36 (input signal) and 37 (ground) on the DB37 
input/output connector. 

How it works:
Every minute (except at the top of the hour) WWV broadcasts a 800 millisecond
1000Hz tone. On the A to D card is a phase locked loop tone decoder with a 
bandpass filter tuned to 1000Hz. The output of the tone detector is connected
to a input port that can be read by SDR.  SDR detects this 800 millisecond 
tone and uses it to keep track of the time. It does this by averaging several
minutes worth of pulses and a lot of digital filtering.  About 2/3 of the time
it took to write SDR has been the time keeping software code. 

Short-wave Receiver and WWV:
The short-wave receiver should be a digital type so it will stay tuned to the
WWV station you select. Radio Shack has a portable digital receiver for under
$100.00 (Model DX-375). You will probably need to have an external antenna.
This can be a long piece of wire run between two trees or on top or your roof.
For SDR to keep correct time it needs to have good reception of one of the 
WWV frequencies. 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. I 
normally keep my receiver tuned to 5.0Mhz because I get better, and longer, 
reception at night.

Startup:
Before  starting SDR you need to set the time and date on your system within 
5 seconds of local time. You can do this by listening to WWV and setting your
local time at the top of the minute. After setting the time start SDR. SDR 
will read the time from the system, and from there on, keep track of time 
using the 1 millisecond generated by the A to D card. It is important to 
remember that the only information SDR gets from  WWV is the minute mark. You
must set the system date and time to +- 5 seconds of local time for the time
to lock and have good time information for your event files. 

After starting up SDR, and connecting the audio to the A to D card, you will 
need to adjust the audio level on the receiver. First enter the Debug Mode
by pressing the D key. This mode will display the status of the time keeping 
and other debug messages. The messages are displayed on the last line of the 
display and are also written to a file called SDR.LOG. To exit the Debug mode
press the D key again.

Start with the volume control about 1/3 of the way up. At the top of the 
minute you should see the following sequence on the debug line:

06/15/96 15:39:01 Not Locked - Lock Count:1  Diff:435ms
06/15/96 15:41:01 Not Locked Count:2  Width:812  Diff:423ms Avg:432ms
06/15/96 15:43:01 Not Locked Count:3  Width:828  Diff:436ms Avg:427ms
...
The format of the line is:
  Date/Time of message.
  Not Lock Count: Number of good locks.
  Width: Width of the 800 ms pulse detected by the tone decoder.
  Diff: This is the current 800 ms pulse start offset in time from the 
        running average.
  Avg: This is the running average of the start of the 800 ms pulse.

When the Not Locked Count gets to 6 the system will be locked to WWV. This 
process may take from 6 to 10 minutes, if you have very good reception to WWV,
to several hours if  you have poor reception. If you are not getting any time
messages, and you are getting good reception to WWV, you should increase the 
volume a little. At some point on the volume control you should start to see 
the time messages. This will take some time and patients. If you can't get 
the time to lock see the trouble shooting section below.

When SDR is locked you will see the following messages on the debug line:
06/15/96 17:35:01 Lock Time:1 01:23 Locks:4/12 Diff:3ms Avg:-4ms
06/15/96 17:38:01 Lock Time:1 01:26 Locks:5/12 Diff:-10ms Avg:-3ms
06/15/96 17:39:01 Lock Time:1 01:27 Locks:6/12 Diff:-8ms Avg:-4ms
06/15/96 17:42:01 Lock Time:1 01:30 Locks:8/12 Diff:-1ms Avg:-4ms
....
The format of the line is:
  Date/Time of message.
  Lock Time: is the time in days hours minutes that SDR is locked to WWV.
  Locks: current number of good 800 ms pulses  /  last number of good pulses.
         This is changed/updated every 15 minutes.
  Diff: This is the current pulse start offset in time from the running 
        average.
  Avg: This is the running average of the start of the 800 ms pulse.

One Second Output: 
SDR produces a one second output square wave at pin 18. The rising
edge of this output marks the beginning of each second. You can use this
output to monitor how well SDR is keep time. If you have a o-scope you
can monitor this output and compare it to some other reference. One reference
can be the audio output from your WWV receiver and looking for the 1 second
tick you hear (this tick is 5 cycles a 1000hz). You can compare this to the one
second output and see how far you time is off.   

 
PC-Labs 711s Users:
It is possible to use the WWV time option with your PC-Labs card. To do
this you must build the circuit in the file TONEDET.GIF. The output of
the tone detector should go to Input Port 0 or pin 1 of the digital input
connector on the A/D card. The bandpass filter and tone detector should be 
tuned for a 1000hz tone.
 

Trouble Shooting:
 
No Time Messages:
  Make sure you are in the Debug mode. The "D" key toggles the debug mode on 
  or off.  Make sure you have the audio out (speaker or earphone output) 
  connected to the  A to D card. Adjust the volume level until you start 
  getting the time messages. Make sure you are tuned to one of the WWV 
  stations and getting good reception.

Getting Time Messages but Never Locks:
  Your system maybe losing interrupts. Make sure you removed all TSR's 
  (except the TSR needed by SDR). Your disk control maybe disabling 
  interrupts when writing to the
  disk. Try another disk control. I use IDE controls and have tried several 
  different types, all with no problems.

Please feel free to contact me if you are having any problems.

Larry Cochrane
www.seismicnet.com/contact.html