FLDIGI Users Manual  3.21
Digiscope Display - WWV mode

The WWV mode is used to measure the offset of the sound card oscillator. It does this by comparing the timing loop for the sound card measurements against the clock tick signal that is transmitted by WWV and WWVH. The sampling rate for the sound card should be set to "native". The sound card samples the signal and returns the values in 512 blocks. This block sampling is what sets the basic timing mechanism for the thread that reads the sound card, sends data to the waterfall, and sends data to the modem signal processing functions. A process of filtering is used that simultaneously reduces the sampling rate. Most modern soundcards will use 44100 or 48000 as the native smampling rate. That sample rate in down converted to 1000 using a decimation in time type FIR. The resulting signal is then power detected and further filtered with a filter called a moving average filter. The moving average is very good at detecting the edge of a pulse such as the 1 second tick transmitted by WWV. This output is then displayed in a manner very similar to a FAX signal. Each scan line represents the received signal over a 1 second interval. The bright white line is the time tick. You can see a very slight slope from left to right as the signal goes from top to bottom of the display.

Open the configure dialog box to the "SndCrd" tab. You are going to be adjusting the "Rx corr Rate" while you observe the effect of this control on the slope of the time tick line.

Tune in WWV or WWVH on 2.5, 5.0, 10.0 or 15.0 MHz in the AM mode. This seems to give the best signal view. Select the WWV modem and allow the data to begin to accumulate in the digiscope display. When you can clearly see the bright tick line, move the cursor to the bottom of the line and left click at that position. That will resync the digiscope display and put the ensuing tick marks at the center line red graticule.

Then right click anywhere in the digiscope display. That changes the zoom level to show more detail regarding the slope of the time tick line. The zoom level increases by a factor of 5. Right clicking again restores the original zoom level. I recommend making the adjustments to the Rx corr Rate control in the x5 zoom level.

If the slope of the time tick line is positive you will need to apply a negative value to the Rx corr Rate. If it is negative then a positive correction is needed.

Start with a correction of 0 ppm and observe the slope. Try a value of 1000 ppm and observe the slope. Again, try a -1000 ppm correction and observe the slope. The following are some observations made on 10 MHz WWV, DCF-77 and RWM under less than ideal conditions.

wwv3.png
WWV corrected 20 minute trace 5x scale

-1000 ppm WWV
5x scale

0 ppm WWV
5x scale

+1000 ppm WWV
5x scale
+120 ppm WWV
5x scale

0 ppm DCF-77
1x scale

0 ppm DCF-77
5x scale

+1000 ppm DCF-77
1x scale

+65 ppm DCF-77
5x scale

RWM uncorrected
1x scale

RWM
+25361 ppm
1x scale

RWM
+25361 ppm
5x scale


You can see that my sound card requires a positive correction since the slope is negative with a 0 ppm entry. The required correction of +120 ppm was determined by guessing the needed correction to be close to 1/10 of the -1000 ppm slope and then adjusting for a steady track along the red graticule. The DCF-77 images were provided by Walter, DL8FCL. The RWM images were provided by Andy G3TDJ.

You can left click on the tick line anytime you want to recenter the signal. That will aid in making your visual observation.

When you are finished, the Rx corr Rate entry is the correct one for your sound card. Save the configuration for future fldigi use.

Andy also provided information on the RWM transmissions:

RWM details extracted from http://www.irkutsk.com/radio/tis.htm

Station RWM - Main characteristics

Location: Russia, Moscow

55 degr. 44' North , 38 degr. 12' East

Standard frequencies : 4996, 9996 and 14996 kHz

Radiated power: 5kW on 4996 and 9996 kHz; 8kW on 14996 kHz

Period of operation: 24 hours per day, except 08.00-16.00 msk for maintenance as below:

on 4996 kHz : 1st wednesday of the 1st month of quater;

on 9996 kHz : 2nd wednesday of the 1st month of the quater;

on 14996 kHz : 3rd wednesday of each odd month;

Coverage: 20 degr. - 120 degr. East

35 degr. - 75 degr. North

Time signals A1X are given every second of 100 ms duration with a frequency of 1 Hz. Minute pip is extended to 500 ms.

Hourly transmission schedule

m:s - m:s

00:00 - 07:55 – MON signals (no modulation)
08:00 - 09:00 – transmitter is signed off
09:00 - 10:00 – station's identification is sent by Morse Code
10:00 - 19:55 – A1X signals and identification of DUT1+dUT1
20:00 - 29:55 – DXXXW signals
30:00 - 37:55 – N0N signals (no modulation)
38:00 - 39:00 – transmitter is signed off
39:00 - 40:00 – station's identification is sent by Morse Code
40:00 - 49:55 – A1X signals and identification of DUT1+dUT1
50:00 - 59:55 – DXXXW signals


See Transmitting Simulated WWV Timing Tone


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