[time-nuts] WWVB New Modulation scheme...

Burt I. Weiner biwa at att.net
Fri Oct 26 00:46:41 EDT 2012


If I sent these thoughts before, pardon me as I'm suffering from CRS Syndrom.

Part of the normal work I provide is off-air measurements of 
broadcast stations.  Would you believe, they pay me to do 
this!  Anyway, I measure AM carriers, FM carriers, and TV, both 
analog and digital pilots, as well as other stuff.  For FM stations I 
also measure any subcarrier including the RDBS signal at 57 kHz.

My method for measuring is described 
Here:  http://www.k5cm.com/k6OQK%20FMT%20NEW.htm  The methodology 
described here is used for primarily for AM Broadcast signals, but 
can be used for FM subcarriers also.

Some time ago I tried measuring the RDS signal in the broadcast FM 
baseband  With my trusty 3586B looking at the baseband of my FM 
receiver I tried tuning the 3586B SLM to 57 kHz and looked at the 
resultant 15625 Hz I.F. out of the 3586B as described in the above 
article.  It was a useless mess.  Due to the nature of the 57 kHz RDS 
modulation, which is not FM, I just couldn't get a meaningful 
measurement.  Rats!

One day while poking around and looking at the 57 kHz RDS signal, 
rather than using 20 Hz bandwidth, which is really about -3 dB at 6 
Hz, I used the 3.1 kHz bandwidth position.  Lo and behold, I now had 
a very well defined display that looked very similar to the eye 
pattern used for examining digital transmissions.  I was allowing 
both sidebands into the 3586B and was now able to measure down to 
milliHertz resolution with no problem.  All I had to do was adjust my 
3336B (read the above linked article) to get the pattern to stand 
still as either a circle or and X on the scope.

I still need to dig into this a bit farther because the RDS signal 
really does not have a carrier as such.  It has the two sidebands 
depending on the phase shift determined by the data.  However, I do 
know that RDS generators that are GPS referenced give me a stationary 
pattern when I'm tuned to 57000.000 Hz.  The RDBS signal is supposed 
to be locked to the third harmonic of the 19 kHz pilot, which has a 
+/- 2 Hz tolerance.  The truth is that they're not always locked, and 
I know of several stations that do not feed a Pilot reference to the 
RDBS generator.  FM broadcast RDS injection levels are typically 
about 2-5 percent of +/- 75 kHz deviation, but it is not a FM signal 
when you look in the FM baseband.  Anyway, just a thought that maybe 
if you look at the WWVB signal with a wider bandwidth it might be 
interesting.  I really need to take the WWVB loop off of my 
Symetricom 8170 and hook it to my 3586B SLM and see for myself.

For what it's worth,

Burt, K6OQK


Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
K6OQK 





More information about the time-nuts mailing list