A "Local" Frequency Standard

How do you know if you are transmitting on the right frequency? How do you know when to start the bulletin? How do you correct the deviation on your rig to the new(ish) 12.5kHz channel standard?

For a great many years we would tune into WWV or ZUO and "Zero-beat" the "standard" 5.0MHz or 2.5MHz against an uncalibrated oscillator. This method would be good to a few Hertz and would usually hold for a few days on H.F. On VHF and UHF the calibration would be several tens of Hertz out due to frequency multiplication. Until the advent of crystal controlled digital frequency counters this was pretty much the only method available to a radio amateur.

Most modern counters have a stable and accurate reference source internal to the counter. Most bench type counters have an external connecting socket to attach a more accurate reference standard. Portable devices used to allow the user easy adjustment to the internal reference. But on the unit that Stuart showed me the other day, this was definitely not an option.

You have an extremely accurate "Frequency Standard" transmitted all around you - the SABC TV transmissions. The line scan frequency is the easiest to use as it is only 15625 Hertz. This can be 'picked up' using a loop of wire placed against the rear of the television set. This means that you don't have to open the television set. (See the above section) The frequency is accurate as it originates from a rubidium standard used inside the SABC. This "standard" frequency used to be referred to the National Standard Frequency provided by the CSIR. I was informed in the 1980's that the differerence was of the order of 10^-23 (10 to the power -23). To most of us that means that it has an equivalent accuracy.

Due to the national coverage of the SABC, this will function quite happily over most of the South Africa. This method however, has the advantage of being 'low cost' and generally available. An alternative to this method is the use of the clock frequency used internally by the GPS system. The GPS satellite network may suffer outages due to sun storm activity and availablity of satellites. Not everyone can afford a GPS either.

This 'locked' signal can then be used to 'pull' a 1 or 10 Megahertz crystal oscillator. A simple block diagram is shown below. The circuitry can be CMOS as the frequencies involved are not high. A suitable phase detector is the 4046 chip, which has been used before for phase locking radios to a crystal oscillator.

Whilst the frequency standard used by the SABC TV transmission can be traced back to the "Standard Frequency" for South Africa. Unfortunately a "Time Standard" cannot. So a clock run by the "local standard" will never lose or gain time by any perceptable amount. Unfortunately it will probably never indicate the "correct time"! Remember the old quip about the clocks that lost a second a day and the other was stopped?

[For a Wiki definition of 'time standard', go to; http://en.wikipedia.org/wiki/Time_standard]

Time Standards in Africa

So what? You may ask. In Africa the accurate time of day is not relevant. Well the measurement of time has some vital purposes. Aircraft navigation is one of them. For if you cannot measure how far you have travelled, you will need to know your speed and last known position as well as the elapsed time. Not too difficult when using a GPS system, but if the GPS is out of commission...

Intriguingly in the UK, the best selling items of a "scientific" nature are the clocks that synchronise to the 60kHz signal transmitted from Anthorn. It used to be Rugby but was moved more centrally quite recently. Unfortunately even this is way out of range for South Africa.

Our local stations ZUO and ZSC which supposedly transmit on 2.5MHz, 5.0MHz and 4.291MHz as well as 8.461kHz are either not heard or ignored for a variety of reasons. Can you remember ever hearing these stations?

[The Americans still run WWV check out the NIST Station WWV.

Go to; http://tf.nist.gov/stations/wwv.html]

Use the Internet

Of course we have the Internet these days. So you can obtain an accurate time signal from various servers on the net using the Network Time Protocol. Well you could if your firewall allowed you. With a huge number of people connecting in Europe recently, it was possible for the router manufacturers to set the time in firmware. The router can be programmed in factory to use NTP to access a "time server" when it connects to the Internet. Only problem there was that the designers chose an obscure time server at a university which had a slow and paid-for connection. The massive number of routers all over Europe that enquired the time from the server, took up all the bandwidth. Costing the university a fortune.

[There are local as well as overseas "Time Servers". The list is at;

http://www.ntp.org/

Local time servers and information; http://www.time.za.net/