Workshop GPS Reference

Hello everyone. For a few years now I have had Trimble Thunderbolt GPS Antenna and Receiver. I used this to provide an Atomic Standard 10Mhz reference from the GPS (Global Positioning Satellites) flying around the earth to calibrate my frequency counters here in the workshop. I got involved in GPS as a reference source when we had a problem at work nearly 30 years ago. Broadcast TV stations use high quality synchronising pulse generators (SPGs) to provide a stable, in the day, black and burst reference signal that all the TV stations equipment locks to. At the company I work at we had two sites, West London and Southampton. When we where analogue transmission then and the remote Southampton site locked their SPGs to one of our West London incoming analogue video channels. Below is my GPS antenna and receiver.

All this worked fine. Every so often our master West London SPG's would be checked for frequency accuracy against a couple of stations that had higher quality Rubidium standard oscillators in their stations. Most company's master oscillators where temperature controlled quartz as ours were. Our master SPG's for those who would like to know were Tektronix units, an SPG271 and a TSG271 into a changeover switch in case of an SPG failure. When we went from analogue transmission using fm modulation, single channel per satellite transponder to digital transport streams, multiple cannels per transponder, this is when we ran into a problem. Some transport streams being generated in West London and some in Southampton off two different SPG systems now not locked to each other anymore due to the loss of the analogue video feeds. Your digital decoder in your set top box works using a 27Mhz reference clock derived from the channel multiplex you are receiving it is quite clever as it uses an open loop control system but that is another story. For those who know, a multiplex stream from London when arriving at Southampton would then be re-multiplex with services from the local Southampton encoders, which is where the problem was being generated, gets quite complicated. For those thinking why not maintain the analogue video line? These where provided by BT, just about the only provider in those days. One analogue line from London to Southampton cost £2m pounds a year to rent and this was in 1990. Not really an option. This was the problem I was given to look at. I knew these up and coming cellular masts that where now appearing everywhere had some sort of common reference system to lock them together using the new GPS positioning satellites. I obtained the name Rapco that made these oscillator units and after contacting them borrowed a couple of units. These gave a 10Mhz reference out, and used the Trimble Thunderbolt Antenna and receiver. This unit also gave 10Mhz out so I didn't need the Rapco units just the Trimble antennas and receivers. The Tektronix SPGs could lock to a continuous 2, 5 or 10Mhz sinewave. So a Trimble receiver at London and one at Southampton did the job! These units where £1500 each, much cheaper than a BT line. This kept the two different SPG systems locked the same as each other. For those who know, no phase adjustment but not wandering relative to each other and this solved our problem. We did have a problem with all the Tektronix SPG's locking to a CW reference, you could pull them up in frequency if you where low but if you where high in frequency they would go even higher. Tektronix had to fix this one way phase lock problem as it was a design fault in their equipment, although I had to install and test the fixes, one SPG at a time, easier when you have an SPG changeover switch. The Trimble GPS unit I have used is one of these original antenna/receivers that where used for the above implementation.

As time moved on a firm called Leitch came up with this new "GPS" disciplinable SPG which we moved to. These where extremely good as you where guaranteed to be within +/-50nS of each other at two different locations, anywhere in the world. These independent units could now be phase adjusted to each other, back almost to analogue locking standards but with no physical wire between them. Our next implementation of SPGs was from a firm called Evertz with their MSC-5600 GPS disciplined SPGs and at present we are using their second generation MSC-5601 SPGs now moving over to their new third generation MSC-5700 GPS disciplined SPGs as these have signals that comply with the new broadcast 2206 and 2110 ip carried video/audio signals. That will give you plenty of reading to do if you are interested in this.

Anyway back to the workshop. I still had a Leitch 1601 SPG at work in my work bench rack which I used for testing Synchronisers now no longer required so it has come home with me and I decided to put this in my local equipment rack here in the workshop. I have wired the 10Mhz, 1PPS coaxes up to this SPG and added the RS232 delivered Coordinated Universal Time (UTC) from the Trimble receiver. The GPS antenna is connected via an F-type 75ohm CF100 coax. Here is the result, all GPS locked.

Happy to answer any questions on this. I have worked on Broadcast Television Stations reference systems for over 40 years. Considering I started off as a self employed Television Engineer moving on to domestic video and then into broadcast. I remember being in my domestic television workshop one day sat at the bench, listening to Radio 1 in those days, I was 20, and wondering "I wonder how they connect the turntable to the mixer, what kind of lead do they use? phono's?" never mind all the things like levels and impedances. After a year had passed there I was on my BBC Engineering TA course at the BBC Engineering training centre at Wood Norton. The rest is history.

A bit off topic but hope you enjoy.