Time Passes 1

“Time Passes, but will You?” – Graffito on university lavatory wall.

Time does seem to just go on and on, and there is nothing much we can do about that. There are some restricted senses in which we can make it look as if the pace at which time passes varies. “Look as if” that is, to a particular circuit.

I have two examples. One clever one which was not thought up by me, and another one which was not quite as clever as it looked, which was thought up by me. We will look only at the first one in this post.

I was leading a small development team working on a driverless towing tractor system. These battery electric towing tractors followed a wire set in epoxy in a slot in the concrete warehouse floor. The “guidepath” was the magnetic field around the wire from a 6.25kHz sine wave current. Part of the system was that two way communications were required between the controller on the tractor and the central system controller. The communications scheme chosen for the communication from the tractor to the central control (We called this the “Up” direction) was phase reversal keying of a carrier at (if I recall correctly)  15.625kHz, which was obtained at the transmitter by multiplying the guidepath frequency by 2.5 with a phase locked loop. The transmit modulator was an exclusive OR gate. I was fortunate to engage Roger Riordan, a consulting contractor at the time. Roger is famous for having invented a gyrator (Google “Riordan Gyrator”) and will be known to some readers as the person who set up and ran the business which offered the “VET” anti-virus software for many years. Roger was assigned the task of designing a transmitter to impress the signal onto the guidepath wire. His scheme was to use a winding around a ferrite rod of the type used for a.m. radio reception. The arrangement was roughly as shown here.

In this sketch, “1” is the guidepath wire set in epoxy in a slot about 4 mm wide and 10 mm deep cut in the floor. I do not recall how many turns were wound on the rod, but the full winding had a capacitor across and resonated with a high Q. The drive was by way of a tap near the bottom end of the coil.

All that is just background. Now for the real story. As the Q of the tuned circuit was high, it was necessary to tune it. I do not recall the capacitor value, but it might have been 470nF or so. Certainly far too large to make much impact with a trimmer capacitor. Roger’s trick was to make the tuned circuit “pause” for a short while each cycle. In this way, an LC combination with a resonance period that was a little short, could have that period extended to match the excitation frequency. I used to like to say that “Time stands still for the resonant circuit”.

Consider this circuit, which we will imagine for the moment to be made of ideal components.

This seems to be a funny place to put a switch, and one would imagine that closure of the switch would be a disaster. It would discharge the capacitor pretty promptly! The trick is, that the switch is closed when the capacitor has no charge anyway: at a voltage zero crossing. When the switch is closed, the inductor current flows through the switch, and as we are considering ideal components, this current flows forever, until the switch is opened, when the resonance of the L and the C will go on just as if it hadn’t been interfered with. If we use real world components, there will be losses, but if the switch is not kept closed for too long, the loss will not be too great. The picture below shows the voltage waveform that we get with a short duration switch closure.


The time tD is added to the natural period of the LC to give a new “synthetic” resonant period. The transmitted waveform will look like this:

The delay time was only short: just long enough to ensure that the entire production run could be tuned to the transmit carrier frequency. The distortion of the waveform was of no importance in this application.

Here is a block diagram of the scheme. There could well be records extant with more details, but they are not to hand as I write this.

I have corresponded with Roger recently about this clever trick, just to make sure that I should not have been giving someone else credit for the idea. Roger tells me that it was all his own. He also played a major part in the receiver design. There were many troublesome aspects to that project, but once we worked out what phases to expect the signal at at the receiver, this communications channel facility was a trouble free design.

I will come to the other example of the interfering with the passage of time for a circuit in another posting.


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