THE TIMEBASE

Introduction

Before we start it would be as well to define exactly what waveform we are after. First we have the region marked as 'S' in the diagram opposite which is the portion that scans the CRT. Here we need a linear ramp, preferably of quite high voltage swing for driving the CRT plates. Secondly, we have the flyback period 'F' whose time we strive to minimise. The third region 'I' is an idle period where we would like to wait for a trigger pulse to come along and start another ramp.

At the heart of the design of almost all oscilloscope timebases is the principle of charging and discharging a capacitor. The charging waveform is illustrated above. As can be seen the ramp produced is pretty non-linear. However if we were to only operate the circuit over a small part of the ramp (as shown circled) we can get a significant improvement in the linearity. We also need some mechanism to rapidly partially discharge the capacitor at the end og the scan portion.

1) Gas Devices

a) The Neon Bulb

Enter the neon bulb. These bulbs contain low presure neon gas with two electrodes inserted. Once a certain voltage (the "striking" voltage, VS) is reached, the neon gas ionises and the bulb conducts heavilly. Assuming the current is limited in some way to avoid destruction of the bulb, the neon maintains a constant voltage across it, VE, which is less than the striking voltage. However, in order to remain in conduction, a certain minimum current needs to be maintained otherwise the neon gas de-ionises and the bulb ceases to conduct. If we place one of these bulbs accross our basic R-C network then once the capacitor charges to VS the neon strikes, discharging the capacitor to the voltage VE. If we make resistor R large enough, the minimum current required for the neon to remain conducting can not be maintained and the neon ceases to conduct. The circuit then repeats the charging cycle and so we obtain an oscillation, as illustrated.

We now have a crude waveform suitable for use as a timebase. However it does suffer from several shortcomings :-

b) The Thyratron

The thyratron is a gas filled triode. If the valve is held cut-off by a suitably large -ve voltage applied to the grid, the thyratron behaves in a similar fashion to the neon bulb. However we are not limited to just gasses since the heater can cause the evaporation of other non-gaseous elemnets such as mercury. Mercury has a quite low extiguishing voltage which allows a much larger amplitude of oscillation compared to the neon bulb, although this must be at the sacrifice of scan non-linearity. However, the device is also basically a triode. If the voltage on the grid is raised above its cut-off voltage the triode will begin to conduct as per normal triode action. If there is sufficient voltage on the anode then this conduction can cause ionisation to occur even if the anode voltage were below the striking voltage.

The cut-off voltage of a triode is not a constant value. Its value varies with the applied anode voltage as shown in figure 5. Thus if we vary the grid voltage can can cause the thyratron to rigger when its anode voltage has reason to some value below its striking voltage. We can thus use the grid to control the amplitude of the generated waveform.
The effect is illustrated in figure 6. With the grid held very negative (V1), the waveform reaches the striking voltage 'A' as if we were dealing with a simple bulb. If we raise the grid voltage to V2 then when the anode voltage reaches 'B' the cut-off voltage becomes more negative than V2, so conduction starts due to triode action triggering ionisation.

If we now add a capacitor (CT in figure 3) we can feed synchronising pulses into the grid. These pulses are such that they raise the grid potential and should this occur near what would have been the trigger point then firing will occur synchronous with the trigger pulses. This is illustrated by the pulses added to V2 in figure 6 ; the amplitude of the first two pulses is insufficient to cause conduction as the cut-off voltage is not yet negative enough (i.t. the anode voltage is too low). The third pulse is just sufficient to cross the cut-off voltage and hence triggering occurs at point 'C' instead of 'B' as would have been the case in the absence of the trigger pulses.

Triggering, good amplitude, what a shame about the non-linearity. What we need to do is instead of driving the capacitor via a resistor we actually need to drive it with a constant current.

Enter the pentode. The pentode has a very high slop resistance, that means that its anode current is completely independant of the applied anode voltage (OK, its not perfect and this only applies at higher anode voltages). The anode current is controlled by the grid, so by appropriate choice of grid bias voltage we have in fact a constant current sink.

If we simply replace the timing resistor with this constant current generator then the cathode would be connected to the capacitor and hence vary with the ramp waveform. However, the bias voltages on Vg and on the screen need to be set with respect to the cathode which would make biasing somewhat complicated. We can get around this by turning the thyratron circuit "upside down" so that the timing capacitor now connects to HT and the timing resistor would have one end grounded. The result is shown in figure 8. Altering P1 varies the grid bias of the pentode, hence varying the current and therefore speen of the ramp. P2 generates the grid bias for the thyratron thus controlling the size of the waveform, though it should be noted that the P2 resistor chain acts as a load on the timing capacitor and hence influences the linearity.

We may have addressed the issue of linearity but there is still the issue of limited speed. The fundamenal limit is the ionisation/deionisation time of the gas, so clearly the gas must go !

NEXT PAGE : The "Puckle" Timebase