The Cathode Ray Tube Many logic analysers and some DSOs use magnetically deflected c.r.t.s either as monochrome or colour. This is the type of display technology used in TV sets.
In the c.r.t. storage oscilloscope, the cathode ray tube is basically similar to the electrostatistically deflected type of tube described below; but with the addition of one or more storage meshes.
The cathode ray tube is the main component of an oscilloscope. A cathode ray tube consists basically of an electrode assembly mounted in an evacuated glass vessel. The electrodes perform the following functions:
The potential at the focus electrode is adjusted to obtain a very small round spot on the end of the tube. Unfortunately, if no other control were provided, it would often be found that the focus control setting for minimum spot width was different from that for minimum spot height. This is avoided by providing an astigmatism control. In the case of a simple cathode ray tube this consists of a potentiometer that adjusts the voltage on the final anode and screen relative to the deflection plate voltages. Alternate adjustments of the focus and astigmatism controls then permit the smallest possible spot size to be achieved. With more complicated tubes using a high post deflection acceleration ratio another electrode is often needed. This is a 'geometry' electrode and is connected to another preset potentiometer, which is adjusted for minimum 'pincushion' or 'barrel' distortion of the display. When an electron beam passes between two horizontal plates that have a potential difference of V volts between them it is deflected vertically by an amount:
where
L = Length of the plates
D = distance between the plates and the point on the axis where the deflection is measured
d = distance between the plates
Va = acceleration voltage applied to the beams at the level of the plates
K = a constant relating the charge of an electron to its mass
Brilliance or intensity modulation (also called Z modulation) is obtained by the action of a potential applied to the cathode or grid hat controls the intensity of the beam. Generally, a change of 5V will produce a noticeable change of brightness, while a swing of about 50V will extinguish a maximum intensity trace. The beam is normally extinguished during 'flyback' or retrace; by means of an auxiliary 'blanking' electrode, which can deflect the beam so that it no longer passes through the deflection plates and hence does not reach the screen.
TUBE SENSITIVITY
The deflection plates of a c.r.t. are connected to amplifiers, which can be relatively simple design when the required output amplitude is low; it is therefore desirable for the tube sensitivity to be as high as possible. To enable the amplifier to have a wide bandwidth, the capacity between the plates must be kept low, so they must be small and well seperated. On the other handm in order to obtain a suitably clear trace of a signal with low repitition frequency (or single shot) the energy of the beam must be high. But the ideal tube must be:
Short (not cumbersome) : D small
Bright (high acceleration voltage) : Va large
And with low acceleration deflection-plate capacity: L small, d large.
This gives the tubes with very low sensitivity, considering the formulae:
The requirements for high sensitivity contradicts the terms of the equation. Practical cathode ray tubes are therefore the result of a compromise. However, techniques have been developed to improve a selected parameter without prejudice to the others.
Post deflection acceleration (p.d.a) is one of these; To improve the trace brightness while retaining good sensitivity, it is arranged that the beam passes through the deflection system in a low energy condition; post deflection acceleration is then applied to the electrons. This is achieved by applying a voltage of several kilovolts to the screen of c.r.t.
Spiral p.d.a is a development of the basic p.d.a technique, and consists of the application of the p.d.a. voltage to a resistive spiral (500M Ohms) deposited on the inner tube surface between the screen and the deflection system. The unformity of the electric field is improved, which reduces distortion. In addition to the effect of the p.d.a. field between the deflection plates is weaker, so the loss in sensitivity caused by this field is reduced.
The use of a field grid, avoids any reduction in sensitivity caused by the effect of the post deflection acceleration field. A screen is interposed between the deflection system and the p.d.a; this makes the tube sensitivity independent of the p.d.a, a significant benefit. The screen must, of course, be transparent to the electrons and is formed from a very fine metallic grid. With this system we reach the domain of modern cathode ray tubes.
The next development is the electrostatic expansion lens. By modifying the shape of the field gird it is possible to create, with respect to the other electrodes, an electric field that acts on the beam in the same way as a lens acts on a light beam. It is therefore possible to increase the beam deflection angle, for example by a factor of 2 which improves the sensitivity by the same amount.
The field can also be formed by quadripolar lenses.
For example, if the sensitivity of a spiral tube is 30 V/cm in the X-axis and 10 V/cm in the Y axis, then the sensitivity of a lens fitted tube, for the same trace brightness, may be 8V/cm in X and 2V/cm in Y or even better.
To improve the sensitivity by modifying the deflection system it is necessary to do one of two things:
Reduce the distance between the plates, increasing the capacity between them; in addition it must be possible to deflect the beam without it striking them.
Lengthen the plates, again increasing the capacity, however, the transit time involved limits the application of this idea.
The transmit time is the time taken for an electron to pass through the deflection system