Analogue Oscilloscope

- each different type of oscilloscope has its own advantages and disadvantages. To choose a scope for a given application it is necessary to know what is available.

Although analogue oscilloscopes are now deemed to be old technology, they are still used in a number of areas.

Older analog scopes may be all that is available, especially as a result of cost restrictions, or for other reasons.

However, these older analogue instruments are still able to provide a good account of themselves and they should not be ignored. Often they may be put to one side in a stock of laboratory equipment. However they can still be used to good effect in many situations, some people preferring to sue them against more advanced digital scopes.

A typical analogue oscilloscope display showing the screen and trace

Analogue scope basics

The key to the operation of an analog scope is its display. It uses the cathode ray tube or CRT. This form of display was for many years the only viable form of display that could be used to display images. Accordingly it was used in television sets for many years, although other forms of display including LCDs, LEDs and many other format are now used, but these all require digital signal inputs to the display.


Note on Cathode Ray Tube:

A cathode ray tube consists of a number of elements. There is an electron gun that generates an electron beam that is fired along the length of the tube. This beam passes by deflection plates that are used to deflect the beam, as a result of electrostatic attraction and repulsion, and finally the beam hits a phosphor coating on the "screen" creating a small dot of light.


The analogue scope uses the cathode ray tube to display signals in both X (horizontal) and Y (vertical) axes. Typically the Y axis is the instantaneous value of the incoming voltage and the X axis is ramp waveform.

As the ramp waveform increases in voltage so the trace moves across the screen in a horizontal direction. When it reaches the end of the screen, the waveform returns to zero and the trace moves back to the beginning.

Diagram of the basic outline of an analogue oscilloscope showing the cicuit blocks including X and Y amplifiers, CRT display, ramp generator.
Basic analogue oscilloscope block diagram

Using this approach it can be seen that as the X axis corresponds to time, and the Y axis to amplitude. In this way the familiar plots of waveforms can be displayed on the cathode ray tube.

The cathode ray tubes use electrostatic deflection to cause the electron beam position and hence the trace position to change. This method rather than the electromagnetic deflection system used in many CRT based television sets is used to ensure that a sufficiently high speed of operation is attained. The magnetic deflection used in televisions does not enable the very high speeds needed for oscilloscopes to be achieved.

Trigger capability

In order to ensure that a steady waveform is displayed on the display, it is necessary to set the ramp waveform to start at the same point on each cycle of the incoming signal to be monitored. In this way the same point on the waveform will be displayed at the same position on the display.

To achieve this the trigger circuit is used to start the ramp. The trigger picks off the signal from the incoming signal and when a particular voltage level is reached, it starts the ramp. This trigger point is adjustable on most oscilloscopes.

More detailed block diagram of an analogue oscilloscope showong elments including the X & Y amplifiers, attenuators, sweep logic, display, etc.
Major blocks of analogue oscilloscope diagram

Blanking amplifier

A form of blanking is required to ensure that when the ramp or time-base circuit flies back to restart the trace again, it does not cause any illumination on the screen. In order to prevent this happening, a blanking amplifier is used to blank the screen during this fly-back phase. It simply takes the reset element of the ramp to generate a pulse that is applied to the grid of the cathode ray tube. This inhibits the electron flow and effectively blanks the screen for this period.

Analogue scope controls

To assist in making the trace as clear as possible, intensity and focus controls are included. The focus ensures that the dot that scans the screen remains as sharp as possible and in this way it can deliver a clear trace. The intensity control is required because the intensity of the dot or trace varies according to the speed at which the scan is made. Controlling the intensity enables a clear trace to be obtained.

When the scan is very slow the dot is seen to traverse the screen and it is difficult to visualize the waveform. As the speed increases, it ceases to be seen as a dot, but instead it traces out a line and the signal waveform, which when triggered correctly remains static on the screen. The trace may be scanned across the screen many times a second. In many instances it my traverse the screen 100 000, 500 000 or more times a second.

However as the writing speed increases, the trace becomes steadily more dim, and ultimately becomes difficult to see despite the intensity control. For higher frequency signals faster writing speeds are required, and as a result analogue oscilloscopes have a limited frequency range. Typically the maximum frequency that can be seen by an analogue oscilloscope is around 1 GHz. Above this other types of oscilloscope are required.

Advantages and disadvantages

Despite the fact that technology has moved on and digital scopes are tending to dominate the market, there are still many areas where the analogue oscilloscope can provide very valuable service.

Advantages:

  • Cost:   Analog scopes are generally much less expensive than their digital counterparts. The technology is well established and is therefore less expensive than leading edge technologies where large levels of development costs have to be recovered in addition to the component and production costs being higher
  • Performance:   Analog oscilloscopes are able to provide a good level of performance that is more than adequate for many laboratory and service situations.
  • In company availability:   It is often found that analog oscilloscopes may be available in an equipment store when all the other digital scopes are in use. Provided that their performance is satisfactory, the analogue option may provide an ideal way forwards.

Disadvantages:

  • High end performance:   In view of the way in which they operate using analogue technology, these oscilloscopes are not able to provide all the capabilities of many of the high end digital oscilloscopes.
  • Ranges available:   In view of the bias towards digital oscilloscopes, oscilloscope manufacturers and suppliers have focussed on the newer digital scopes. Accordingly the ranges of analogue scopes available are much less than they were some years ago. Nevertheless some are still available new, and others from used test equipment suppliers. Often it is possible to pick up some very good deals from used test equipment suppliers, provided that approved or trustworthy suppliers are used and the proper safeguards are in place.

Analogue storage scope

It is sometimes necessary to be able to display a signal for a period of time. One situation where this may be required is for signals that have a very long period and the normal persistence of a display would mean that the trace would decay before the whole waveform was complete. A storage facility could also be required for single shot applications where the single trace would need to be displayed over a period of time to examine the trace.

For these and many other situations, it is necessary to have a storage facility on the scope where it can display the trace for longer than would normally be possible.

Analogue storage scopes use a special cathode ray tube with a long persistence facility. A special tube with an arrangement to store charge in the area of the display where the electron beam had struck, thereby enabling the fluorescence to remain for much longer than attainable on normal displays.

These cathode ray tubes had the facility to vary the persistence, although if very bright traces were held over long periods of time, they would have the possibility of permanently burning the trace onto the screen. Accordingly these storage displays needed to be used with care.

By Ian Poole


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