Electrical indicating and test instruments

 6.3 Cathode ray oscilloscope

The cathode ray oscilloscope is probably the most versatile and useful instrument available for signal measurement. In its basic form, it is an analogue instrument and is often called an analogue oscilloscope to distinguish it from digital storage oscilloscopes which have emerged more recently (these are discussed in section 6.4). The analogue oscilloscope is widely used for voltage measurement, especially as an item of test equipment for circuit fault-finding, and it is able to measure a very wide range of both a.c. and d.c. voltage signals. Besides measuring voltage levels, it can also measure other quantities such as the frequency and phase of a signal. It can also indicate the nature and magnitude of noise that may be corrupting the measurement signal. The more expensive models can measure signals at frequencies up to 500 MHz and even the cheapest models can measure signals up to 20 MHz. One particularly strong merit of the oscilloscope is its high input impedance, typically 1 MΩ, which means that the instrument has a negligible loading effect in most measurement situations. As a test instrument, it is often required to measure voltages whose frequency and magnitude are totally unknown. The set of rotary switches that alter its timebase so easily, and the circuitry that protects it from damage when high voltages are applied to it on the wrong range, make it ideally suited for such applications. However, it is not a particularly accurate instrument and is best used where only an approximate measurement is required. In the best instruments, inaccuracy can be limited to ±1% of the reading but inaccuracy can approach ±10% in the cheapest instruments. Further disadvantages of oscilloscopes include their fragility (being built around a cathode ray tube) and their moderately high cost.

The most important aspects in the specification of an oscilloscope are its bandwidth, its rise time and its accuracy. The bandwidth is defined as the range of frequencies over which the oscilloscope amplifier gain is within 3 dBŁ of its peak value, as illustrated in Figure 6.11. The -3 dB point is where the gain is 0.707 times its maximum value.

* The decibel, commonly written dB, is used to express the ratio between two quantities. For two voltage levels V₁ and V₂, the difference between the two levels is expressed in decibels as 20 log10 (V₁/V₂). It follows from this that 20 log10 (0.7071) = -3 dB

 In most oscilloscopes, the amplifier is direct coupled, which means that it amplifies d.c. voltages by the same factor as low-frequency a.c. ones. For such instruments, the minimum frequency measurable is zero and the bandwidth can be interpreted as the maximum frequency where the sensitivity (deflection/volt) is within 3 dB of the peak value. In all measurement situations, the oscilloscope chosen for use must be such that the maximum frequency to be measured is well within the bandwidth. The -3 dB specification means that an oscilloscope with a specified inaccuracy of ±2% and bandwidth of 100 MHz will have an inaccuracy of ±5% when measuring 30 MHz signals, and this inaccuracy will increase still further at higher frequencies. Thus, when applied to signal-amplitude measurement, the oscilloscope is only usable at frequencies up to about 0.3 times its specified bandwidth.

 The rise time is the transit time between the 10% and 90% levels of the response when a step input is applied to the oscilloscope. Oscilloscopes are normally designed such that:

                                                Bandwidth × Rise time = 0.35

Thus, for a bandwidth of 100 MHz, rise time = 0.35/100 000 000 = 3.5 ns.

An oscilloscope is a relatively complicated instrument that is constructed from a number of subsystems, and it is necessary to consider each of these in turn in order to understand how the complete instrument functions.