The Multiplier Oscilloscope
One of the latest oscilloscope features is the multiplication of signals. With this feature, it is possible to study instantaneous power. For instance, during the switching transients in logic circuitry, the collector voltage can be seen as a function of time. Also, the collector current can be shown on the screen. The product of these parameters is then a measure of the collector dissipation. But, it is difficult to study the instantaneous power from the screen. For this, the analog multiplier provides a solution.
Analog multiplication is by no means new. In early days, it was performed in analog computers with the aid of operational amplifiers equipped with tubes. Hence, the circuitry suffered from drift. The principle was simple: quarter-square multiplication. Both the sum and the difference of the signals VA and V8 to be multiplied were squared (by diode/resistor networks), then subtracted and scaled according to
(VA + VB) 2 – (VA – VB) 2 = 4VAVB
Other techniques were also used.
The introduction of monolithic integrated circuits eliminated most of the imperfections of the older multiplier. Use is made of the fact that matched pairs of transistors can be produced easily with a high degree of equality. The type of multiplier discussed in the following paragraphs is the type used in the Philips range of oscilloscopes. The heart of the multiplier is the gain cell. For this circuit, it can be mathematically derived that the current ratio
/ 1 1 + a
/ 2= 1 – a
Thus, the ratio of the collector currents / 1 and / 2 is determined by the ratio of the external currents. Hence, the term current-ratio multiplier is often met. It should be noted that the ratio is independent of the bias current because it does not appear in the above equation.
At the front panel of the oscilloscope, two new control knobs OXA and OXB are present. These controls are used to balance out any dc components present at the multiplier inputs. These dc components may come from the input preamplifiers. Because of the special drift compensation circuits in these amplifiers, these balance controls seldom need to be readjusted.
The incorporation of the multiplier into an oscilloscope may be realized, which applies to the Philips oscilloscope PM 3265. The electronic switch in channel A is in the A x B mode. The channel A signal is fed to the multiplier as is the channel B signal, and the output of the multiplier is fed to the electronic channel selector. By means of this selector, the display at the screen can be:
- A only
- B only
- A and B alternate
- A and B chopped
- A + B (position ADD)
- A x B (position MULTIPLY)
- A x B and B chopped or alternate
In this last position, the chopped or alternate mode is dependent on the MTB TIME/01v setting. From 0.5 s to 2 ms it is automatically switched to the chopped mode, and from 1 ms to 2 µs it is in the alternate mode. The upper position of the switch the channel A signal is fed directly to the channel-mode selector and the multiplier is not in use. The output of the channel selector is fed to the delay line and from there to the CRT vertical deflection plates via the final Y amplifier. A second output from the channel-mode selector leads to the composite signal amplifier. This amplifier provides two signals:
- The product A x B to a BNC rear connector (Y OUT) of the oscilloscope. It may be used for other purposes.
- A composite trigger signal for the MTB. Typical applications can be found in power measurements of switching transients in digital circuits, but also in switched sine-wave thyristor control, phase detection, frequency doubling, etc.