Experiment 1 - Oscilloscope input resistance

Important Note!

For experiment #1 you should just use a potentiometer and some leads. There is no need to keep the circuit for the first experiment once you have finished using it. However for the later experiments you must keep and hand in your assembled circuit. Label each circuit board with your name, your bench number, and the day you are doing the lab. If you don’t hand these labelled circuits in with your book you will not be able to get any marks for the experiment. You have been warned! Hand in all the circuits with your book at the end of the series of lab afternoons.

We have provided some photographs to show you what your circuits should look like. To see the pictures for the current experiment, click on the image of a camera near this text.

Similar pictures are provided for the other experiments. Use these picturesas a guide when assembling your circuit, but remember that in some cases you may be using different actual components. The photographs are just a guide to show you how components should be laid out on the board.

Experiment 1 — Oscilloscope input resistance.

This section has two purposes. It should help you get used to your equipment. It also helps you understand the concept of input and output resistances.

Any piece of equipment which accepts input signals will require both a voltage and a current to make it work. This is because every signal must convey some energy/power — except the trivial case of the signal “0 Volts”. When you apply an input voltage to, say, an oscilloscope, it must also draw a small current to make it recognise that a signal has arrived.

The amount of current required by something to make it respond to a given voltage depends upon how it has been designed and built. We don't need to bother about these details, instead we can pretend that a resistor has been connected between its input terminal and earth, somewhere inside its box. The better a 'scope or voltmeter is, the smaller the current it needs to register a given voltage - i.e. the higher its input resistance. The 'scope will probably have an “AC/DC/Ground” switch for each input. You can force the 'scope to show where 0V is on the screen by setting this to “Ground”. Then set it back to “DC” to use the 'scope - just measure the number of divisions between where Ground is and the point on the trace whose voltage you want to measure. For most measurements, these controls should be left on “DC”. The “AC” setting is useful when you want to watch small variations of a relatively large voltage level, but it tends to alter the shape of some a.c. waves.

Build the circuit shown in diagram 3 and connect it between the power supply and 'scope as shown. By adjusting the potentiometer you can apply any D.C. voltage from 0 to +15V to the scope. Set the voltage initially to 0 and adjust the vertical position of the trace to sit on a convenient line.

Measure the input current into the scope for three or four different input voltages. You can use the 'scope itself to measure the voltage. Use these values to calculate the 'scope input resistance.

What is the significance of the order of magnitude of the 'scope resistance,

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