Connecting the CRO to a Circuit

The CRO like any other measuring device, is useless if there is no method available to transfer the signal we wish to measure to the CRO itself.
As far as the CRO is concerned there are three types of cable that can be used to connect the CRO to the circuit of which we wish to take readings. These are:

Coaxial cable is the most convienient connection device for taking readings.
As the diagram below shows, one end of the cable has a BNC connector which fits directly onto the input of the CRO.
While the other end splits into both a Red Positive and Black Negative cable.

The above cable due to it's thickness contains a great deal of Stray Capacitance, this in turn causes the distortion of extremely small signals.

Things to Remember About Coaxial Cable
Always connect the Negative cable (Black) to the GROUND point of the circuit. This type of cable is not useful for very small signals (around 1mV). The most convienient of the above types of cable as we only require one cable for each input channel of the CRO.

This type of cable differs from both the Coaxial and the X10 probe in two ways:
1. We require two 3mm cables for each CRO input (one positive and one negative).
2. The 3mm cables cannot be connected directly to the CRO, this means that we require the use of a BNC to 3mm adaptor. This adaptor is shown below on the right of the 3mm cables.

When we look at the diagram above we see that the adaptor has two terminals. The terminal marked as (-) in BLACK is the Negative Terminal and should always be connected to the GROUND node of the circuit we are taking readings from.
Now the other terminal marked as (+) in RED is the Positive Terminal and should be connected to the node at which we wish to find the voltage.
Finally the BNC connector needs to be connected to the CRO input itself.

Here we have the most sophisticated of the connection devices, it is really only used when we require very accurate measurements.

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Revised: Friday, 17 July 1998