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Signals and Circuits

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I was given an unwanted oscilloscope and to test it I bought a function generator kit from Maplin. I built it and connected an oscilloscope test probe to the output terminals of the generator and a nice sine wave appeared.

The test probe consists of a BNC plug at the oscilioscope end and a red positive and black negative clip at the other end. I assume the red connects to the centre pin and thus the central wire in the coaxial cable whereas the black connects to the shield part of the coaxial cable.

The terminal block on the generator has two outputs, and are not indicated as negative or positive. Whichever way I connect the test probe, I get a signal. Fine

Qu 1: Why is there two connections on the signal output?

However, If I unclip the black clip, thus removing the connection to the shield part, I still get a signal, however it is roughly half the amplitude.

I always though, that BNC / coaxial connections simply send a signal. I.e. the core and shroud aren’t used to complete circuits. This seems to be the case as a signal appears on the oscilloscope if only the core of the coaxial (via the red clip) is connected to either side of the terminal block.

Qu 2: Why therefore is the signal halved?

Qus 3: When you use a BNC to send a signal from one device to another (for example a camera to a video recorder), surely you aren’t creating a complete circuit between the two devices? If you removed the outer shroud, wouldn’t it just make it more probe to interference? The video signal would not be reduced would it?

I’m baffled.


Dear baffled, in fact the shield on a coax does complete the circuit connect between the signal sender and signal receiver. To illustrate this principle try this simple experiment. Take an ordinary battery, the type does not matter. Connect the 'red' lead of your oscilloscope to the postive terminal of the battery. You would not normally see any deflection in the trace of your oscilloscope. Now, connect both the red and black leads to your battery. Now, you should see the trace deflect in proportion to the battery's voltage. Reverse the connections, and observe that the deflection is the negative of what it was before.

The reason you can see a signal from your signal generator is because the return path that should have been provided from the coax shield is provided by your house wiring..... somewhere. Both of your intstuments are plugged into your house mains wiring, and getting the reference from the house wiring system. Due to the various uncontrolled impeadances of your house's system, the resultant signal you observe is 1/2 what you would normally expect.
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What is the frequency of the signal you were observing from the function generator? A high frequency signal can find a return path through the capacitance of the circuit to ground.

You always have to have a complete circuit with a return path for a signal. With a coax cable, the shield does indeed provide the return path (as well as providing shielding for the center conductor).
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