I'm setting up an electronics lab so I can increase my knowledge of electronics. I've bought some equipment of ebay ('scope, power supply, signal generator etc.,) but don't know what sort of BNC cables I should get.

Do I need to worry about impedance when connecting the equipment together and buy (for example) 50 ohm cables or would straight through work OK?

## Best Answer

You seem to have the false impression (see my highlight in the above quote) that a 50 ohm cable will incorporate 50 ohm resistors. A 50 ohm cable (coax or otherwise) is telling you about the impedance it will present when using RF signals (greater than 1 MHz) and has nothing to do with real resistance values measured using a multimeter (for example).

All cables have a characteristic impedance (including "straight through" cables) and its impedance is represented by the formula: -

$$Z_0 = \sqrt{\dfrac{L}{C}}$$

Where L is the distributed inductance per metre and C is the distributed capacitance per metre. If you looked at some cables in their data sheets you might see typical values such as: -

Take the square root of the ratio and you get 50 ohms (nothing to do with actual resistive components).