# Impedance mismatch

When I connect an oscilloscope to a function generator use both oscilloscope cable (one connect to the scope and one connect to the FG), I got 700mVpp when outputing a sine wave of 1Vpp and 10kHz.

But when I use the BNC cable connects to the FG, and the scope cable connect to the scope I got 2Vpp output with the same input.

What is the exact calculation behind this?

• Some scopes have a variable gain pot built into the volts per cm switch - check that it's set up properly on both channels or use a cable that is somewhat less than 500 metres long! – Andy aka Oct 15 '14 at 20:54
• Can you clarify what you mean by the "oscilloscope cable" versus the "BNC cable". – Barry Oct 15 '14 at 21:15
• I calculate you don't quite know what you are doing :^) If by 'scope cable you mean 'scope probe... then you don't use a scope probe as the output of a FG. – George Herold Oct 15 '14 at 23:12

## 4 Answers

If you're saying that the open-circuit output of the function generator is 1VPP and that when you connect it to the scope the scope reads 700mVPP, then if your scope's input impedance looks like 10 megohms paralleled by some few picofarads:

EDIT: 1 megohm paralleled by some few picofarads:

the generator impedance looks like 428k ohms, so clearly something's very wrong.

Do you have your scope's impedance set at 50 ohms, perhaps, and with half of 1VRMS coming from a 50 ohm function generator you're reading 0.7V peak from the scope?

• I see.an accepted answer and an indecipherable question and I ask Wtf is this all about? LOL – Andy aka Oct 15 '14 at 22:38
• @Andy EM Fields is a Jedi mind reader! – JYelton Oct 16 '14 at 6:54

What you might be seeing is Root Mean Square, which is calculated as the amplitude $a$ over the square root of 2:

$$\frac{a}{\sqrt2}$$

For amplitude values of 1, this works out to ~0.707.

Check the settings on your scope; perhaps it is showing you a measured RMS value instead of peak to peak? (As in, a text value, rather than what's displayed on the grid. My scope can show various measured values aside from the normal waveform grid view.)

• I think he's saying he sees 700mVp-p on one channel and 1Vp-p on another where the cable may be longer. – Andy aka Oct 15 '14 at 20:55
• Maybe a voltage drop, then -- but I don't see where two cables are mentioned. – JYelton Oct 15 '14 at 20:56
• Just reading between confused lines J!! – Andy aka Oct 15 '14 at 20:58
• @Andy: With a cable that long it'd probly take a couple of seconds for the signal just to get through it! ;) – EM Fields Oct 15 '14 at 22:09

You say you are outputting 1VPP, and I assume when you say you're "seeing" 700mV on the scope that you are also speaking of PP. If that is the case then there is no calculation to it. Either the FG or the scope is simply out of calibration.

Just to add to what JYelton said, read more about RMS values, but the ${\sqrt2}$ factor is generally only applicable to sine waves (might be others too, have to do the math). For example, a square wave would have RMS value equal to its Peak.

• I think he meant measuring the function generator output with the scope probe, and then connecting them directly with a BNC-to-BNC cable. – nurchi Oct 15 '14 at 21:26