Holding scope probes causes notable changes, why?

I was playing with my oscilloscope and I noticed if I touched just one lead with my finger, it would turn into almost a sine wave, and if I touched the other probe it would significantly dampen any noise at all. And with no fingers it was kind of giving some garbage in between the two scenarios described.

I know the human body acts like an antennae and it also has capacitance; so it is one, both, or neither of these reason for the behavior I was seeing?

wes000000:
I was playing with my oscilloscope and I noticed if I touched just one lead with my finger, it would turn into almost a sine wave, and if I touched the other probe it would significantly dampen any noise at all. And with no fingers it was kind of giving some garbage in between the two scenarios described.

I know the human body acts like an antennae and it also has capacitance; so it is one, both, or neither of these reason for the behavior I was seeing?

Scopes typically have a very high input impedence (unless you have the type with a 50 ohm switchable input impedence). THis means that it takes VERY little current to make an appreciable change. Since your body acts as a good antenna, often what you are picking up is stray power line noise (50 or 60Hz depending on your country as well as multiples there of due to distortion products). I am a bit confused by other probe do you mean the ground clip or do you mean the probe for the other channel?

often what you are picking up is stray power line noise (50 or 60Hz depending on your country as well as multiples there of due to distortion products)

Good to know, very interesting!

I meant ground when I said 'other probe'.

wes000000:

often what you are picking up is stray power line noise (50 or 60Hz depending on your country as well as multiples there of due to distortion products)

Good to know, very interesting!

I meant ground when I said 'other probe'.

Oscilloscope probes are in a way "differential". This means they measure the difference between the signal on the probe and ground. Note I put differential in quotes because there are true differential probes which measure the difference between a positive and negative lead (negative is no longer grounded). So when you aren't touching the probe, the probe by itself is picking up the mains noise. When you grab the ground connection, you provide a better antenna for the ground connection. This shunts more noise into ground and serves to better offset the noise picked up by the probe itself.

As an aside, it is important to remember that the ground lead is ACTUALLY connected to ground. It is VERY easy to kill a scope with ground loops, which is why for power supply work a true differential probe is often recommended.

Very interesting, and makes a lot of sense.

I've never heard of a ground loop, what is that exactly?

Basically, where the ground of two instruments is not quite at the same potential. As a result current can flow from one device to another via the ground line. It is a common source of noise in audio.

Wikipedia can explain it better if you wish to read more:

Very interesting. You learn something new everyday!

Oscilloscope probes are in a way "differential". This means they measure the difference between the signal on the probe and ground. Note I put differential in quotes because there are true differential probes which measure the difference between a positive and negative lead (negative is no longer grounded). So when you aren't touching the probe, the probe by itself is picking up the mains noise. When you grab the ground connection, you provide a better antenna for the ground connection. This shunts more noise into ground and serves to better offset the noise picked up by the probe itself.

As an aside, it is important to remember that the ground lead is ACTUALLY connected to ground. It is VERY easy to kill a scope with ground loops, which is why for power supply work a true differential probe is often recommended.

What a really great explanation. Congratullations.

A ground loop is typically a large area loop (via two or more pieces of equipment and their earth connections to the mains wiring) which will pick up mains hum and noise via electromagnetic induction. Induced voltages are proportional to flux linkage and flux linkage depends on both field strength and area.