Coax Cable Deformation Test

I'm not familiar with the term "freely terminated". How do you know the scope has a 50 Ohm input impedance? And finally I have never seen an "ideal" conductor. Methinks such a conductor would have zero resistance and reactance, making the claim "75 ohm impedance" irrelevant.

What do you mean the other end is freely terminated?

Is this an open circuit or are you shorting the cable?

Does it look like this?

hi, yes it is a open circuit. and it looks something like what you had posted.

Yep, a microphonic cable effect. The center conductor and the shield create a capacitor. When the value of this capacitor changes from the deformation of the cable, charges move. This moving charge (electric current) induces a voltage across the input impedance of your oscilloscope. Most of the time this is a hard to detect phenomena but depending on how hard one whacks the cable and how that cable is constructed it can be found. It also depends on the input impedance of the amplifier circuit. Are you sure that the internal 50 ohm termination was ON when you saw this?

Image removed at request of original poster.

This was the effect that was seen when the coax cable was hit with a pendulum at a force of 81.13 N. There was no current that was passed into the cable but how does the oscilloscope show around 15.8 mV ? I am little confused on the physics behind. The input impedance of the oscilloscope is 50 ohms and i am not sure if the internal 50 ohm of the oscilloscope was turned on or not. I didn't have any amplifier circuit.

OK. Maybe some Physics of a condenser microphone will help. I'm not thrilled with the Hyper Physics page there. Page 8 and 9 of this presentation is a little better.

What complicates the situation is the capacitor has a net charge of zero. Many people get hung up with two zeroes in an equation like C=Q/V. So what happens when the the charge (Q) is a constant that is anything else but precisely zero. A change in the capacitance with time will induce an inverse change in the voltage.

There's no saying that the o-scope's *input* does not have some small voltage on it, also, regardless of whether there is anything connected to it. You're very likely right: there are likely no exactly-zero voltages anywhere in the setup.

Hi europium mkII:

Cable insulator itself is an electret material and often holds charge within generated by triboelectric forces and applied high voltages from previous measurements. Even fresh cables have such stored charge as they are tested at high voltages for corona current. Movement generates Microphonic Noise.

See http://en.wikipedia.org/wiki/Electret_microphone

Electret charge can be discharge by heating and can be increased by polarizing voltage to charge the electret.

By hitting the cable one changes capacitance and signal is like piezo response. You can notice that push has a waveform and deform shape takes its own time and hence plastic ball like bouncing waveform is seen by hitting the cable.

Waveform differs hen you jerk the cable.

Redfred got it right, so I have stayed out of this for now, but...

The effect is called Singing Capacitor (Piezoelectric Effect).

Hi Anonymous Hero:

Singing Capacitor- nice idea.

Difference here is that charge is also trapped inside insulator dielectric and it can't leak out but on movement can induce voltage which is seen as noise. I think noise frequency may have something to do with the tuning fork like resonance frequency of the string. I tried with PTFE RG188 cables. I also tried RG series 50 ohms and 75 ohms cables with and without resistive termination. Sometime you see burst initially.

Some cables are calm but not free from noise. Low noise cables are sturdy an sometime rigid with copper tube top.

Shyam did a post on something similar some time ago:-

http://170.207.226.52/thread/66960/Instrumentation-Special-Tribo-Electricity-and-Micro-Phonic-Noises

The pictures no longer seem to exist.

Hi Randall:

Here are some recorded noises that are Microphonic

There have been many times that I have tried to source ideal conductors for projects. What is the manufacturer's name and part number of the ideal conductor?

Try one of the Yttrium Barium Copper Oxides (YBCO) for a nearly ideal conductor. I recommend having this conductor bathed in liquid nitrogen instead of trying to transfer heat through another solid media. The localized heating concerns and risk of a runaway quench is not worth it.

Look for supper conducting wires and operate on low defined operating temperature. For the rest of the conducting wires there are numerous tricks involved like high frequency travels on surfaces and use Litz wires etc.

Unless you and your cable are in a Faraday cage, cable is also an antenna. This gives enough AC potential to get modulated by cable capacitive piezoelectric effects or connector bad contact induced noise, even if scope input is really close to zero bias. S.M.