Blog Post Actions
Group Actions
Engagement
Author: peteroakes
Date Created:
Views: 72
Likes: 1
Comments: 1
Related
Recommended

33622A - Lets play with Ohms Law, Characteristic Impedance and other fun things.

peteroakes

Link back to Primary review; http://www.element14.com/community/roadTestReviews/1744

 

Refresher for those new to all this

 

Ohms Law. Given a simple circuit, ohms law is as simple as

 

 

So if I have a simple cable, Coax, UTP or even Twin lead, they all on the face of it have simple properties

 

 

They all have at least two signal carrying wires and one or more layers of insulation

COAX: A=insulation (PVC), B=screen and return path, C=insulation / dielectric and D= main signal wire.

UTP: several pairs of insulated wire twisted together and an overall outer insulator of PVC (Typically)

Twin Lead: a pair of wires separated by a constant distance managed by the insulator / spacer material

In all cases the wire is typically copper or a copper alloy, other materials are sometimes used in special cases (Aluminum, Silver, Gold, Other metals)

Measurements of Coax, and UTP with a multi meter


Most newcomers to Electronics look at a piece of wire like this

 

All have similar properties if measured with a multi-meter (Near zero resistance for all but the thinnest or longest wires). A typical newcomer approach which is great for DC to lower frequencies or where the rate of change is not high (Sine wave VS Square wave for example) Guess what comes out of most Micro-Controllers ?

So now lets play with the 33622A and see what we can uncover and find out how far we can bend Ohms law

Effect of incorrect or no termination of a cable on a Sine wave at various frequencies

As you can see, with a sine wave there is less visual effect from reflections and more of what appears to be changing amplitude of the signal giving the illusion that there is  an uneven frequency response of the cable. Of course this is not true and the illusion is actually due to the reflected signal adding to or subtracting from the source signal as its phase changes over frequency (Better known as a Standing Wave)

 

Note, this video was recorded off the cuff so for a little while I made an incorrect assumption on what was causing this effect on the signal, I got there in the end

Its all about the interaction of reflections interfering with source signal creating a standing wave on the wire and depending on the phase and frequency either cancelling out the signal or adding to it (With everything in between too ) at the point of connection to the scope

 

Now I repeat a similar experiment but using a fast pulse and later I increase the pulse width to the point where it  exceeds the reflected time. At this point you will see the source signal appear to increase in amplitude to the level of the real source voltage (V source)  present before the output impedance (RS in the previous diagram)

 

And now a similar experiment but this time playing with the termination resistance to demonstrate the effect of short and open circuits on a pulse signal and everything in-between. It shows how with unlucky timing due to frequency, risetimes and cable/wire length you can get corrupted data signals


 

OK, so three videos and three different but similar effects, the is clearly showing that a piece of COAX or in reality any length of wire at high frequencies is not just a resistive element between a source and a load


it clearly has an active and frequency dependent component. The realist is it is a complex component as shown in the following simplified diagram

 

now we also need to remember that a capacitor and an inductor are also not as simple as there shown above

 

As you can see, if I filled out the above transmission line diagram it would be very complex indeed (That's why I kept it simple)

 

so what have we learned in all of the above videos

1. At low frequencies a transmission line is a fairly simple component and mostly low value resistance but it is still important to account for the source signal impedance or you will not get the signal values you expect and reflections still exist if the rise time of your signals are hgh

2. At high frequencies a transmission line presents its own load on a source but again it is still important to terminate the line in order to avoid corruption in the signal

3. The Agilent 33622A is an awesome educational tool to discover the properties of cables, transmission lines and the importance of correct termination

Anonymous
Parents Comment Children
No Data