Fast Video Covers Coax Velocity Factor

We once saw an interview test for C programmers that showed a structure with a few integer, floating point, and pointer fields. The question: How big is this structure? The correct answer was either “It depends,” or “sizeof(struct x).” The same could be said of the question “What is the speed of light?” The flip answer is 186,282 miles per second, or 299,792,458 metres per second. However, a better answer is “It depends on what it is traveling in.” [KB9VBR] discusses how different transmission lines have different velocity factors and what that means when making RF measurements. A cable with a 0.6 velocity factor sees radio signals move at 60% of that 186,282 number.

This might seem like pedantry, but the velocity factor makes a difference because it changes the actual measurements of such things as dipole legs and coax stubs. The guys make a makeshift time domain reflectometer using a signal generator and an oscilloscope.

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Video: Putting High Speed PCB Design To The Test

Designing circuit boards for high speed applications requires special considerations. This you already know, but what exactly do you need to do differently from common board layout? Building on where I left off discussing impedance in 2 layer Printed Circuit Board (PCB) designs, I wanted to start talking about high speed design techniques as they relate to PCBs.  This is the world of multi-layer PCBs and where the impedance of both the Power Delivery Network (PDN) and the integrity of the signals themselves (Signal Integrity or SI) become very important factors.

I put together a few board designs to test out different situations that affect high speed signals. You’ve likely heard of vias and traces laid out at right angles having an impact. But have you considered how the glass fabric weave in the board itself impacts a design? In this video I grabbed some of my fanciest test equipment and put these design assumptions to the test. Have a look and then join me after the break for more details on what went into this!

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Poor Man’s Time Domain Reflectometer

A time domain reflectometer, or TDR, is an essential piece of test gear when working on long cables. The idea is simple: send a pulse down the cable and listen for the reflection from the far end. The catch is that pesky universal constant, the speed of light.

The reason the speed of light is an issue is that, in a traditional system, the pulse needs to be complete before the reflection. Also, time is resolution, so a 1 GHz sampling rate provides a resolution of about 10 centimeters. [Krampmeier] has a different design. He sends variable length pulses and measures the overlap between the outgoing and reflected pulses. The approach allows a much simpler design compared to the traditional method.

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Hackers Measure Cable Lengths With Time Domain Reflectometers

[android] has built up a fast edge pulse generator for time domain reflectometry (TDR). TDR is a neat technique which lets you measure cable lengths using electrical signals and can also be used to locate faults within the cable.

TDR works by sending a pulse down the cable. When the pulse reaches the end of the unterminated cable it is reflected back to the source. By monitoring the delay between the original pulse and its reflection you can determine the length of the cable. We’ve seen projects that use TDR before, and it’s often used in telecoms industry to locate faults in long cable runs.

You can try TDR in your lab using only a scope to observe the delay and a function generator to create the pulse. However, the technique works a lot better with pulses that have very fast rise times. So [android] built a fast edge pulse generator based on [w2aew]s design. Then added googly eyes for good measure. His build works great and is a nice demonstration of the technique.