Re: Any transmission line experts?
- From: "Dave (from the UK)" <see-my-signature@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 03 Mar 2006 10:27:39 +0000
Geoff C wrote:
I have an EMI problem with a high speed system. There is a pair of wires which are taken from a high speed driver to a high impedance electrostatic plate which resides in a vacumm. The plate is enclosed in a metal enclosure so does not emit, but the wires do. They are an untwisted pair about 4 inches of which is exposed, between 2 metal boxes. The signal on the wires is about 500 volts with a 1 nanosecond rise time and about 48 khz repetition rate. The rise time may not be compromised.
I have tried coax type shielding, but this slows down the rise time. I was thinking the next step is a rigid conduit about 2 inch diameter, which may have a higher impedance/lower capacitance than a coax. Also, twisted pair is worth trying, but again I cannot afford to slow down the signal.
You should have no problem with coax if your driver can drive the impedance of the cable. It is not capacitive or inductive, but resistive (assuming it is lossless).
I have regularly sent sub ns pulses down coax. (i.e. risetime around 50ps, FWHM around 300ps). I used a microwave step recovery diode to generate the pulses. That would not generate 500V though!! I did some of this during my PhD too.
I used a driver that had an output Z or 50 Ohms, a load of 50 Ohms and 50 Ohm coax. Of course, the output voltage drops a factor of 2 from the source, as half the voltage is dumped across the source impedance of the driver.
If you use large coax, it will support higher order modes, so behaves quite differently at high frequencies than low ones. Semi-rigid coax is regularly used to 10's of GHz.
PS
I am the author of http://atlc.sourceforge.net/ which someone else mentioned.
--
Dave K
Minefield Consultant and Solitaire Expert (MCSE).
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