Re: Any transmission line experts?

"Dave (from the UK)" <see-my-signature@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote in news:44081a1d@xxxxxxxxxxxxx:

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.


My load is capacitive, that is unavoidable 'cos its a type of deflection
plate. I'd like to know what the effect of twisted pair is on the Zo as
apposed to parallel conductors.
.

Relevant Pages

  • Re: Any transmission line experts?
    ... wires which are taken from a high speed driver to a high impedance ... electrostatic plate which resides in a vacumm. ... I have tried coax type shielding, but this slows down the rise time. ... You should have no problem with coax if your driver can drive the ...
    (sci.electronics.design)
  • Re: Any transmission line experts?
    ... The plate is enclosed in a metal enclosure so does not emit, ... 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. ... You should have no problem with coax if your driver can drive the impedance of the cable. ...
    (sci.electronics.design)
  • Re: Counterpoise theory
    ... You can think of a counterpoise as "the ... Equal-but-opposite currents flow out of the coax (from the ... impedance and the presence of nearby conductors) and you get a good ...
    (rec.radio.amateur.antenna)
  • Re: The Characteristically 50 Ohm Impedance Coax Cable is only 50 Ohms Nominal when . . .
    ... The cable itself has a characteristic impedance of some ... > impedance of the coax. ... The energy has to propagate through the circuit. ...
    (rec.radio.shortwave)
  • Re: Transmission Line Question
    ... TL) are an electrical quarter wave, and the balun (the TL formed by the ... outside of the coaxes in parallel and the boom etc) is a quarter wave. ... The impedance transformation with paralleled coax is no problem. ...
    (rec.radio.amateur.antenna)