In message ,
Terry Casey writes
In message o.uk on Mon, 11
Apr 2011 12:10:59 +0100 (BST)
Dave Liquorice wrote:

On Sun, 10 Apr 2011 13:49:44 +0100, Terry Casey wrote:

A terminator is just a 75 ohm resistor.

But not just any 75 ohm resistor. Many use a spiral track on an
insulating former, nice little inductor... Heaven knows what the
actual impedance will be at UHF frequencies.

Something that it doesn't seem easy to find data on - but there is a graph on
the Vishay data sheet at

http://www.vishay.com/doc?20135

Blank screen!

which indicates that reactance versus resistance for a 0.35W 75 ohm
carbon film
resistor is virtually unity up to 1GHz!

Indeed.

While cracked carbon resistors potentially are likely to have the lowest
inductance, they are neither high tolerance no high stability.
Surprisingly, some spiral cut metal film resistors don't seem to have
excessive inductance. That said, I recall an impromptu production-line
change to a certain type of resistor (Electrosil, I believe) causing
serious problems at as low as 200MHz.

I suppose that today's tiddly chip resistors are too small to have much
inductance . Mind you, I'm surprised they are big enough to have any
resistance!

As for making good terminations, even though the resistor has some
inductance, the secret is to space the resistor at the right distance
above a groundplane. This adds capacitance, distributed along the length
of the resistor. In effect, the inductive resistor becomes part of a
transmission line, and a good wideband match can be maintained to the
cut-off frequency.
--
Ian