Re: Tektronix 50 Ohm Terminator / Attenuators

Fabio Trevisan

Hi Guys,
Indeed this whole thing of 50Ohm input and output impedance gets confusing.

I start thinking of e.g. the 5x attenuator, how exactly would be the
voltage divider values (and a parallel input resistor, or a series output
resistor), but I can't seem to solve the equation on my head.

I know that the divider must present 50 Ohm impedance to the source, and
that to calculate its 5:1 ratio one must take in account that it must have,
at the same time, 50Ohm output impedance and be already loaded by 50Ohm at
its output...
Fine up 'til this point, but when I try to sort out the output impedance
and consider that it also reflects part of the source impedance and part of
the input impedance... It mind blows me.

I know it's a matter of looking at it by Kirchoff's and Thevenin's rules...
but it's a little counter-intuitive at first.

Does anyone have the actual values of the resistors of a would-be 50Ohm 5X
attenuator? Not that I`m going to build it, but just to see if it agrees to
my rational?



2017-05-25 9:52 GMT-03:00 Chuck Harris cfharris@... [TekScopes] <

I think when Dick read: "50 ohms from either side to
ground", he read it as meaning 50 ohms from the left side,
*and* 50 ohms on the right side, which would be 25 ohms

I think the OP meant 50 ohms on the input side, *or*
50 ohms on the output side...

English can be difficult for us natives.

-Chuck Harris

David @DWH [TekScopes] wrote:
On Wed, 24 May 2017 14:05:44 -0700, you wrote:

..." Feedthrough terminations have a short from input to output and 50
of resistance from either side to ground."... .
Doesn't that make a 25 Ohm termination ?

73, Dick, W1KSZ
When the feedthrough termination is used on the transmission line,
then the impedance at that point is halved so it is 25 ohms. But it
is still a 50 ohm termination; the two 50 ohm impedances are in
parallel at that point yielding 25 ohms.

The same thing happens when the 50 ohm feedthrough termination is used
on the output of a 50 ohm attenuator; the impedance at that point
becomes 25 ohms.

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