¿ªÔÆÌåÓý

ctrl + shift + ? for shortcuts
© 2025 Groups.io
  1. TekScopes
  2. Topics

Need for Termination Feedthrus

 

As I gather up all the stuff I need to test and maintain my newly purchased 2235 I am wondering how necessary it is to use a 50 ohm feedthru termination when sending test and calibration signals into the scope. Can one not construct their own device that perform the same purpose as a factory-built feedthru?

And, can anyone recommend a good book that goes into detail on the theory of construction and operation of oscilloscopes? The ones I have gotten at the library are pretty superficial.

Thanks,

Phil...
 

On Sat, 19 Jan 2013 20:19:08 -0000, "Philip" <ndpmcintosh@...>
wrote:

As I gather up all the stuff I need to test and maintain my newly purchased 2235 I am wondering how necessary it is to use a 50 ohm feedthru termination when sending test and calibration signals into the scope. Can one not construct their own device that perform the same purpose as a factory-built feedthru?
A T-connector with a termination on one end will work for
oscilloscopes that lack internal switchable terminations which
generally only have a bandwidth that extends to 100 MHz anyway.

And, can anyone recommend a good book that goes into detail on the theory of construction and operation of oscilloscopes? The ones I have gotten at the library are pretty superficial.
Jim Williams' book has a great section on oscilloscope vertical
amplifiers among other things:
 

Sure but the question remains--why? If I know that the signal amplitude is within range for the task and my cables are short then why would I want to stick an additional 50 ohm feedthru in the signal path?

And thanks for the book recommendation. Ant other will be welcome also.

--- In TekScopes@..., David wrote:

On Sat, 19 Jan 2013 20:19:08 -0000, "Philip"
wrote:

As I gather up all the stuff I need to test and maintain my newly purchased 2235 I am wondering how necessary it is to use a 50 ohm feedthru termination when sending test and calibration signals into the scope. Can one not construct their own device that perform the same purpose as a factory-built feedthru?
A T-connector with a termination on one end will work for
oscilloscopes that lack internal switchable terminations which
generally only have a bandwidth that extends to 100 MHz anyway.

And, can anyone recommend a good book that goes into detail on the theory of construction and operation of oscilloscopes? The ones I have gotten at the library are pretty superficial.
Jim Williams' book has a great section on oscilloscope vertical
amplifiers among other things:
 

On Sun, Jan 20, 2013 at 11:08 AM, Philip <ndpmcintosh@...> wrote:
Sure but the question remains--why? If I know that the signal amplitude is within range for the task and my cables are short then why would I want to stick an additional 50 ohm feedthru in the signal path?
To avoid reflections from the unterminated input of the scope.

Try connecting a fast rise generator (10MHz square wave would be fine)
to the scope through an unterminated cable, then add a terminator.
You'll see a huge difference.
RonC
 

Here's a little reading material.
It'll help you understand why using terminators and matching impedance, plus using attenuators, is a good thing.







Basically to keep "ringing" from distorting your signal the transmission (coax) needs to have the same impedance(ohms)on both ends, the input and at the end connected to the DUT. Otherwise you will get part of the signal you sent from your signal generator reflected back to it which will affect the waveforms. Same as having too much SWR on a CB radio.

You can build your own pads to match the impedance and attenuators.
Here's an attenuator, with pads, that I built, along with some links to the information behind it.
RonC
 

One last link, this explains the 50 ohm impedance and measuring RF and microwave frequencies.

Good read!

1 - 6 of 6