[time-nuts] TAPR TICC and TADD-2 Minis

[Bob kb8tq]

Hi

Scope basics:

It is not unusual for a scope to have an input channel with a capacitance 
of 10 to 20 pf. That does not sound like much but ….

A 10 MHz square wave has significant energy at 30, 50, 70, 90 MHz (and on up).
Unless a coax is terminated at all these frequencies they will be reflected and you
will get a ring. 

So 20 pf at 10 MHz is 795j ohms. At 90 MHz 88j ohms. A feed through termination 
probably does a reasonable job at 10 MHz …. it’s pretty awful  by the time you get 
to 90 MHz. It is somewhere in the “ummm …. errr …” range at 30, 50, and 70 MHz. 

At 10 MHz, 50 + 795j ohms will give you an 18 db return loss. ( the reflected signal 
will be 18 db below the incoming signal). A voltage at 1/10 the input is 20 db down. 
Not all of the reflected signal will be “ring”, it has to bounce off the source first. How
much bounces depends a lot on how good the *source* match is. 

Bottom line - for a *good* 50 ohm termination, you need to do things another way. 
One of the reasons there are compensated scope probes is to get around this
issue. There are a variety of them out there with a range of capabilities. Yes, the
fancy fet probes cost more than something simple ……. 

A 10 db pad ahead of the feed through is another way to deal with this. It should at
lest get you into the 20 db return loss range.

Even scopes that *have* 50 ohm inputs may or may not have *good* 50 ohm inputs. 
Is that clearly spelled out in the specs? Often it isn’t …. sorry about that. 

Bob

> On Jun 1, 2020, at 2:29 AM, Frank O'Donnell <time at inkbox.net> wrote:
> 
> On 5/31/20 8:14 PM, Charles Steinmetz wrote:
>> Doing as you are now -- feeding the T2-Mini output through coax to the input of your scope -- you will either need to switch the scope input impedance from 1 Mohm to 50 ohms (if the scope has that feature -- sadly, many digiscopes do not), or use a 50 ohm feedthrough terminator on the scope input.
> 
> Thanks for the comments. Attached is a screen shot with the Thunderbolt's 10 MHz output again going through the T2-Mini to the scope, but with a 50-ohm feed-through terminator placed on the scope's input.
> 
> Frank
> 
> <tadd2-mini-50ohm-term.jpg>_______________________________________________
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