Rise time measurment

How direct is that? Is there a cable? If so, how long and what type?

What's the termination? 1Mohm//15pF or 50ohm//15pF?

The rule of thumb is the risetime is 0.15/BW, so it should be about 1ns.

A 350 MHz scope should have a 10:90 rise time of about 1 ns.

The input capacitance of a hi-Z scope might be 15 pF, and that makes a

750 ps time constant with 50 ohms, and RC defined steps take a while to get to 90%. Is the scope set to 50 ohms?

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

The usual spec is tr = 0.35/BW (BW is the -3dB bandwidth). If that's 350MHz, the rise time should be about 1.0 ns. My 182-page 2465A manual has Performance Characteristics, and gives the step-response as 1.0ns.

Are you using a 50-ohm terminator adapter?

When you take a rise-time measurement, by setting the cursors to 10 and 90%, any start / stop slowness in the waveform will degrade the answer.

--
 Thanks, 
    - Win

I've tried switching to a known better quality 50ohm termination just to be sure, your Grace, but there's not been much improvement. Here's a photo and the scope is already maxed-out at 5ns/div despite which it's still tricky to set the cursors precisely. Obviously no point hitting X- MAG as any remaining accuracy would go straight out the window.

"Direct" as in the OED definition - IOW nothing in-between.

No, it's a *direct* connection.

I know what you're getting at, but there is *no* cable involved!!

I'm using a Suhner 50 ohm termination (*directly* attached to the tee)

I'm out by 75% then.

Yes, now go back to bed, John. :-)

It is regarded as bad form to get testy with someone that is trying to get you to help yourself.

So it /isn't/ actually directly attached.

Why are you using a tee+external 50ohm, and not the 2465A's internal 50ohm?

In my experience there is a noticeable difference in the HF response (and therefore) risetime between the internal

50ohm termination, an external in-line 50ohm termination, and an external Tee+50ohm termination.

But that difference isn't sufficient to explain your figures.

When was it last calibrated and tweaked?

There's someone on the TekScopes group that does it. And allegedly there's a fleabay seller that boasts of turning

2445Bs into 2465Bs :(

Sorry, the 0.15 was a typo (should be 0.35), but the 1ns is correct.

You're going to have to explain why the *10 mag would be less accurate than your measurement. It appears to me that you haven't selected the 90% point.

It looks like PEBCAK.

That's too little horizontal deflection, you need the 10x.

--
 Thanks, 
    - Win

Excuse me for being helpful. I'll be more careful in the future.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

That, combined with a slow sweep speed, and measuring the 0% and 100% points, might.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Having seen his post and picture afterwards, I can only agree! I even made a subsequent post to that effect.

The details /do/ matter, and it can be depressingly difficult to extract them from people.

Brilliant! It's spot-on now. Thanks, Win.