74AC14 TDR

What's the period/frequency of the ringing? What's the load capacitance? What's the power/ground lead inductance between the AC14 silicon and the load capacitance and/or the decoupling capacitor?

Quite possibly that C and L is resonating; you can see that effect in a bog-standard *10 "high" impedance probe with a 6" ground lead.

Is that a 2-layer board? Looks poorly bypassed, with all thru-hole parts. And the scope pickoff is a stub.

Use a slower scope and the ringing will go away!

You might kluge in some better bypassing on the IC, and maybe jumper over gaps in the ground pours.

torsdag den 12. juli 2018 kl. 00.30.13 UTC+2 skrev amdx:

looks like that is just how it is,

Is the scope set for input termination of 50 ohms?

Looks like you got just about what the website promised. You should be able to determine the advertised parameters for cables longer than the ring time.

Some things to think about... That's likely a T-connection where three things (unknown, sense, stimulus) come together. Everything needs to be properly terminated in both directions. That's hard do with that configuration. Most anything you do makes it worse.

What's the transient response of your scope with a real calibration generator?

What's the input impedance of your scope? If it's 1M in parallel with some pF, you've already shot yourself in the foot. Put some QUALITY cable in between the scope and the TDR. Use a 10X 50 ohm attenuator (an expensive/good one) and a terminator on the scope too...or select 50 ohm input if available.

You have the same problem on the unknown end. That BNC is an open transmission line. Terminate it.

BNC connectors suck. They're "transmission line adjacent". Last pulse generator I designed shipped with a high quality cable with high quality BNC connectors. It wouldn't meet transient response spec at the other end of random crap cables.

Then there's the problem of generating a square edge with a device not optimized to do that. Current pulses into a 50 ohm resistor work well. Voltage pulses are too sensitive to load.

I'd say you got your $15 worth. You can obsess over it or just use it for the designated purpose.

It shouldn't be!

It certainly should be.

Nope, IF, you're going to terminate it, do it at the far end of the cable, but then you don't get a reflection, so there's nothing to measure. Measured a couple of 6 or 8 ft RG58 cables using .66 VF and got within an inch, but then there's another inch of connectors, so very good agreement. Then I have 75 ohm cable about 1/8" coax, it measures .70 VF. Mikek

Spice it and see.

Or try it.

No. This kind of generator is a 50 ohm voltage source. Adding a 50 ohm scope in parallel makes it 25 ohms. A 50 ohm TDR should be a 50 ohm step generator and a hi-Z scope.

Or a 50 ohm scope and a current step, which is how most classic TDR scope heads worked.

With volts of drive from a CMOS gate, most scopes have gain to burn. So you could do something like

gate----55r---BNC---450r------------coax--------50R scope.

which is a 50 ohm TDR at the BNC, and drives the coax and 50 ohm scope, with 10:1 attenuation.

It shouldn't be, because it looks like a Tek 475. Ancient thing doesn't have internal termination. (IIRC the 485 and most 2xxx's did, though.)

Tim

FYI:

1. Dropbox says it's a file, not an image, so it has to be downloaded.
2. Dropbox? Really?
3. It's 710x483 which is fine, but it's saved in the worst possible image format, 24-bit PNG. It's literally a zipped bitmap. Use JPEG, usually around 90-95 quality factor, and you'll see file size drop to maybe 50k. Only use PNG for reduced-color line drawings.
4. Try to set up square shots, especially for scope shots! Look for the reflection of the camera off the screen/bezel, and center that on the screen and in the camera viewfinder. Turn off ambient lights to eliminate the reflection, then take the picture. Adjust exposure until the trace is easily visible, not burned out -- automatic settings are usually poor on high-contrast displays. The graticule intensity seems to have kept contrast reasonable so that only a little trace is burned out (just barely, around the top right), which works as well.
5. And always focus the picture. (Probably fine on the present case -- this is general advice.) Don't crowd the subject, that can only ever make things worse. Preferably, take the picture from a distance and use optical zoom to enlarge it -- this also reduces barrel/fisheye distortion, and abberation of the lens. Some postprocessing may still be necessary to correct for the spherical CRT face. All the more reason to move into the 20th century with a digital scope -- even better, one with floppy drive (if you're really old fashioned), serial or GPIB interface so you can save a screenshot directly.

Tim

You use a 50 ohm through termination for a scope that does not have a 50 ohm input setting. That is if you want to get rid of the overshoot. Take a look at the service manual for a 475 Tek scope if you care to see how to do it. Even the short connection between the pulse source makes a short unterminated (by 1 Megohm and 20 pF) transmission line.

On page 5-35, item 31, Adjust HF Compensation. Note the 50 ohm termination.

I admit this is just nit picking but if you want to do it right, RTFM.

Good advice. And use the self timer with a tripod.

The scope is a Tek 2465 CTS, it does have an internal switched 50 ohm termination. IF, I used the internal 50 ohm termination and then put a 50 ohm coax cable on the TDR, it would have 25 ohm termination for as long as it takes for the signal to get to the end of the cable and return to the scope. It doesn't make any difference as far as I can tell, other then the signal is about 1/2. The ringing doesn't get any better with a 50 ohm termination, it changes in the same proportion as the signal.

For those interested here are two videos showing what I'm following.

Does "No quibble returns" mean you can return it for any reason and they won't quibble or they won't accept returns based on "quibbles"?

Ok, I'll try to give you better pictures next time. I had to avoid a window that was reflecting off the screen, That was some of the angle, I did turn down the intensity of both my trace and the scale illumination. My camera has auto focus only. I'm not sure my tripod will go high enough. Here's a JPG, >

I'm have no desire to send it back, it works. What do you want it to do? Mikek

When we get to a bandwidth that is outside the children toy league, there is no such thing as a hi-Z scope.

The proper way is a splitter and 50 Ohm devices. See Fig. 11 in <

When the generator/scope side is not perfectly matched, everything after the first large reflection is just dirt. And you usually do not have a long enough precision air delay line. Maybe in the VNA cal kit.

cheers, Gerhard