74AC14 TDR

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  I ordered a built 74AC14 TDR square wave generator.
  It has overshoot and ringing, 2 or 3 cycles.
Is there any way to tame it with out increasing rise time?
I added a 33pf across scope side input, it helped but increased
rise time.
Before and after pictures, >  
https://www.dropbox.com/s/t5y0jfrmbxaqs0o/risetime.png?dl=0


  Not sure it matters, I would just like a nice rising edge to start with.



  Here's the TDR unit I bought, click through pictures to see layout
of connectors.
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                  Mikek


Re: 74AC14 TDR
On 11/07/18 23:30, amdx wrote:


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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.

Re: 74AC14 TDR

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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.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: 74AC14 TDR
torsdag den 12. juli 2018 kl. 00.30.13 UTC+2 skrev amdx:
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looks like that is just how it is, https://youtu.be/CQTGhcjE7Ww

Re: 74AC14 TDR

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Is the scope set for input termination of 50 ohms?




Re: 74AC14 TDR
wrote:

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It shouldn't be!


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: 74AC14 TDR

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It certainly should be.



Re: 74AC14 TDR
On 7/11/2018 8:38 PM, tom wrote:
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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

Re: 74AC14 TDR
wrote:

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Spice it and see.

Or try it.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 74AC14 TDR
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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

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: 74AC14 TDR

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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.

http://w140.com/tek_475_tek.pdf

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.





Re: 74AC14 TDR
On 7/12/2018 2:36 AM, tom wrote:
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  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.

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Re: 74AC14 TDR
On 7/11/2018 3:30 PM, amdx wrote:
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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.








Re: 74AC14 TDR

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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.




--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: 74AC14 TDR
Am 12.07.2018 um 04:15 schrieb John Larkin:
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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
<  https://kh6htv.files.wordpress.com/2015/11/an-05c-10ps-tdr.pdf

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




Re: 74AC14 TDR
On Thu, 12 Jul 2018 20:53:33 +0200, Gerhard Hoffmann

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Well, you could also do

gates--100r--BNC--50r--------coax------50r scope

But we're talking $18 TDR boards here, not shock-line step generators
and 50 GHz scopes.



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: 74AC14 TDR
On 7/12/2018 1:53 PM, Gerhard Hoffmann wrote:
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  Is that what is known as a humble-brag?
                               Mikek

  I bought the Fast Edge Pulse Generator to find the Characteristic  
impedance of 253ft of wire laying on the ground. I can do that by  
adjusting the termination at the far end until I don't have any step
in the square wave. Also, with it unterminated, I can find out the VF
of the wire, knowing the length. I hope to reduce the VF down to around  
0.55 using several coils along the length of the wire.
  Here is a very short video showing how the wire on ground is  
directional. The problem, this is about as low in frequency as the  
antenna goes before losing directionality. I hope with coils inserted in  
the wire I can reduce the VF making the antenna seem longer.
   I just wanted to have some measurements as a starting point to see  
how the characteristics were changing as I made the physical changes.
  Just having fun.
  I gave up on the camera, the auto focus, goes through focusing, but  
then stops at the wrong place.
  Wife dropped her phone in the water, so I don't have my phone to take  
any pictures, They may be better.


Re: 74AC14 TDR
On 7/12/2018 3:41 PM, amdx wrote:
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The Link,




  The problem, this is about as low in frequency as the
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Re: 74AC14 TDR
On 7/12/2018 3:42 PM, amdx wrote:
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  Also, when I get the characteristic impedance, properly terminated, I  
will add an additional ground rod at each end to see if that reduces my
ground resistance.

Re: 74AC14 TDR
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

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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