74AC14 TDR

Started by amdx July 11, 2018
  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.
> https://www.ebay.com/itm/152587007700
Mikek
On 11/07/18 23:30, amdx wrote:
>  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. >> https://www.ebay.com/itm/152587007700 >
                 Mikek
>
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.
On Wed, 11 Jul 2018 17:30:07 -0500, amdx <nojunk@knology.net> wrote:

> 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. >> https://www.ebay.com/itm/152587007700 > Mikek
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 jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
torsdag den 12. juli 2018 kl. 00.30.13 UTC+2 skrev amdx:
> 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. > > https://www.ebay.com/itm/152587007700 > Mikek
looks like that is just how it is, https://youtu.be/CQTGhcjE7Ww
"amdx" <nojunk@knology.net> wrote in message 
news:pi60di$can$1@dont-email.me...
> 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. >> https://www.ebay.com/itm/152587007700 > Mikek >
Is the scope set for input termination of 50 ohms?
On 7/11/2018 3:30 PM, amdx wrote:
> 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. >> https://www.ebay.com/itm/152587007700 > Mikek >
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.
On Wed, 11 Jul 2018 19:30:35 -0400, "tom" <tmiller11147@verizon.net>
wrote:

> >"amdx" <nojunk@knology.net> wrote in message >news:pi60di$can$1@dont-email.me... >> I ordered a built 74AC14 TDR square wave generator. >> It has overshoot and ringing, 2 or 3 cycles. >> Is there any way to tame it with out increasing rise time? >> I added a 33pf across scope side input, it helped but increased >> rise time. >> Before and after pictures, > >> https://www.dropbox.com/s/t5y0jfrmbxaqs0o/risetime.png?dl=0 >> >> >> Not sure it matters, I would just like a nice rising edge to start with. >> >> >> >> Here's the TDR unit I bought, click through pictures to see layout >> of connectors. >>> https://www.ebay.com/itm/152587007700 >> Mikek >> > >Is the scope set for input termination of 50 ohms? > >
It shouldn't be! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message 
news:4d9dkd1qu2sft6tgv6dp537omcil567df0@4ax.com...
> On Wed, 11 Jul 2018 19:30:35 -0400, "tom" <tmiller11147@verizon.net> > wrote: > >> >>"amdx" <nojunk@knology.net> wrote in message >>news:pi60di$can$1@dont-email.me... >>> I ordered a built 74AC14 TDR square wave generator. >>> It has overshoot and ringing, 2 or 3 cycles. >>> Is there any way to tame it with out increasing rise time? >>> I added a 33pf across scope side input, it helped but increased >>> rise time. >>> Before and after pictures, > >>> https://www.dropbox.com/s/t5y0jfrmbxaqs0o/risetime.png?dl=0 >>> >>> >>> Not sure it matters, I would just like a nice rising edge to start >>> with. >>> >>> >>> >>> Here's the TDR unit I bought, click through pictures to see layout >>> of connectors. >>>> https://www.ebay.com/itm/152587007700 >>> Mikek >>> >> >>Is the scope set for input termination of 50 ohms? >> >> > > It shouldn't be! > > > --
It certainly should be.
On 7/11/2018 8:38 PM, tom wrote:
> "John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message > news:4d9dkd1qu2sft6tgv6dp537omcil567df0@4ax.com... >> On Wed, 11 Jul 2018 19:30:35 -0400, "tom" <tmiller11147@verizon.net> >> wrote: >> >>> >>> "amdx" <nojunk@knology.net> wrote in message >>> news:pi60di$can$1@dont-email.me... >>>> 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. >>>>> https://www.ebay.com/itm/152587007700 >>>> Mikek >>>> >>> >>> Is the scope set for input termination of 50 ohms? >>> >>> >> >> 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
On Wed, 11 Jul 2018 21:38:19 -0400, "tom" <tmiller11147@verizon.net>
wrote:

> >"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message >news:4d9dkd1qu2sft6tgv6dp537omcil567df0@4ax.com... >> On Wed, 11 Jul 2018 19:30:35 -0400, "tom" <tmiller11147@verizon.net> >> wrote: >> >>> >>>"amdx" <nojunk@knology.net> wrote in message >>>news:pi60di$can$1@dont-email.me... >>>> 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. >>>>> https://www.ebay.com/itm/152587007700 >>>> Mikek >>>> >>> >>>Is the scope set for input termination of 50 ohms? >>> >>> >> >> It shouldn't be! >> >> >> -- > >It certainly should be. >
Spice it and see. Or try it. -- John Larkin Highland Technology, Inc lunatic fringe electronics