74AC14 TDR

Started by amdx July 11, 2018
On Wed, 11 Jul 2018 16:45:58 -0700, mike <ham789@netzero.net> wrote:

>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.
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
"tom" <tmiller11147@verizon.net> wrote in message 
news:pi6bcd$1le$1@dont-email.me...
>> >> It shouldn't be! >> > > It certainly should be. >
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 Website: https://www.seventransistorlabs.com/
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
Website: https://www.seventransistorlabs.com/

"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 >
"Tim Williams" <tiwill@seventransistorlabs.com> wrote in message 
news:pi6soc$7gf$1@dont-email.me...
> "tom" <tmiller11147@verizon.net> wrote in message > news:pi6bcd$1le$1@dont-email.me... >>> >>> It shouldn't be! >>> >> >> It certainly should be. >> > > It shouldn't be, because it looks like a Tek 475. Ancient thing doesn't > have internal termination. (IIRC the 485 and most 2xxx's did, though.) > > Tim > > --
You use a 50 ohm through termination for a scope that does not have a 50 ohm input setting. That is if you want to get rid of the overshoot. Take a look at the service manual for a 475 Tek scope if you care to see how to do it. Even the short connection between the pulse source makes a short unterminated (by 1 Megohm and 20 pF) transmission line. http://w140.com/tek_475_tek.pdf On page 5-35, item 31, Adjust HF Compensation. Note the 50 ohm termination. I admit this is just nit picking but if you want to do it right, RTFM.
"Tim Williams" <tiwill@seventransistorlabs.com> wrote in message 
news:pi6t93$a7g$1@dont-email.me...
> FYI: > 1. Dropbox says it's a file, not an image, so it has to be downloaded. > 2. Dropbox? Really? > 3. It's 710x483 which is fine, but it's saved in the worst possible image > format, 24-bit PNG. It's literally a zipped bitmap. Use JPEG, usually > around 90-95 quality factor, and you'll see file size drop to maybe 50k. > Only use PNG for reduced-color line drawings. > 4. Try to set up square shots, especially for scope shots! Look for the > reflection of the camera off the screen/bezel, and center that on the > screen and in the camera viewfinder. Turn off ambient lights to eliminate > the reflection, then take the picture. Adjust exposure until the trace is > easily visible, not burned out -- automatic settings are usually poor on > high-contrast displays. The graticule intensity seems to have kept > contrast reasonable so that only a little trace is burned out (just > barely, around the top right), which works as well. > 5. And always focus the picture. (Probably fine on the present case -- > this is general advice.) Don't crowd the subject, that can only ever make > things worse. Preferably, take the picture from a distance and use > optical zoom to enlarge it -- this also reduces barrel/fisheye distortion, > and abberation of the lens. Some postprocessing may still be necessary to > correct for the spherical CRT face. All the more reason to move into the > 20th century with a digital scope -- even better, one with floppy drive > (if you're really old fashioned), serial or GPIB interface so you can save > a screenshot directly. > > Tim > > -- > Seven Transistor Labs, LLC > Electrical Engineering Consultation and Contract Design > Website: https://www.seventransistorlabs.com/ > > "amdx" <nojunk@knology.net> wrote in message > news:pi60di$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 >> >
Good advice. And use the self timer with a tripod.
On 7/12/2018 2:36 AM, tom wrote:
> "Tim Williams" <tiwill@seventransistorlabs.com> wrote in message > news:pi6soc$7gf$1@dont-email.me... >> "tom" <tmiller11147@verizon.net> wrote in message >> news:pi6bcd$1le$1@dont-email.me... >>>> >>>> It shouldn't be! >>>> >>> >>> It certainly should be. >>> >> >> 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 >> >> -- > > 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. > > > >
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.
> https://www.youtube.com/watch?v=9cP6w2odGUc&t=214s > https://www.youtube.com/watch?v=Il_eju4D_TM
On 07/11/2018 06:30 PM, amdx wrote:
> &nbsp;I ordered a built 74AC14 TDR square wave generator. > &nbsp;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 > > > &nbsp;Not sure it matters, I would just like a nice rising edge to start with. > > > > &nbsp;Here's the TDR unit I bought, click through pictures to see layout > of connectors. >> https://www.ebay.com/itm/152587007700 >
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Mikek
>
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"?
On 7/12/2018 2:40 AM, tom wrote:
> "Tim Williams" <tiwill@seventransistorlabs.com> wrote in message > news:pi6t93$a7g$1@dont-email.me... >> FYI: >> 1. Dropbox says it's a file, not an image, so it has to be downloaded. >> 2. Dropbox? Really? >> 3. It's 710x483 which is fine, but it's saved in the worst possible image >> format, 24-bit PNG. It's literally a zipped bitmap. Use JPEG, usually >> around 90-95 quality factor, and you'll see file size drop to maybe 50k. >> Only use PNG for reduced-color line drawings. >> 4. Try to set up square shots, especially for scope shots! Look for the >> reflection of the camera off the screen/bezel, and center that on the >> screen and in the camera viewfinder. Turn off ambient lights to eliminate >> the reflection, then take the picture. Adjust exposure until the trace is >> easily visible, not burned out -- automatic settings are usually poor on >> high-contrast displays. The graticule intensity seems to have kept >> contrast reasonable so that only a little trace is burned out (just >> barely, around the top right), which works as well. >> 5. And always focus the picture. (Probably fine on the present case -- >> this is general advice.) Don't crowd the subject, that can only ever make >> things worse. Preferably, take the picture from a distance and use >> optical zoom to enlarge it -- this also reduces barrel/fisheye distortion, >> and abberation of the lens. Some postprocessing may still be necessary to >> correct for the spherical CRT face. All the more reason to move into the >> 20th century with a digital scope -- even better, one with floppy drive >> (if you're really old fashioned), serial or GPIB interface so you can save >> a screenshot directly. >> >> Tim >> >> -- >> Seven Transistor Labs, LLC >> Electrical Engineering Consultation and Contract Design >> Website: https://www.seventransistorlabs.com/ >> >> "amdx" <nojunk@knology.net> wrote in message >> news:pi60di$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 >>> >> > > Good advice. And use the self timer with a tripod. > > > >
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, > https://www.dropbox.com/s/48i8fmfp9mhy1o5/delayed%20sweep%20jpg.jpg?dl=0 I'll go away and lick my wounds! Mikek :-)
On 7/12/2018 1:03 PM, bitrex wrote:
> On 07/11/2018 06:30 PM, amdx wrote: >> &nbsp;&nbsp;I ordered a built 74AC14 TDR square wave generator. >> &nbsp;&nbsp;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 >> >> >> &nbsp;&nbsp;Not sure it matters, I would just like a nice rising edge to start >> with. >> >> >> >> &nbsp;&nbsp;Here's the TDR unit I bought, click through pictures to see layout >> of connectors. >>> https://www.ebay.com/itm/152587007700 >>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Mikek
>> > > 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"?
I'm have no desire to send it back, it works. What do you want it to do? Mikek
Am 12.07.2018 um 04:15 schrieb John Larkin:
> On Wed, 11 Jul 2018 16:45:58 -0700, mike <ham789@netzero.net> wrote: > >> 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. > > 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.
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