Author Topic: Transmission Line Avalanche Pulse Generator  (Read 40647 times)

0 Members and 1 Guest are viewing this topic.

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Transmission Line Avalanche Pulse Generator
« on: January 27, 2013, 01:22:36 am »
Hi Group,

I would like to share some details of a Transmission Line Avalanche Pulse generator that I built.

The challenge is to try and generate a flat-topped waveform.

Here is a 4ns pulse being displayed on a Tektronix 7104 1GHz oscilloscope.


The rest of the story is in the pdf document attached.

Jay_Diddy_B
« Last Edit: January 27, 2013, 01:24:24 am by Jay_Diddy_B »
 
The following users thanked this post: Andie

Offline muvideo

  • Frequent Contributor
  • **
  • Posts: 393
  • Country: it
Re: Transmission Line Avalanche Pulse Generator
« Reply #1 on: January 27, 2013, 03:36:22 am »
Nice, thank you for sharing, it's always fascinating
the exploration of such high speed events.

I had success using a STC945, but since I'm cheap,
I built a simple discrete step-up for the power supply :)

The pulse profile I obtained was similar to yours,
but slightly longer at 10nS total, I was limited by
my plugin speed:
http://www.flickr.com/photos/14336723@N08/7248441944/#

Fabio.
Fabio Eboli.
 

Online free_electron

  • Super Contributor
  • ***
  • Posts: 6936
  • Country: us
    • SiliconValleyGarage
Re: Transmission Line Avalanche Pulse Generator
« Reply #2 on: January 27, 2013, 05:39:53 am »
Good show old man ! ( quote from the addams family movie )

As i need to send out the PCB for the JW pulser this weekend i will adapt it so it has a transmission line on it. don;t know if it will work... i already had the layout adapted so it only uses traces ont he top. so i can use the bottom side of the board to do the wave guide.

i recalculated and will use 25 mil track with 8 mil gap . don't know how long i will be able to make the track ... i'll do the layout so that you can solder the 1 meg resistor into te classic layout or solder it into the waveguide and 'dot' the other end ento the ring around the collector

(see the JW pulse generator topic here)



Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline CarlG

  • Regular Contributor
  • *
  • Posts: 152
  • Country: se
Re: Transmission Line Avalanche Pulse Generator
« Reply #3 on: January 27, 2013, 06:14:45 am »
Nice work & good report!
 

Offline BravoV

  • Super Contributor
  • ***
  • Posts: 5356
  • Country: 00
Re: Transmission Line Avalanche Pulse Generator
« Reply #4 on: January 27, 2013, 09:20:17 pm »
Thanks for sharing this, and what a bunch of nice scopes you got there !  :-+
 

Online Mechatrommer

  • Super Contributor
  • ***
  • Posts: 8249
  • Country: my
  • reassessing directives...
Re: Transmission Line Avalanche Pulse Generator
« Reply #5 on: January 27, 2013, 10:31:06 pm »
if no abberation there, the rise time should be around 400ps guesstimating from my eyeball.
edit: i agree to prolonge the ON period to greater than 10ns minimum, so there's no dispute on rise time for slower DSO. at 4ns pulse, a 100MHz DSO will not reach max volt reading by the time the pulse is going down.
« Last Edit: January 28, 2013, 04:46:48 am by Mechatrommer »
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

alm

  • Guest
Re: Transmission Line Avalanche Pulse Generator
« Reply #6 on: January 28, 2013, 03:15:19 am »
Which is about right for a 1 GHz scope. You would need a sampling scope to properly characterize this circuit.
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #7 on: January 28, 2013, 03:56:44 am »
i did similar efforts, finally ended up with 2m of RG316, based on AN94 version http://www.linear.com/docs/4183

My results are so far ok, you can compare to your TDS700 pics to see the difference.

EDIT: i'm using Mini Circuits 20db attenuator between the pulser and scope to optimize the damping.



« Last Edit: January 28, 2013, 04:00:30 am by tinhead »
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #8 on: January 28, 2013, 04:56:44 am »
Hi,
I am working on a new version. The new version will have a pre-trigger so that I can use my Tektronix SD-24 sampling head in an 11801A  mainframe. This is a 20GHz scope. The sampling scope requires a pre-trigger.

I have an attenuator built in to my board.

I hope to have some results later today.

Stay tuned ....

Jay_Diddy_B
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #9 on: January 28, 2013, 10:54:36 am »
Hi,
I am going to share some preliminary pictures and test results. I will take some better scope shots when I am finished.

The results were measured with a BFR505 transistor. The BFR505 will avalanche with around 30V.

The scope shots were taken on a Tektronix 11801 scope with a SD-24 sampling head. This combination has a bandwidth of 20GHz and a risetime of 17.5 ps  :-+

Blank Board component side:


Blank Board solder side:



Rise time timebase 200ps / div. Measured risetime is around 100ps



Full pulse at 2ns / div



It will be interesting to see if the waveform cleans up when I put in all the 'Z' wires. Have left them out because I want to experiment with them.


Photograph of the assembled board, all the important stuff is the lower left corner.


Photograph showing the SMD transistor



Component side. The 74HC14s are used to generate the pre-trigger timing.



I will share more details later.

Jay_Diddy_B

« Last Edit: January 28, 2013, 10:59:48 am by Jay_Diddy_B »
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #10 on: January 28, 2013, 01:42:32 pm »
Hi Group,

I have done a little more work. I have installed about 1/2 of the 'Z' wires that connect the top and bottom ground planes. I have also taken some scope shots with some other scopes.

Tektronix TDS 754A 500MHz Bandwidth


Tektronix DSA602A with 11A71 Plugin 1 GHz


These were taken with a Tektronix 11801 and SD-24 sampling system. 20 GHz BW , 17.5ps RT.








I have also attached the schematic in pdf format.

Jay_Diddy_B

« Last Edit: January 28, 2013, 01:44:37 pm by Jay_Diddy_B »
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #11 on: January 28, 2013, 02:42:49 pm »
I have also taken some scope shots with some other scopes.

Tektronix TDS 754A 500MHz Bandwidth



mixed feelings ... yours looks like Figure 11 in AN94, where my compared looks like Figure 13. The rise time difference (71ps) is "huge", these scopes are normally good up to ~600MHz, these 735ps are in my opinion not good enough.
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #12 on: January 28, 2013, 02:45:25 pm »
JDB,

I can't find a ferrite bead noted on a schematic. I'm trying to google 'we74279203'. Could you please share the manufacturer/model?

Thank you,
Oleg.
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #13 on: January 28, 2013, 03:12:08 pm »
WE is "Wurth Elektronik", simply search for number
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #14 on: January 28, 2013, 06:18:04 pm »
Hi,

for mazurov:

The Ferrite bead is from Wurth Electronik. Some of these parts are available from Digikey. This part is very similar

Digikey 732-1602-1-ND

The bead is not super critical.

for tinhead:

My risetime has not been peaked. I don't have a capacitor on the collector of the transistor, only the transmission line. It opens up a much bigger question as to why the risetime on an oscilloscope looks the way it is.

If a scope had a single pole at the bandwidth frequency then the waveform would look exponential.

Oscilloscopes have been engineered to have a Gaussian Response.

Here is a SPICE model of a Gaussian Filter




The reponse in the Frequency Domain




The response in the Time Domain




One of the properties of the Gaussian filter is that as you reduce the risetime on the input the risetime on the output reaches a minimum. After this point the risetime on the output becomes fixed.

HP made some Transistion Time limiters based on this property. The model numbers are 15432 through 15435

Link http://www.home.agilent.com/en/pc-1000000488%3Aepsg%3Apgr/transition-time-converters?nid=-536902323.0.00&cc=CA&lc=eng


What is interesting about this, is that the waveform from my pulse generator looks flater on a low bandwidth scope than it does on a higher bandwidth model.

The bottom line is that you should not see overshoot unless the input is overshooting.

Is there a Gaussian Conspiracy ?  :-//

Jay_Diddy_B
« Last Edit: January 28, 2013, 06:22:16 pm by Jay_Diddy_B »
 

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1045
  • Country: fi
Re: Transmission Line Avalanche Pulse Generator
« Reply #15 on: January 28, 2013, 06:26:05 pm »
Hi,

Here is a SPICE model of a Gaussian Filter



Jay_Diddy_B

Isn't a Gaussian filter supposed to be like cascaded RC instead of cascaded LC?

Regards,
Janne
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #16 on: January 28, 2013, 07:26:30 pm »
Hi,

This application note from Picosecond Pulse Labs discusses the Gaussian Filter.

http://www.picosecond.com/objects/AN-7a.pdf

By strict definition the filter I implemented is 'near Gaussian' I modelled the G10 filter as described in the Picosecond app note.

Jay_Diddy_B
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #17 on: January 28, 2013, 11:17:16 pm »
Oscilloscopes have been engineered to have a Gaussian Response.

The rise time of this pulse generator is in ps range, this is where a typical digital scope respond flat and not gaussian anymore

http://cp.literature.agilent.com/litweb/pdf/5988-8008EN.pdf

therefore, and this is my opinion and why i've set damping in my generator as i did, it does not make sense to "cut" the signal to match gaussian response.

I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #18 on: January 29, 2013, 10:32:52 pm »
Hi,
 It is a very interesting application note from Agilent. It basically say that oscilloscopes fall into two groups Gaussian and Flat response. They present the argument that the Flat Response instruments will give more accurate risetime measurements.

The essential difference is that in the Flat Response instruments the analog front end does not restrict the bandwidth, and an anti-aliasing filter dominates the response of the instrument.

The application note references Infiniium 58400 series of oscilloscopes. So I did a little digging into the specifications. This is what I found:



This suggests that only the top of the line 2.5GHz 54846A is flat response. All the other members are Gaussian.

I also looked at the Tektronix TDS520 manual. I believe that all the 500MHz TDS scopes have similar performance, but I might be wrong. And this is what I found there:





These specifications and the second formula provided by Tektronix suggest that the Tektronix TDS 500 series are in fact Gaussian.

It is also interesting to note that the risetimes are not measured directly, but calculated from the bandwidth measurement.


Jay_Diddy_B
« Last Edit: January 29, 2013, 10:44:00 pm by Jay_Diddy_B »
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #19 on: January 30, 2013, 10:16:00 am »

These specifications and the second formula provided by Tektronix suggest that the Tektronix TDS 500 series are in fact Gaussian.


right, TDS500/600/700 belongs for sure to Gaussian response group, the newer series TDS2000/TDS3000 as well.
TDS7000 should be however Flat response model.

I did some measurments of TDS700(with 250MHz LPF enabled) and Tekway TDS1202B -> this is a chinese DSO, with 200MHz BW. It seems to be as well in flat group, from the values it does match Butterworth. The same input stage is being used in lot of chinese DSOs, another example is Rigol DS1102e (all these Rigol hardware clones made by ATEN and Siglent).



It could be insteressting to see, properly measured, frequency respond of Rigol DS1102E.
« Last Edit: January 30, 2013, 10:38:05 am by tinhead »
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline PA4TIM

  • Super Contributor
  • ***
  • Posts: 1115
  • Country: nl
  • instruments are like rabbits, they multiply fast
    • PA4TIMs shelter for orphan measurement stuff
Re: Transmission Line Avalanche Pulse Generator
« Reply #20 on: February 01, 2013, 11:44:44 am »
http://www.pa4tim.nl/wp-content/uploads/2010/11/calibratiescoop.png
Picture of bandwidth test using a sinewave. The Rigol DS1102E is included.
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
http://www.youtube.com/user/pa4tim my youtube channel
 

Online Mechatrommer

  • Super Contributor
  • ***
  • Posts: 8249
  • Country: my
  • reassessing directives...
Re: Transmission Line Avalanche Pulse Generator
« Reply #21 on: February 01, 2013, 03:42:03 pm »
can you post it at http://www.eevblog.com/forum/reviews/frequency-response-of-your-dso/ including the excel file? whats the relation between dBmV, mVtt, mV rms and dB columns? noob question here i hope you can reply in that thread, thanks.
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #22 on: February 03, 2013, 11:15:13 am »
Hi Group,

I have been busy adjusting my Transmission Line Avalanche Pulse Generator. Here are the latest results:

Picture 1 - Measured with Tektronix SD-24 in 11801 mainframe 20 GHz sampling scope.
Full pulse 1ns/div



Picture 2 - Measured with Tektronix SD-24 in 11801 mainframe 20 GHz sampling scope.
Risetime 200ps/div



Picture 3 - Measured with Tektronix SD-24 in 11801 mainframe 20 GHz sampling scope.
Risetime 100ps/div
Measured risetime is 82ps  (light travels 25 mm in 82ps, a signal on the board about 17mm)  :-+



Picture 4 - Measured with Tektronix 11A71 plugin in DSA602A. (1GHz BW)
risetime 200ps/div



Picture 5 - Measured with Tektronix TDS754A 500MHz scope.
risetime 500ps/div

Measured risetime is consistent with measured bandwidth = 340 / .568 = 619 MHz.




Construction



The important area:




I have attached a pdf file of the schematic

Jay_Diddy_B

« Last Edit: February 03, 2013, 11:17:54 am by Jay_Diddy_B »
 

Offline krenzo

  • Regular Contributor
  • *
  • Posts: 102
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #23 on: February 03, 2013, 11:59:44 am »
Jay, you can use this program to get screenshots from your 11801.  I use it with my 11801B and a serial-to-usb cable.  I have the source code if you're interested.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #24 on: February 03, 2013, 12:21:20 pm »
Krenzo,

Interesting!!

The screenshots from the DSA602A are generated by a program that I wrote to convert the Tektronix bitmap file to TIF files. My DSA602A does not have the latest firmware in it.

I was planning on doing the transfer electronically, but for now I use I use a floppy. The 11801 does not have a floppy, so I used a camera. I was going to use GPIB with a prologix adapter. It is not very often that I use the 11801. It is not a very good general purpose scope.  :D

I will look on the back and see if it has RS232.

Many thanks !!!

Jay_Diddy_B
 

Offline tinhead

  • Super Contributor
  • ***
  • Posts: 1801
  • Country: 00
    • If you like my hacks, send me a donation
Re: Transmission Line Avalanche Pulse Generator
« Reply #25 on: February 03, 2013, 01:55:41 pm »
now it looks perfect, good job!
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline marshallh

  • Supporter
  • ****
  • Posts: 1458
  • Country: us
    • retroactive
Re: Transmission Line Avalanche Pulse Generator
« Reply #26 on: February 03, 2013, 04:23:33 pm »
Very nice captures and pictures. Thanks for sharing.
Verilog tips
BGA soldering intro

11:37 <@ktemkin> c4757p: marshall has transcended communications media
11:37 <@ktemkin> He speaks protocols directly.
 

Offline muvideo

  • Frequent Contributor
  • **
  • Posts: 393
  • Country: it
Re: Transmission Line Avalanche Pulse Generator
« Reply #27 on: February 03, 2013, 07:53:01 pm »
Very nice result!
Do you have an idea about how sensitive is the pulser
behaviour to the particular transistor or pcb?

Just to understand if it's easily reproduced,
I updated my "todo list" with your pulser :)
It will be a nice playground for learning to use
old sampling scope plugins.

Could you post the layout, if you did already,
sorry, I've missed it.

Thanks,
Fabio.
Fabio Eboli.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #28 on: February 03, 2013, 09:40:25 pm »
Hi,

I believe that the layout is relatively insensitive to the board material. This is based on using a calculator on the coplanar waveguide. The properties of the coplanar waveguide seem to be dominated by the gap between the trace and the ground plane. This why I chose coplanar waveguide over stripline.

I used standard FR4.

The layout seems critical. To remove the ringing on the pulse I used a Tek SD-26 in the TDR mode to look for impedance discontinuities.

By far the bigest improvement came from removing this thin ground track in the attenuator section.



The BFR505 was not selected. It was manufactured by NXP and bought from Digikey.

The next step would be to buy some proper end launch SMA connectors and make a second one.

Jay_Diddy_B
« Last Edit: February 03, 2013, 09:42:39 pm by Jay_Diddy_B »
 

Offline mrflibble

  • Super Contributor
  • ***
  • Posts: 1878
  • Country: nl
Re: Transmission Line Avalanche Pulse Generator
« Reply #29 on: February 03, 2013, 10:30:28 pm »
By far the bigest improvement came from removing this thin ground track in the attenuator section.

Stupid question: why was that trace there in the first place? As in, what would it's intended function be?
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #30 on: February 04, 2013, 02:02:48 am »
Hi,

Mechatrommer is correct. I just did a copper pour in the design of the pcb with the spacing set at 20 thou. I have a lot of experience at moderate frequencies, but I am new to GHz+ frequencies.

To me this is a learning excercise.

Jay_Diddy_B
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #31 on: February 07, 2013, 11:58:40 pm »
Hi Group,

All the previous scope photographs that I have posted were made with a BFR505 transistor.
I have now swapped the transistor with the MMBT2369A, this is the SMD version of the 2N2369A.

Here are the new pictures:









These pictures were taken with a 20 GHz scope (SD-24 in 11801).

Regards,

Jay_Diddy_B
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #32 on: February 08, 2013, 10:36:00 am »
It is strange that this transistor avalanches that slowly. I have a pulser made out of 2N2369A in TO-18 and it's about twice as fast.

Another strange thing is that top is ramping up.

 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #33 on: February 12, 2013, 09:15:50 pm »
I built BFR505-based pulser and here are my results. I'm using 1m of good quality coax generating ~10ns wide pulse. Rigid cable gives better pulse but the longest piece I have is 6in. It seems that 20ns pulse width is the limit  for BFR505 - first 3 lasted about 10 minutes, I had to stop since I only have 2 left. 10ns seems to be safe.

The 200ps screenshot shows the risetime of 7a29p amplifier which is about right.

Next step is going to be wiring a pre-trigger from S-52 and check the risetime with 7S12 sampler.
« Last Edit: February 12, 2013, 09:20:22 pm by mazurov »
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #34 on: February 12, 2013, 09:44:25 pm »
Hi mazurov,

What repetition frequency are you using?

What is the voltage of the power supply?

I did my testing at 20kHz.

I will be interested to see the results from the 7S12. Which sampling head do you have?

Regards,

Jay_Diddy_B
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #35 on: February 12, 2013, 10:01:34 pm »
This one self-avalanches every 16.6 uS, so about 60KHz. The power supply is at 53V.

I have S-6 and S-4 heads, will try both. The biggest question is whether 800ps risetime 1V pre-trigger pulse from S-52 will be enough to trigger the transistor.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #36 on: December 10, 2013, 03:39:38 pm »
Hi Group,

With the low cost of having boards made by itead, $15.00 for 10 boards 5 x 10 cm, I have modified my design to fit on a 5 x 10cm board.

I order the new boards about two weeks ago, I have been tracking them. I will probably get to try the new boards on the weekend.

Here are pictures of the new board:





Jay_Diddy_B
 

Offline mazurov

  • Frequent Contributor
  • **
  • Posts: 448
  • Country: us
Re: Transmission Line Avalanche Pulse Generator
« Reply #37 on: December 11, 2013, 03:54:42 am »
Are you planning on selling some of the boards?
 

Online nctnico

  • Super Contributor
  • ***
  • Posts: 15264
  • Country: nl
    • NCT Developments
Re: Transmission Line Avalanche Pulse Generator
« Reply #38 on: December 11, 2013, 09:28:23 am »
If that serpentine trace is supposed to be a matched impedance trace then the clearance is way too narrow.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online free_electron

  • Super Contributor
  • ***
  • Posts: 6936
  • Country: us
    • SiliconValleyGarage
Re: Transmission Line Avalanche Pulse Generator
« Reply #39 on: December 11, 2013, 09:35:48 am »
its not the impedance. This thing is a long capacitor.
one thing that concernsme is i don't see an opening in the soldermask... that's going to leak...
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #40 on: December 11, 2013, 10:33:15 am »
Hi,

The dimension of the transmission line are the same as the line that was used to generate this pulse:





I used this calculator:

http://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm



I am also concerned about having solder mask on the line. I realized this after I sent it out. I will have to what happens when I get the boards back. I can always try scraping away the solder mask  :(

Jay_Diddy_B


« Last Edit: December 11, 2013, 10:34:52 am by Jay_Diddy_B »
 

Offline BravoV

  • Super Contributor
  • ***
  • Posts: 5356
  • Country: 00
Re: Transmission Line Avalanche Pulse Generator
« Reply #41 on: December 11, 2013, 01:14:17 pm »
I am also concerned about having solder mask on the line. I realized this after I sent it out. I will have to what happens when I get the boards back. I can always try scraping away the solder mask  :(

Let me guess, precision scraping using the Protomat ?  >:D
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #42 on: January 02, 2014, 01:50:21 pm »
Hi group,

I am ready to share some results obtained from the new version of the board made by itead.

TDR Testing

A Tektronix SD24 sampling head was used in a Tektronix 11801 Mainframe. The SD24 has specified reflected rise time of 35ps.

Test setup:

The board was connected to the SD24 with a short length of semi-rigid coaxial cable.



The attenuator on the board was shorted. The avalanche transistor was also shorted collector emitter. This allows the transmission line on the board to be measured with the TDR.



TDR Results



The step rise is the start of the pulse. The first aberration is the SMA connector on the SD24. There is a smooth section which is the 100mm length of semi-rigid cable. The next group of aberrations is the SMA connector, R7 and Q1 components. After this is the transmission line on the board. There are small aberrations corresponding to the bends on the transmission line.

It can be seen that the transmission line on the board is a close match to the 50 Ohm coax.


Pulse Testing

In order to get the best pulse shape I had to play around with the attenuator located on the PCB. Originally I had a single 20dB attenuator. This did not work very well. There was a very fast spike on the output corresponding to a fraction of pF in parallel with the resistor. I split the attenuator into two 6dB sections. I also found it was necessary to add a matching network.

Here is a picture of the attenuator section:



Here is a picture of the pulse generated:



The rise time is around 60ps  :-+



Jay_Diddy_B

« Last Edit: January 02, 2014, 01:52:40 pm by Jay_Diddy_B »
 

Offline BravoV

  • Super Contributor
  • ***
  • Posts: 5356
  • Country: 00
Re: Transmission Line Avalanche Pulse Generator
« Reply #43 on: January 02, 2014, 04:24:35 pm »
Thanks for sharing, damn ... 60ps  :o , so the solder mask doesn't give you any trouble  ?
 

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1045
  • Country: fi
Re: Transmission Line Avalanche Pulse Generator
« Reply #44 on: January 02, 2014, 07:23:14 pm »
The effect of the solder mask is that it makes the impedance slightly lower and is more lossy than air. But the effect is quite negligible in this case. However, produced pulse looks somewhat like that FR4 dielectric and skin effect loss is becoming visible (tilting of rising edge near flat top). That is not surprising considering the edge speed. I wonder, would this benefit of considerably better substrate, like teflon or Rogers etc.

Also, since edge speed is very fast, there might be some non-TEM waveguide modes which cause distortion on the edge even if the impedance match is perfect.

Regards,
Janne
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #45 on: January 02, 2014, 10:26:21 pm »
Hi Janne and the group,

I did a little reading on TDR measurements. Tektronix has a nice document 55W_14601_2.pdf

Link: www.tek.com/dl/55W_14601_2.pdf

In this document there is this formula which gives the relationship between the reflection coefficient and the impedance:



The Tektronix 11801/SD24 makes this easy by giving a cursor readout of the impedance.



I have one cursor on the semi-rigid coax and the other cursor on the straight section of the coplanar waveguide. The measured impedances are 51 and 47 Ohms respectively.



If I move the second cursor to the bend on the coplanar waveguide, the measured impedance drops to 46 Ohms as shown in this measurement.



The impedance is slightly lower than 50 ohms. This could be due to the solder mask as suggested by Janne or the dielectric constant of the FR4 is not 4.7 as used in the calculation.

I also believe that some loss is visible in the TDR measurements.

I am sure that using an RF substrate instead of FR4 would give better results. I am also sure that building the high speed attenuator from ordinary 0603 and 0805 ($0.002) resistors is not the best. The idea is to try and see what you can do with ordinary parts.

It is quite remarkable that the Tektronix SD24 TDR is able to measure these discontinuities. The SD24 had a MRSP of $17,875 and the 11801 was about $20K (1990 dollars).

Jay_Diddy_B



« Last Edit: January 02, 2014, 10:32:38 pm by Jay_Diddy_B »
 

Offline DL8RI

  • Regular Contributor
  • *
  • Posts: 166
  • Country: de
Re: Transmission Line Avalanche Pulse Generator
« Reply #46 on: January 02, 2014, 10:45:04 pm »
Hi,

Quote
I am sure that using an RF substrate instead of FR4 would give better results.
Yes. With this signals you are far beyond the capabilities of FR4. I have never characterized FR4 up to this Frequency therefore I can't give you any numbers, but the tan-delta and permitivity will be quite off. I'd suggest you try some Rogers-Material or go for a semi-rigid-line instead of the CPW.
« Last Edit: January 02, 2014, 10:47:44 pm by DL8RI »
 

Online nctnico

  • Super Contributor
  • ***
  • Posts: 15264
  • Country: nl
    • NCT Developments
Re: Transmission Line Avalanche Pulse Generator
« Reply #47 on: January 03, 2014, 04:31:22 am »
The fiber glass weaving in the FR4 is probably also visible in the TDR trace. For circuits running at several GHz on FR4 it is recommended to have the traces run at 45 degree angles to even out the impedance differences caused by the fiber glass.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #48 on: January 03, 2014, 06:24:36 am »
The fiber glass weaving in the FR4 is probably also visible in the TDR trace. For circuits running at several GHz on FR4 it is recommended to have the traces run at 45 degree angles to even out the impedance differences caused by the fiber glass.

I am not experienced enough at these high frequencies to know if it is the fibre glass weave. I do know that more exotic materials are normally used for these frequencies.

At the self-generated frequencies the effect is not visible.

Is anybody aware of a place that will make boards using Rogers material at a reasonable price? The few places I looked wanted a lot of money.

I also looked for some SMD attenuators. I found these Susuma ones on Digikey:

Link: http://www.susumu-usa.com/pdf/products_38.pdf

I don't think that they are really good. Any alternatives?


Jay_Diddy_B
 

Offline marshallh

  • Supporter
  • ****
  • Posts: 1458
  • Country: us
    • retroactive
Re: Transmission Line Avalanche Pulse Generator
« Reply #49 on: January 03, 2014, 06:34:33 am »
I was wondering if the impedance changes in the coplanar waveguide section were caused by glass weave (I don't think so, since each 6 bends was clearly defined and consistent) or by a non-TEM mode. How would routing the turns as 45 degree beveled right angles instead of arcs affect that?

I remember reading just a bit ago somewhere, maybe PCB-Pool, doing a pool of Rogers (or was it alumnium?)
Verilog tips
BGA soldering intro

11:37 <@ktemkin> c4757p: marshall has transcended communications media
11:37 <@ktemkin> He speaks protocols directly.
 

Offline jahonen

  • Super Contributor
  • ***
  • Posts: 1045
  • Country: fi
Re: Transmission Line Avalanche Pulse Generator
« Reply #50 on: January 03, 2014, 07:01:26 am »
Howard Johnson states that even making perfectly rounded corners does not mitigate the excess capacitance (lower impedance regions!) at the corners. To mitigate the effect, one must remove much more material. That explains why in many microwave PCB's there are quite chamfered corners, but they are still 90 degree ones:

Quote
Layout professionals often point out that modern layout systems already round off all the outside corners, assuming that this rounding eliminates the square-corner effect. It doesn't. Rounding the corners removes 21% of the copper in the corner. Edwards[1] shows that you must remove 70 to 90% of the copper from a right angle bend to neutralize (to first order) the excess capacitance. Rounding removes only a small fraction of the required amount of copper. Rounded-corner right-angle bends work well in digital designs not because they are rounded, but because the corners are too tiny to cause significant problems in the first place.

[1] T. C. Edwards, Foundations of Interconnect and Microstrip design, 3rd ed.,John Wiley & Sons, 2000, ISBN 0-471-60701-0

Regards,
Janne
 

Offline jscm2000

  • Newbie
  • Posts: 2
Re: Transmission Line Avalanche Pulse Generator
« Reply #51 on: March 03, 2014, 12:08:37 am »
????1ns??????????????
 

Offline G0HZU

  • Super Contributor
  • ***
  • Posts: 2576
  • Country: gb
Re: Transmission Line Avalanche Pulse Generator
« Reply #52 on: March 03, 2014, 11:19:19 am »
Interesting thread...

I can only suggest PCB places in the UK that deal in laminates like Rogers for small qty. I expect that they deal worldwide though.

Very local to me is Trackwise in Tewkesbury.
http://www.trackwise.co.uk/

We sometimes also use Labtech but I haven't used them for a few years now.

http://www.teledynelabtech.com

I'm pretty sure they are still based in Wales. For large qty multilayer PCBs with Rogers on the outside and FR4 inner layers we use Graphic or Exception but these places will be very expensive even for a basic Rogers board.

I can mill Rogers boards here at home on a T-Tech Router but not to the accuracy you need. I can normally mill track widths to about 1-2 thou accuracy but it isn't guaranteed and it sometimes takes a few attempts. I think you have a milling machine as well so I guess this isn't an option.

Have you tried simulating the meanders on something like Sonnet EM? you can probably do it with the free trial version as I think it allows up to 32Mb these days if you register for the free trial upgrade to 32Mb.
http://www.sonnetsoftware.com/

I can't advise on the precision SMD attenuators as we tend to use fairly average spec ones for SMD use.

Note: It's worth asking about etching tolerance for Rogers boards. Labtech are pretty good at getting low tolerance. We sometimes need ultra high accuracy in the PCB for things like notch filters in the GHz region or simple microwave BPFs. The more expensive places like Exception or Graphic are the top choice for this though. They can also advise on what they think the characteristic impedance will be of the finished GCPW transmission line.



« Last Edit: March 03, 2014, 11:32:37 am by G0HZU »
 

Offline G0HZU

  • Super Contributor
  • ***
  • Posts: 2576
  • Country: gb
Re: Transmission Line Avalanche Pulse Generator
« Reply #53 on: March 04, 2014, 06:32:00 am »
I had a look at what SMD attenuators we have in our system and the usual suspects are either Barry or MiniCircuits.

The GAT range from Minicircuits are quite good and they are very small and cheap. But they aren't really what I would describe as precision attenuators.

http://217.34.103.131/pdfs/GAT-20+.pdf

The other option is to stick with SMD resistors and choose to make your attenuator using resistor values that have parasitics that can hopefully mimic a lossy transmission line when placed on your PCB.

The best behaved resistors in 0805 package tend to be in the 100 to 150 Ohm range as their impedance stays faithful up to the higher frequencies and you can make a pretty decent attenuator if you make up the values using combinations of these resistances. But the PCB will spoil this a bit once you place the resistors down on it and you may need to do some subtle matching to get very low return loss up at a few GHz.

Also, when asking about etching tolerance it's best to specify the copper weight because the available accuracy from the PCB maker probably won't be the same for different copper weights on the Rogers Laminate.  I tend to use Rogers 4003C or 4350 here at home on the milling machine. It is very hard on the tools though. Notably more wear compared to FR4.

In the old days (before they closed down) the best laminate for milling was GIL GML1000 as it was very easy on the tools and had decent RF performance. I've still use it as I still have some left but you might be able to buy some cheaply as obsolete stock and mill the board yourself or send out the GML1000 to a PCB place.
You can get it in 0.02" or 0.03" or 0.06" thicknesses.

http://www.apcircuits.com/resources/information/gml10060.pdf
« Last Edit: March 04, 2014, 06:45:27 am by G0HZU »
 

Online Marco

  • Super Contributor
  • ***
  • Posts: 3749
  • Country: nl
Re: Transmission Line Avalanche Pulse Generator
« Reply #54 on: May 18, 2014, 05:33:55 am »
Sorry for the necro, but in case you're still reading this ... did you ever run the sampling scope with envelope instead of averaging so you could determine the jitter?
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #55 on: February 09, 2015, 10:43:05 am »
Hi group,

I spent a little time in the lab today playing with the Transmission Line Pulse Generator. This is the latest version:




The performance can be influenced by the way you build the circuit. This is a picture of the unit I built today:





And the performance, the rising edge, using a Tektronix SD24 in a 11801 mainframe (20 GHz):




and the big picture showing the pulse:




Regards,

Jay_Diddy_B
« Last Edit: February 09, 2015, 10:44:37 am by Jay_Diddy_B »
 
The following users thanked this post: Andie

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #56 on: February 09, 2015, 01:32:58 pm »
Curious dip, seems to suggest the first... ah what is it... few millimeters? is kind of wonky.

Go figure! ;)

Nice 100ps edge, not bad at all.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #57 on: February 10, 2015, 01:48:43 pm »
Curious dip, seems to suggest the first... ah what is it... few millimeters? is kind of wonky.

Go figure! ;)

Nice 100ps edge, not bad at all.

Tim

Tim and the group,

I determined that the spike on the leading edge came from the potentiometer. It was not really inductance, it was the time that it took for the signal to reach the pot. This was new to me. I found it by setting the pot to 0 Ohms. Those pots have a folded construction inside.

The dip following the peak, I believe is called 'dribble-up'. I am not sure what physical properties cause it.

I have done some fiddling with the circuit to reduce the distortions.

Here is the latest schematic:




A photograph of the output section:



This is the inductor that I used, it has a self-resonance frequency of 3.2 GHz:




These are the latest waveforms:








It has taken a lot of experimenting to get this waveform.

Regards,

Jay_Diddy_B



« Last Edit: February 10, 2015, 01:50:46 pm by Jay_Diddy_B »
 
The following users thanked this post: Andie

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #58 on: February 10, 2015, 02:07:53 pm »
Yeah pretty much... good luck adjusting anything at this level.  Either it's made perfectly, or it stinks.

Also, this is two layers still?  Probably some of the remaining bumpiness comes down to mismatch between resistors (tiny!) and traces (fat), plus higher order TEM modes in the PCB, or even across the width of the trace.  Without getting really, really involved (many spins, or EM simulations?), going to 4 layer would probably be the next major improvement.

Even the best Tek generators had some residual bumps.  I suppose at that point, you have to wonder if it's the generator, or the scope, or cables, or connectors... or bondwires, or what?

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #59 on: February 10, 2015, 02:21:37 pm »
Tim,

I am using 2 layer 0.062 (1.6mm) FR4 boards made by Itead. I am still using the original boards that I made, 10 boards for $15.00 so nothing exotic at all. I scrapped away the solder mask and made some cuts in the copper.

The resistors are standard 5% carbon film that I bought in a binder from eBay. I have been using 0402 and 0603.

The SMA connector that I used on this board is nothing special either some cheap ones that I bought from eBay.

http://www.ebay.ca/itm/10pcs-connector-SMA-male-plug-solder-PCB-clip-edge-mount-straight-M-/281161178633?pt=LH_DefaultDomain_0&hash=item4176831e09

I am not using any cables, I have the board mated to directly to the Tektronix sampling head. The scope may not be perfect, but I can check the scope with 17.5ps rt generators in the TDR head.


I will not try and make any more improvements. I think I will attempt to build a second one the same.

Regards,

Jay_Diddy_B
« Last Edit: February 10, 2015, 02:36:33 pm by Jay_Diddy_B »
 

Offline 3roomlab

  • Frequent Contributor
  • **
  • Posts: 781
  • Country: 00
  • A long long time ago, in a land far far away ...
Re: Transmission Line Avalanche Pulse Generator
« Reply #60 on: October 14, 2015, 12:42:02 am »
Hi,

The dimension of the transmission line are the same as the line that was used to generate this pulse:

I used this calculator:

http://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm

I am also concerned about having solder mask on the line. I realized this after I sent it out. I will have to what happens when I get the boards back. I can always try scraping away the solder mask  :(

Jay_Diddy_B

i know this is an old thread, i was fiddling with the web calculator, i was thinking what if the whole trans-line was shrunk in size? ( i have zero experience in HF design)

example
Er = 4.2
width = 0.915mm (36mil)
gap = 0.153mm (6mil)
PCB = 1mm

Eeff = 2.696
Zo = 50.99

would shrinking the entire circuit help in improving the overall rise time?
« Last Edit: October 14, 2015, 01:22:00 am by 3roomlab »
1) Hayao Miyazaki - "there is no nuclear power here (at studio ghibli)"
2) Feynman - "What Do You Care What Other People Think?"
3) is it possible that the speed of light (a million years ago) is faster than it is now? or slower?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #61 on: October 14, 2015, 01:12:10 am »
Rise is largely limited by the switching device, and things immediately around it (lead/bondwire lengths, traces and pads).  As far as I know, ~200ps is characteristic of avalanche physics and device geometry, so you're doing quite well down there.

Better risetime can be had from a step recovery generator, but these are also more tricky to drive (an already fast pulse is needed) and lower amplitude (a few volts? not that you need much).  On the upside, jitter is better.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline 3roomlab

  • Frequent Contributor
  • **
  • Posts: 781
  • Country: 00
  • A long long time ago, in a land far far away ...
Re: Transmission Line Avalanche Pulse Generator
« Reply #62 on: October 14, 2015, 01:28:05 am »
Rise is largely limited by the switching device, and things immediately around it (lead/bondwire lengths, traces and pads).  As far as I know, ~200ps is characteristic of avalanche physics and device geometry, so you're doing quite well down there.

Better risetime can be had from a step recovery generator, but these are also more tricky to drive (an already fast pulse is needed) and lower amplitude (a few volts? not that you need much).  On the upside, jitter is better.

Tim

ok so i suppose the "shape" change of the trace/PCB affects something else (like what Jay said in earlier post) the jitter-y-jagged-noise of the output signal? (even if i build this i dont think i will see it properly in action, i have no fast scopes, so its more of a curious question)
1) Hayao Miyazaki - "there is no nuclear power here (at studio ghibli)"
2) Feynman - "What Do You Care What Other People Think?"
3) is it possible that the speed of light (a million years ago) is faster than it is now? or slower?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #63 on: October 14, 2015, 01:38:25 am »
The length of the TL determines the length of the square pulse, and its impedance and dispersion control how flat the pulse is, and what the falling edge looks like (dispersion causes an anomalously slow falling edge, "drool" as it were).  A periodically varying TL (as one might expect from a sinuous trace) will have humps or ripples, the amplitude of which depends on how far off the impedance is.  Rounded or chamfered corners are better than solid square corners.  Even a gradual bend should be reduced in width, slightly.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #64 on: October 14, 2015, 06:08:13 am »

i know this is an old thread, i was fiddling with the web calculator, i was thinking what if the whole trans-line was shrunk in size? ( i have zero experience in HF design)

example
Er = 4.2
width = 0.915mm (36mil)
gap = 0.153mm (6mil)
PCB = 1mm

Eeff = 2.696
Zo = 50.99

would shrinking the entire circuit help in improving the overall rise time?

The challenge with making the transmission narrower, is manufacturing tolerances. You can check what happens if the gap is 5mil, 6mil or 7mil.

Free-Electron in his variation used a narrower transmission line than I did.
http://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/msg186193/#msg186193


I do not have free_electron's version to test in my lab. My fast scope (20 GHz) requires a trigger pulse at least 40ns in front of the pulse that you are looking at so I could not test this version anyway.

Regards,

Jay_Diddy_B


 

Online Kalvin

  • Super Contributor
  • ***
  • Posts: 1695
  • Country: fi
  • Embedded SW/HW.
Re: Transmission Line Avalanche Pulse Generator
« Reply #65 on: October 14, 2015, 07:10:49 am »
Jay_Diddy_B,

Did you perform any tests or simulations what would happen if the ground areas around the transmission line was removed or pulled away from the transmission line considerably so that the EM-field would be almost completely confined between the transmission line (top layer) and the ground plane (bottom layer). To me it seems that the ground areas with the patched vias might reduce pulse quality.

For example, using the transmission line calculator http://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm with the following parameters

4.2
3.3 mm
5 mm - 10mm
1.5 mm

will give Z0 48.91 ... 50.43. Also, the tolerance to the track width is quite good. For example varying the track width between 3.1mm ... 3.3mm changes Z0 between 52.84 ... 48.54 (gap width = 10mm).
« Last Edit: October 14, 2015, 07:28:26 am by Kalvin »
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #66 on: October 14, 2015, 07:26:05 am »
Jay_Diddy_B,

Did you perform any tests or simulations what would happen if the ground areas around the transmission line was removed or reduced considerably so that the EM-field would be almost completely confined between the transmission line (top layer) and the ground plane (bottom layer). To me it seems that the ground areas with the patched vias might reduce pulse quality.

Kalvin,

There different kinds of transmission lines used on circuit boards.  There is microstrip which is a single trace over a ground plane. The width of the strip is determined by thickness of the board and the dielectric constant of the board. For a given board there is only one width of trace for given impedance.

I used a construction called co-planar waveguide. In co-planar waveguide the gaps between the strip and the ground plane influence the impedance. You can choose the width of the trace and vary the gap to keep the impedance constant. You can also make tapered sections to go from one width to another width.

There is also stripline where the conductor is between two ground planes.

I checked the impedance of the line using a TDR with 17.5ps rise time. With this TDR I can see impedance discontinuities in mated SMA connectors.

http://www.eevblog.com/forum/projects/transmission-line-avalanche-pulse-generator/msg358382/#msg358382


Regards,

Jay_Diddy_B






 

Online Kalvin

  • Super Contributor
  • ***
  • Posts: 1695
  • Country: fi
  • Embedded SW/HW.
Re: Transmission Line Avalanche Pulse Generator
« Reply #67 on: October 14, 2015, 07:35:39 am »
Jay_Diddy_B, thank you for the explanation. I removed my comment about the pulse quality as the ripple is quite modest. The tolerance to the track width seem to improve when the construction is towards the microstrip.
 

Offline 3roomlab

  • Frequent Contributor
  • **
  • Posts: 781
  • Country: 00
  • A long long time ago, in a land far far away ...
Re: Transmission Line Avalanche Pulse Generator
« Reply #68 on: October 15, 2015, 01:10:01 am »
i put together this based on Jay's PDF. im not making the project cos i have no fast scopes to "calibrate", so i am just mucking around with creating eagle PCB layouts and schematics. i am interested to know if i am in the right ballpark esp on the transmission line and the entry point of the SMA (i editted the footprint so that it is all similar width to the 39mil 50ohm trace throughout, does this make sense?) pls ignore the incomplete via stitching for ground plane. this size of the board is based on the popular 2inchx2inch, and it seems it is also possible to fit in a LT1073 power supply on the flip side left corner.

instead of the OSC in the PDF, i am toying with a 6 gate schmitt trigger as oscillator and R-C phase shifting it to get different rising edges. (or maybe not, schmitt trigger have large performance drift?)

i can see the length of the transmission line at approx 20320mils long, i dont suppose there is any harm if i make it even longer?
(edit, while trying to find some reading material, i saw this PDF called corners-japan, which tells of some interesting experiments about right angled traces)
« Last Edit: October 15, 2015, 03:19:47 pm by 3roomlab »
1) Hayao Miyazaki - "there is no nuclear power here (at studio ghibli)"
2) Feynman - "What Do You Care What Other People Think?"
3) is it possible that the speed of light (a million years ago) is faster than it is now? or slower?
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #69 on: October 15, 2015, 06:31:24 am »
Mind that there will be a very slight amount of coupling between side-by-side traces (very little, with the ground plane filling in the gaps, but it might be discernible on a faster scope?), and the bends have a lower impedance on account of the way the fields distribute across the width of a curve (the velocity is effectively faster on the inside corner), which also contributes some dispersion (falling edge 'drool').  Being in FR-4, it'll also be fairly "drool-y" due to the dielectric properties.  Which you can't really do much about, aside from perhaps an R+L (or a more complex network of some sort) at the termination.

Speaking of networks, I suppose the rise/fall time could be very slightly improved (no more than ~2x in the limit) by adding a small C or R+C in parallel with one or more of the output attenuator (series) resistors.  That might be worth trying, I mean, if you had a scope to play with of course.  (Nothing you'd bother with on the initial layout -- the extra pads would do as much to screw with the response as you'd be trying to fix.  But it's something that can be tried by stacking chip components, a perfectly cromulent prototyping approach.)

So, for what it is, it's fine, and unless you have over 1GHz of scope bandwidth to hand, you probably won't be able to tell the artifacts in the first place!

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline juani_c

  • Contributor
  • Posts: 19
  • Country: ar
    • ssihla
Re: Transmission Line Avalanche Pulse Generator
« Reply #70 on: December 31, 2015, 11:46:29 am »
In AN94, figure 8, the charge line is shown with an open end. Is that "charge line" similar to this "Transmission Line"?
Does the Transmission Line need to have an open end as well? I can't see that in the schematic.

I don't know much of Transmission Lines, sorry if this is a stupid question.

 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 1668
  • Country: ca
Re: Transmission Line Avalanche Pulse Generator
« Reply #71 on: December 31, 2015, 02:14:19 pm »
In AN94, figure 8, the charge line is shown with an open end. Is that "charge line" similar to this "Transmission Line"?
Does the Transmission Line need to have an open end as well? I can't see that in the schematic.

I don't know much of Transmission Lines, sorry if this is a stupid question.



The terms 'charge line' and 'transmission line' have the same meaning in this context. It could be described as a charged transmission line.

The open end of the transmission is connected to resistor that is used to charge the transmission line. From a fast signal point of view, the high value resistor is an open circuit.

The circuit works by first charging the transmission. The charged transmission line is then connected to the output attenuator using an avalanched transistor. The transistor switched in this way is very fast.

The length of the output pulse is twice the electrical length of the transmission line. The speed of the propagation in the transmission is very close to the speed of light for an air line, slower for a transmission line with plastic or epoxy insulation.

The circuit makes a pulse that has a flat top. You need the flat top if you want to measure the rise time of an oscilloscope.

Regards,

Jay_Diddy_B


 

Offline daiguangming

  • Newbie
  • Posts: 2
  • Country: cn
Re: Transmission Line Avalanche Pulse Generator
« Reply #72 on: February 05, 2018, 05:41:08 pm »
hi group
way i rise is not fast ?use ztx415
« Last Edit: February 05, 2018, 06:01:15 pm by daiguangming »
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11693
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Transmission Line Avalanche Pulse Generator
« Reply #73 on: February 05, 2018, 06:54:50 pm »
I have no idea what's going on with that board there...

ZTX415 is slower (and more powerful), check the datasheet.  Waveform seems more or less reasonable.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline Insatman

  • Frequent Contributor
  • **
  • Posts: 253
  • Country: ph
Re: Transmission Line Avalanche Pulse Generator
« Reply #74 on: February 05, 2018, 07:46:24 pm »
I posted about a charge line pulser with <600ps Trise using a simple 2N2222.  http://www.eevblog.com/forum/projects/avalance-jw-charge-line-pulser-for-step-waveforms-~45v-out/msg1405898/#msg1405898.

A photo of the output waveform below.  More detail in origional post including schematic.

Relatively cheap to build.
Retired Pulsed Power Engineer/Physicist...now I just dabble in electronics
 

Offline daiguangming

  • Newbie
  • Posts: 2
  • Country: cn
Re: Transmission Line Avalanche Pulse Generator
« Reply #75 on: February 05, 2018, 11:32:44 pm »
schematic?way fall is slower ?
 

Offline Insatman

  • Frequent Contributor
  • **
  • Posts: 253
  • Country: ph
Re: Transmission Line Avalanche Pulse Generator
« Reply #76 on: February 06, 2018, 01:53:01 pm »
schematic?way fall is slower ?

The fall is almost always slower than the rise on chargeline pulsers.  You can usually reduce it by putting a resistor from the output of the transistor to ground.  The value of which depends on your particular circuit.  I've found for most 50 ohm circuits that something between 50 and 200 ohms usually works best to shape the falling edge.   I'm sure that better methods exist as well.
Retired Pulsed Power Engineer/Physicist...now I just dabble in electronics
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf