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How can I build a simple pulse generator to demonstrate transmission lines

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Re: How can I build a simple pulse generator to demonstrate transmission lines

#11

Post by adrian_b »

KK7NIL wrote: post #2

Nice write up and sneaky introduction to time-domain reflectrometry but I'd like to point out the classic answer to this question is the famous Jim Williams pulse generator: https://github.com/podonoghue/Jim_Williams_Pulse_Generator?t...

Avalanche transistors, like the tunnel diodes mentioned by another poster, had been widely used in the past for generating fast pulses.

However, nowadays it is difficult to find any bipolar transistors that are suitable to be operated in the avalanche mode or any tunnel diodes, because these were fabricated using older technologies that are not suitable for the semiconductor devices that are popular today, so most such fabrication lines have been closed, due to insufficient demand.

Only for extremely few bipolar transistors the characteristics of the avalanche mode operation were specified by their manufacturer, so for most devices using avalanche transistors the transistors for each built device had to be cherry picked by testing many transistors of a type known to include suitable transistors.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#12

Post by greenbit »

HNisCIS wrote: post #7

The real gem is the answer at the bottom about doing the same thing with a bit of transmission line you treat as a capacitor.

Yeah, that's a pretty awesome idea. The only critical spec for the transistor is the time to get into saturation, the actual pulse length coming from the propagation of the discharge of the coax segment from the collector end to the far end. When the capacitance of that coax is discharged, the pulse stops, even with the transistor still on.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#14

Post by ssl-3 »

chimpanzee2 wrote: post #10

Any recommended resources for learning the first principles required to understand this and all the components involved?

For transmission lines: _Similarities of Wave Behavior_, presented by Dr. J. N. Shier (of Bell Labs fame, and whose team invented the phototransistor):

https://www.youtube.com/watch?v=DovunOxlY1k

It's an easy thing to watch at any level, with both brilliant practical demonstrations and supporting math provided.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#15

Post by KK7NIL »

adrian_b wrote: post #11
KK7NIL wrote: post #2

Nice write up and sneaky introduction to time-domain reflectrometry but I'd like to point out the classic answer to this question is the famous Jim Williams pulse generator: https://github.com/podonoghue/Jim_Williams_Pulse_Generator?t...

Avalanche transistors, like the tunnel diodes mentioned by another poster, had been widely used in the past for generating fast pulses. However, nowadays it is difficult to find any bipolar transistors that are suitable to be operated in the avalanche mode or any tunnel diodes, because these were fabricated using older technologies that are not suitable for the semiconductor devices that are popular today, so most su…

A 2N3904 will show similar avalanche behavior and is still widely available.

But, really, I just took the opportunity to talk about Jim Williams and his magnum opus that is AN47!

Re: How can I build a simple pulse generator to demonstrate transmission lines

#16

Post by KK7NIL »

chimpanzee2 wrote: post #10

Any recommended resources for learning the first principles required to understand this and all the components involved?

W2AEW on YouTube was one of my introductions to high-speed analog electronics (and I had the pleasure of working with him later on).

Re: How can I build a simple pulse generator to demonstrate transmission lines

#17

Post by adrian_b »

KK7NIL wrote: post #15
adrian_b wrote: post #11

Earlier quoted context omitted.

Avalanche transistors, like the tunnel diodes mentioned by another poster, had been widely used in the past for generating fast pulses. However, nowadays it is difficult to find any bipolar transistors that are suitable to be operated in the avalanche mode or any tunnel diodes, because these were fabricated using older technologies that are not suitable for the semiconductor devices that are popular today, so most su…

A 2N3904 will show similar avalanche behavior and is still widely available. But, really, I just took the opportunity to talk about Jim Williams and his magnum opus that is AN47!

Indeed, in the now distant past the application notes from companies like Linear Technology, and many others, were a treasure of information from which one could learn more about electronics than from university textbooks.

Sadly, such great technical documentation exists no more. The companies that make such products are no longer your business partners, but they are adversarial entities, whose only goal is how to confuse and to fool their customers into paying as much as possible for products whose quality is as low as possible.

Educating your customers about how to better use your products is no longer a business goal. Another current thread on HN is about the fear that the huge decline in the quality of technical documentation during the last 3 decades will be accelerated by the replacement of professional technical writers with AI.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#18

Post by KK7NIL »

adrian_b wrote: post #17
KK7NIL wrote: post #15

Earlier quoted context omitted.

A 2N3904 will show similar avalanche behavior and is still widely available. But, really, I just took the opportunity to talk about Jim Williams and his magnum opus that is AN47!

Indeed, in the now distant past the application notes from companies like Linear Technology, and many others, were a treasure of information from which one could learn more about electronics than from university textbooks. Sadly, such great technical documentation exists no more. The companies that make such products are no longer your business partners, but they are adversarial entities, whose only goal is how to co…

The market has changed significantly, there's much less need for this kind of education for a 3 cent microcontroller.

I've found ADI still has some great educational material, although that's partly because they've been better at maintaining their webpages from the 90's and 00's, not because they're putting out much new material.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#19

Post by lostlogin »

dylan604 wrote: post #4
hilbert42 wrote: post #3

Decades ago we used a much simpler method. A few 50 or 75 ohm non-inductive resistors and a tunnel diode. Feed any (slow) pulse generator into the diode and make it switch. Tunnel diodes can have sub-nanosecond switching times. We also used this technique to check/measure the rise times of our oscilloscopes.

sometimes, I realize how much is taken for granted these days that when I stop to think about is one of those "whoa" (in a Neo voice) moment. being able to time something in the picosecond range just gives me a wry smile.

I use an MRI scanner.

One day the engineer explained the switching between transmit and receive and the need for it to be very accurate - blasting the receive coil with the massive transmit coil when the receive coil expects a minuscule signal would be bad.

It’s timed in picoseconds. It’s so impressive.

Re: How can I build a simple pulse generator to demonstrate transmission lines

#20

Post by [deleted] »

lostlogin wrote: post #19
dylan604 wrote: post #4

Earlier quoted context omitted.

sometimes, I realize how much is taken for granted these days that when I stop to think about is one of those "whoa" (in a Neo voice) moment. being able to time something in the picosecond range just gives me a wry smile.

I use an MRI scanner. One day the engineer explained the switching between transmit and receive and the need for it to be very accurate - blasting the receive coil with the massive transmit coil when the receive coil expects a minuscule signal would be bad. It’s timed in picoseconds. It’s so impressive.

[deleted]
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