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.
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.
I've not had the opportunity to study circuits of MRI scanners so I'm unfamiliar how they achieve such a high switching speed but I'd also suggest some clever circuitry must be employed to protect the sensitive receiver sensors/electronics from damage during the transmit cycle.
Seems to me we not only have ultra fast picosecond switching involved but also the transmit signal would have be attenuated many orders of magnitude around the receiver to stop it 'frying'. That's very impressive at that switching speed.
When I was a kid, I used to rat old disposals WWII radar equipment for parts to build hobby projects and what truly impressed me perhaps more so than the magnetrons was how the delicate receiver circuit was protected from damage during the transmit cycle which reached a pulse power of over 50kW (in some units power could be as high as 1MW).
In those days the only receiver electronics that would work at the then almost unheard of frequency of 10GHz (3cm wavelength) was a tiny point contact (cat's whisker) silicon diode used as a mixer which was very fragile—even a tiny proportion of 50kW would annihilate it in a fraction of a second.
The solution to protecting the diode was the gas-filled T/R switch, it was not only brilliant in conception but truly eloquent in its simplicity (even nowadays this is greatly underappreciated).
The same waveguide was used for transmission and reception but during the transmit cycle the T/R switch isolated the receiver by effectively short circuiting the signal path to the receiver by utilizing a small fraction of the transmitter powet to ionize its gas. Moreover, the ionization had to strike almost instantaneously—essentially on the leading edge of the 10GHz pulse (I'm unsure of the exact time but at that frequency the first 1/4-cycle (voltage maximum) occurs in 250 picoseconds). During reception the ionization would quench thus opening the signal path to the diode. No other circuitry was necessary although some devices had a bias voltage applied to aid striking.
Here's a photo of a CV-115 type T/R switch from WWII (it does not use a bias voltage). You'll note the circular resonant circuits, they increase the voltage across the spark gap thus aid striking).
This is a great post about the basics of what happens in transmission lines.
If you need really fast rise times, there are cheap pulse generators that are a couple orders of magnitude faster: https://leobodnar.com/shop/index.php?main_page=product_info&... At this level everything has to be optimized including physical geometry.
I am using Leo Bodnar’s fast pulse generator (SMA) in my lectures to teach transmission lines. With sufficient length (I use ~1m) it works quite well to demo with a low cost scope. I originally bought it for TDR with 40GSPS/15GHz scope, which works very well with few orders of magnitude smaller lengths. Old on has upper length limit with 10MHz fixed frequency. There is a new one available with external sync and variable frequency, but I have not bought/tested it.
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.
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.
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.
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.
Great article!
aside: I've never seen Stack Exchange used as a blogpost medium (which normally this kind of write-up would be) and I like it! It's still formatted as Q&A so people with the same question can find it, and what's more, suggest edits or write alternative solutions (as OP explicitly invites here) on equal footing themselves. A collaborative quest for the answer, but not anonymized like a wiki.
For transmission lines: _Similarities of Wave Behavior_, presented by Dr. J. N. Shier (of Bell Labs fame, and whose team invented the phototransistor):
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.
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.
Seems to me we not only have ultra fast picosecond switching involved but also the transmit signal would have be attenuated many orders of magnitude around the receiver to stop it 'frying'. That's very impressive at that switching speed.
When I was a kid, I used to rat old disposals WWII radar equipment for parts to build hobby projects and what truly impressed me perhaps more so than the magnetrons was how the delicate receiver circuit was protected from damage during the transmit cycle which reached a pulse power of over 50kW (in some units power could be as high as 1MW).
In those days the only receiver electronics that would work at the then almost unheard of frequency of 10GHz (3cm wavelength) was a tiny point contact (cat's whisker) silicon diode used as a mixer which was very fragile—even a tiny proportion of 50kW would annihilate it in a fraction of a second.
The solution to protecting the diode was the gas-filled T/R switch, it was not only brilliant in conception but truly eloquent in its simplicity (even nowadays this is greatly underappreciated).
The same waveguide was used for transmission and reception but during the transmit cycle the T/R switch isolated the receiver by effectively short circuiting the signal path to the receiver by utilizing a small fraction of the transmitter powet to ionize its gas. Moreover, the ionization had to strike almost instantaneously—essentially on the leading edge of the 10GHz pulse (I'm unsure of the exact time but at that frequency the first 1/4-cycle (voltage maximum) occurs in 250 picoseconds). During reception the ionization would quench thus opening the signal path to the diode. No other circuitry was necessary although some devices had a bias voltage applied to aid striking.
Here's a photo of a CV-115 type T/R switch from WWII (it does not use a bias voltage). You'll note the circular resonant circuits, they increase the voltage across the spark gap thus aid striking).
I still own one of these switches which sits on a mantelpiece, I often ask visiting techies what it is and most haven't a clue: https://www.radiomuseum.org/tubes/tube_cv115.html
Edit: here's a photo of the 1N23 diode, it's about 2cm long: https://www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/tube...
If you need really fast rise times, there are cheap pulse generators that are a couple orders of magnitude faster: https://leobodnar.com/shop/index.php?main_page=product_info&... At this level everything has to be optimized including physical geometry.
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.
But, really, I just took the opportunity to talk about Jim Williams and his magnum opus that is AN47!
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.
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.
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.