[hilbert42]

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.

[dylan604]

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.

[lostlogin]

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.

[hilbert42]

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).

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...