I came across a lovely little picosecond pulse generator, designed and assembled by SiliconValleyGarage.
It is based on circuit designs by Jim Williams, who had a habit of squeezing every last drop of performance out of relatively low-cost components, frequently by building 3D rats nests which look like they would be hell to turn into equally-performing pcbs.
Anyway, it looks like the person behind SiliconValleyGarage, Vincent Himpe, managed to do just that with a lovely bit of construction, producing a 7ps rise time and an impressively square pulse (measured on a 32GHz scope…. 🙂
The circuit requires a rising voltage (of up to 90V, generated by Himpe from a 3V coin cell) to be applied to a fast npn transistor until it avalanches, and the tough bit is to keep capacitance and inductance to a minimum around the fast signal paths.
Not that Himpe quite managed to make it a normal pcb: the transistor comes in a TO-18 can, which he ingeniously solders legs-uppermost into a purpose-sized plated through hole (left), and the circuit’s output resistor is deliberately tomb-stoned on the board to reach the transistor’s air-bridged emitter lead en-route to an SMA connector (at least, I assume it is an SMA).
While not a normal pcb, it is magnificent, and it looks robust and fairly simple to assemble.
On the back of the pcb using a copper track, Himpe has managed to replace the length of solid coax employed by Williams with a serpentine micro-strip transmission line.
The capacitors surrounding the transistor are an alternative energy store should the transmission line not suite the required waveform.
and an example of the original Williams circuit is in Linear Tech’s AN19 (see page 18)