[sdiy] Transistor questions

[mskala at northcoastsynthesis.com]

On Mon, 3 Mar 2025, Roman Sowa via Synth-diy wrote:

> I don't think there's a way to achieve 3ns rise time in this circuit,
> expecially when done in THT.

Having anything happen in 3ns implies bandwidth of at least about 300 MHz,
i.e. the bottom of the UHF range, which is certainly possible on a
carefully designed through-hole board.  I've seen ham radio projects do
it, and note that ordinary FM broadcast recievers routinely handle(d) 108
MHz signals on through-hole boards, let alone UHF analog television
receivers.  Conventional wisdom today includes exaggerated ideas of the
disadvantages of through-hole.  But the hams would usually use dead-bug
construction instead of a through-hole board for this range.  Getting it
to work is far from a gimmie.

A related issue is that if you achieved it, *would you know*?  You quite
possibly don't have an oscilloscope fast enough to see such an edge.  The
comparator in your oscillator which needs to respond to the edge is likely
not fast enough to care, and if the signal ever has to pass through an
amplifier, or even a cable, designed primarily for audio, it'll round off
the 3ns edge.

I don't understand the calculation that goes from "6kHz fundamental" to "I
need 3ns rise time" but I think that's the step to look at.

-- 
Matthew Skala
North Coast Synthesis Ltd.