Hi everyone,

I've been looking at different oscillator topologies recently including some classic Italian designs. It got me thinking why they use coax rather than coils in oscillators. Are there actually any benefits to using a length of coax as the inductance in an oscillator rather than a coil?

Got any pictures?

teckniqs wrote: ↑Thu Feb 18, 2021 8:48 pm Got any pictures?

Pictures of the schematics or the circuit boards? I haven't built any of those designs yet.

Bton-FM wrote: ↑Thu Feb 18, 2021 8:23 pm Hi everyone,

I've been looking at different oscillator topologies recently including some classic Italian designs. It got me thinking why they use coax rather than coils in oscillators. Are there actually any benefits to using a length of coax as the inductance in an oscillator rather than a coil?

There are two advantages - the lack of a coil (they radiate, require calibrations and are relatively expensive), and a length of coal isn't as microphonic as a coil in an "at frequency" circuit.

I used to use a length of UR176 for the oscillator coil at ~300 MHz in my Band V link transmitters. It eliminated the tricky 1.5 turn coil I otherwise needed. It was in a Colpitts configuration, using a BFR91 as the oscillator transistor, driving another BFR91 as the buffer stage, then into a Lecher-Line tuned doubler using a BFR96. The doubler output then fed a BFR96 amplifier / filter stage, into a grounded base 2N3866 pair for the final, giving around 500mW. The PLL was fed from the emitter of the buffer amplifier, and used a SAB6456 prescaler.
They were neat little boards, and were easy to build and align.