[G0HZU_JMR]

I also dug out a SMD MMBTH10 BJT to do some further testing. These state 650MHz Ft on the bag but this is for unity current gain rather than power gain.

If you look back at marker 4b on the plot in post #20 my VNA data suggests that this device can still go unstable up at around 2.4GHz when biased at 10Vce and 5mA Ic. This is nearly four times higher than the Ft frequency stamped on the bag by Farnell and on the On Semi datasheet. The BJT can still oscillate well past the Ft frequency because an oscillator doesn't need >= unity current gain to meet the requirements for oscillation. It can still produce power gain at much less than unity current gain.

I loaded my VNA derived s2p data model of the MMBTH10 into Genesys and I added a simulated transmission line resonator at the collector and with this resonator added Genesys predicts that this BJT can still deliver a whiff of negative resistance at the base at about 2.4GHz. By adding a small tuning cap at the base there is resonance and a net negative resistance at about 2.4GHz as in the plot below.

I built the oscillator circuit with the resonator and just tested it on a spectrum analyser. Sure enough it oscillated close to the expected 2.4GHz as in the analyser plot below. This shows the power of measuring these devices using a modern VNA. This technology is now coming within reach of hobbyists as the low cost nanovna technology improves year on year.

I suspect that I could get it to oscillate beyond 2.5GHz by optimising the resonator components but I doubt I would get close to 2.7GHz unless I also changed the bias point of the MMBTH10. At the moment it dies just before 2.5GHz if I try and tune it higher in frequency and this is to be expected as my resonator parts introduce ESR losses and the simulation plot below shows there is only a few ohms of negative resistance available at the base of the transistor up at 2.4GHz.