Hi,
I've built a Colpitts oscillator based on the attached circuit diagram.
I'm looking to get it oscillating at 200MHz+, if possible.
I'm using ceramic capacitors with nominal values of 10pF, and an inductor with nominal value 100nH.
The JFET is a J112 (NChannel JFET with Vgs off = -3.6V) - I cannot find its cutoff frequency in the datasheet, so would be interested if anyone can provide it.
All components are through hole (with leads trimmed), and soldered directly together, with no board (essentially, hanging from the power leads). Power is from a bench power supply, with 30cm leads.
Predicted theoretical frequency is approx. 225MHz (with zero allowance for parasitic reactances), but I'm measuring approx. 80MHz on my oscilloscope.
I realise that leads etc. will add inductance, the probe has input capacitance in the order of 20pF, and I’m sure there are many other parasitic capacitances and inductances at play.
I’m wondering what a realistic upper frequency is achievable with this simple direct soldered setup, or what I would need to change in the circuit, or its construction, to get the frequency higher.
Thanks in advance for any input.
Note that this is purely for educational purposes – I’m not interested in buying a purpose made signal generator in that frequency range!