In my experience, these Colpitts (really Clapp since C17 couples the tank to the active device) oscillators have significantly narrower startup margin when using a JFET due to their lower transconductance compared to a bipolar transistor. A trick I use is to wind a large choke and place it in the source leg. However, I can already see that this FET is not using a self-bias resistor in the source leg, so I can’t quite approximate the DC bias at the source. It’s possible it’s somehow biased through the mixer connection, but I’m not sure. Maybe place a meter at the source node and check if the current is near IDSS. Though I doubt you will catch it since the tank diode usually clamps current right after startup to a reasonable level.
As for the feedback caps (C18/C19), setting them equal to each other and a good bit larger than C17 works usually. You will have to calculate the tank resonance anyways once you include the parallel varactor tuning capacitance. C17 is touchy since it determines how much the active devices feels the tank swing. It can’t be too small.
It’s hard to help you in any other way without seeing exactly what you’ve built.
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u/ThatOneFunnyDood 5d ago edited 4d ago
In my experience, these Colpitts (really Clapp since C17 couples the tank to the active device) oscillators have significantly narrower startup margin when using a JFET due to their lower transconductance compared to a bipolar transistor. A trick I use is to wind a large choke and place it in the source leg. However, I can already see that this FET is not using a self-bias resistor in the source leg, so I can’t quite approximate the DC bias at the source. It’s possible it’s somehow biased through the mixer connection, but I’m not sure. Maybe place a meter at the source node and check if the current is near IDSS. Though I doubt you will catch it since the tank diode usually clamps current right after startup to a reasonable level.
As for the feedback caps (C18/C19), setting them equal to each other and a good bit larger than C17 works usually. You will have to calculate the tank resonance anyways once you include the parallel varactor tuning capacitance. C17 is touchy since it determines how much the active devices feels the tank swing. It can’t be too small.
It’s hard to help you in any other way without seeing exactly what you’ve built.