Electronics > RF, Microwave, Ham Radio

[Solved]Type 43 core resonance (Hartley oscillator frequency jump during tuning)

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szoftveres:
Hey All,

Of the few Hartley-oscillators I've built, I've noticed a common symptom: while adjusting the tuning capacitor to the more open setting (least capacitance), the oscillation frequency suddenly jumped - and skipped a small frequency band.

Specifically I've built the attached circuit, (grid leak: 100k and 47pF, Tuning capacitor: 10-330pF with a 5-30pf trimmer, Inductor: 4+17 turns on an FT50-43 core), which is supposed to produce frequencies between ~500kHz and 1700kHz. While inspecting the oscillation on an oscilloscope (very loosely coupled), the frequency goes up and then "hesitates" at around 1500kHz as I keep turning the tuning capacitor, then suddenly jumps up to around 1600kHz and the tuning continues to 1700kHz as I max out the tuning capacitor. It's not producing this behavior when adjusting the frequency the other way.

I've noticed the same thing a while ago with a VHF (transistor) oscillator, and attributed this behavior to the supposedly loose coupling of the lower turns of the air-core coil to the upper turns, but this shouldn't be the case here with a type 43 core material (u=850).

I'm wondering what plays role here, i.e. what could cause this circuit to prefer staying at 1500kHz instead of following the LC tank resonance. One wild guess is that the tube internal (grid to cathode and screen) capacitance forms a series (fixed) resonance point. Any suggestions, especially regarding how to mitigate this thing would be greatly appreciated.

TimFox:
You are seeing strange behavior at the minimum capacitance (maximum frequency) end of the tuning capacitor range.
Is there any possibility that you are not at precisely the minimum where you see the jump.
Also, your circuit should not have a problem with "squegging" (q.v.) unless the time-constant of the grid-leak circuit is too long.
Maybe try reducing the grid resistor by a factor of 5 and see what happens.
Are you monitoring the output with an oscilloscope when the strange jump occurs?

szoftveres:
I still have ~100kHz of tuning range -up to 1700kHz- after this jump happens. No squegging.
Yes, I'm on the cathode with a 1:10 probe (also seen this phenomenon in an electron-coupled mode, i.e. on the anode after the screen grid, which is pretty much 100% isolation)

TimFox:
Is it possible you have a local problem with a bent plate in your multi-plate tuning capacitor?
That could give a non-monotonic capacitance vs. angle.

szoftveres:
Tried with a 10k grid-leak resistor, didn't help.
After the oscillation reached ~1500kHz, the frequency won't change any more, but the amplitude shrinks a tiny bit as I slowly keep tuning upwards - then it abruptly jumps to the new frequency (I don't see this happening when I tune the other way). This makes me think that it's some kind of secondary resonance point.
Checked the capacitor for any mechanical issues, nothing I could point at.

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