Hartley Oscillator: Two Coils, No Cap

Hardly.

The tapped coil (tap is usually lower than center) is an auto-transformer. You need considerable mutual inductance between the parts.

You need a lumped capacitor to make a tuned circuit of the coil and capacitor. I doubt that there is an useful resonance with your idea.

• Why? How much? lose proximity of the 2 coils can give decent mutual inductance. and splitting the larger inductance of the Hartley into two coils that sandwich the smaller can further increase the mutual inductance.

• Really? the above sandwich scheme can bring in a fair amount of capacitance. Using thin high-K insulation (tape) will further add to that, with minimal increase of leakage inductance. May be rather good even in the mid-DC frequencies (~200Mhz region) and up.

If you distribute the capacitance alon the coil, you'll get a transmission line instead of a tuned circuit.

But you can build a perfectly respectable oscillator with transmission line that gives you a 360 degree phase shift / one period delay.

And multiples of that. Then it's up to guess which of the modes responds.

I don't see why not. I have built such one transistor oscillators using two axial dogbone ferrite inductors side-by-side. Mine had an external capacitor but coil self capacitances might do. I found I could get suprisingly big separations like 1cm before oscillation ceased.

piglet

The mutual inductance of a Hartley reduces the impedance seen at the base, but otherwise it's just like a Colpitts. The k factor of an air-core solenoid is no better than 0.7 or so which is pretty poor for an autotransformer.

You can build an oscillator by just hanging an emitter follower on the right tank circuit. The resistance in the emitter circuit makes the input resistance negative.

Cheers

Phil Hobbs

A tapped inductor is not required. Two uncoupled inductors will suffice just as two uncoupled capacitors does with the Colpitts.

See figure 4.7 in Experimental Methods in RF Design by Wes Hayard, Rick Cambell, and Bob Larkin. ISBN # 0-87259-879-9.

(A) You do not need a tapped coil to make a Hartley. You do not need any mutual inductance. You can argue about calling it a Hartley, but a two- coil one-cap oscillator still perks along just fine.

(B) You do not need a lumped cap. That's not to say that the OP's idea will work, but anything that gives the right amount of phase shift will work. Stepping away from Hartley oscillators, there's plenty of oscillators out there that use tuned circuits made of microstrip or coax.

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In fact, circuit designers have been doing this inadvertently for decades, if not a century or so (I think the same thing can happen with a cathode follower in a toob circuit).

Never heard of using a shorted or open stub (of a transmssion line) in oscillators?

Are you positive?

I IMAGINE you are REFLECTING a POSITIVE observation.

This one is my favorite

...it's just a jfet soldered to a piece of wire

I think it worked without the tuning cap but it was >25 years ago so I might be wrong there...

Yeah, but you cheated and built a Hartley. ;)

Cheers

Phil Hobbs

If you let the headset bearings on your hog get really loose, is that a Harley oscillator?

You'll end up with fairly large amount of indeterminate capacitance and low er-Q coils and tank circuit. The lower Q means less frequency stability, dr ift and wander due to external influences that results from a weaker phase- frequency gradient. If you don't care about any of that, go ahead with it, you can still get a working oscillator.