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Hartley Oscillator: Two Coils, No Cap

Started by Kevin Foster July 12, 2016
Would it be possible to build a single transistor Hartley oscillator by 
substituting a pancake (spiral) coil for each leg of the center-tapped 
inductor, and then placing one coil flat on top of the other so the 
insulation between them takes the place of the cap?

If the current in each coil is made to flow opposite to the other, could 
their proximity to each other also be used to vary the overall inductance?

Kevin Foster
On 12.7.16 08:47, Kevin Foster wrote:
> Would it be possible to build a single transistor Hartley oscillator by > substituting a pancake (spiral) coil for each leg of the center-tapped > inductor, and then placing one coil flat on top of the other so the > insulation between them takes the place of the cap? > > If the current in each coil is made to flow opposite to the other, could > their proximity to each other also be used to vary the overall inductance? > > Kevin Foster
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. -- -TV
Tauno Voipio wrote:
> On 12.7.16 08:47, Kevin Foster wrote: >> Would it be possible to build a single transistor Hartley oscillator by >> substituting a pancake (spiral) coil for each leg of the center-tapped >> inductor, and then placing one coil flat on top of the other so the >> insulation between them takes the place of the cap? >> >> If the current in each coil is made to flow opposite to the other, could >> their proximity to each other also be used to vary the overall >> inductance? >> >> Kevin Foster > > > Hardly. > > The tapped coil (tap is usually lower than center) is an > auto-transformer. You need considerable mutual inductance between the > parts.
* 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.
> > 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.
* 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.
>
On 12.7.16 10:26, Robert Baer wrote:
> Tauno Voipio wrote: >> On 12.7.16 08:47, Kevin Foster wrote: >>> Would it be possible to build a single transistor Hartley oscillator by >>> substituting a pancake (spiral) coil for each leg of the center-tapped >>> inductor, and then placing one coil flat on top of the other so the >>> insulation between them takes the place of the cap? >>> >>> If the current in each coil is made to flow opposite to the other, could >>> their proximity to each other also be used to vary the overall >>> inductance? >>> >>> Kevin Foster >> >> >> Hardly. >> >> The tapped coil (tap is usually lower than center) is an >> auto-transformer. You need considerable mutual inductance between the >> parts. > * 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. > >> >> 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. > * 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. -- -TV
On Tuesday, July 12, 2016 at 9:50:00 AM UTC+2, Tauno Voipio wrote:
> On 12.7.16 10:26, Robert Baer wrote: > > Tauno Voipio wrote: > >> On 12.7.16 08:47, Kevin Foster wrote: > >>> Would it be possible to build a single transistor Hartley oscillator by > >>> substituting a pancake (spiral) coil for each leg of the center-tapped > >>> inductor, and then placing one coil flat on top of the other so the > >>> insulation between them takes the place of the cap? > >>> > >>> If the current in each coil is made to flow opposite to the other, could > >>> their proximity to each other also be used to vary the overall > >>> inductance? > >>> > >>> Kevin Foster > >> > >> > >> Hardly. > >> > >> The tapped coil (tap is usually lower than center) is an > >> auto-transformer. You need considerable mutual inductance between the > >> parts. > > * 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. > > > >> > >> 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. > > * 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. -- Bill Sloman, Sydney
On 12.7.16 12:56, bill.sloman@ieee.org wrote:
> On Tuesday, July 12, 2016 at 9:50:00 AM UTC+2, Tauno Voipio wrote: >> On 12.7.16 10:26, Robert Baer wrote: >>> Tauno Voipio wrote: >>>> On 12.7.16 08:47, Kevin Foster wrote: >>>>> Would it be possible to build a single transistor Hartley oscillator by >>>>> substituting a pancake (spiral) coil for each leg of the center-tapped >>>>> inductor, and then placing one coil flat on top of the other so the >>>>> insulation between them takes the place of the cap? >>>>> >>>>> If the current in each coil is made to flow opposite to the other, could >>>>> their proximity to each other also be used to vary the overall >>>>> inductance? >>>>> >>>>> Kevin Foster >>>> >>>> >>>> Hardly. >>>> >>>> The tapped coil (tap is usually lower than center) is an >>>> auto-transformer. You need considerable mutual inductance between the >>>> parts. >>> * 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. >>> >>>> >>>> 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. >>> * 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. -- -TV
On 12/07/2016 06:47, Kevin Foster wrote:
> Would it be possible to build a single transistor Hartley oscillator by > substituting a pancake (spiral) coil for each leg of the center-tapped > inductor, and then placing one coil flat on top of the other so the > insulation between them takes the place of the cap? > > If the current in each coil is made to flow opposite to the other, could > their proximity to each other also be used to vary the overall inductance? > > Kevin Foster
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
On 07/12/2016 09:51 AM, piglet wrote:
> On 12/07/2016 06:47, Kevin Foster wrote: >> Would it be possible to build a single transistor Hartley oscillator by >> substituting a pancake (spiral) coil for each leg of the center-tapped >> inductor, and then placing one coil flat on top of the other so the >> insulation between them takes the place of the cap? >> >> If the current in each coil is made to flow opposite to the other, could >> their proximity to each other also be used to vary the overall >> inductance? >> >> Kevin Foster > > 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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 7/12/2016 12:47 AM, Kevin Foster wrote:
> Would it be possible to build a single transistor Hartley oscillator by > substituting a pancake (spiral) coil for each leg of the center-tapped > inductor, and then placing one coil flat on top of the other so the > insulation between them takes the place of the cap? > > If the current in each coil is made to flow opposite to the other, could > their proximity to each other also be used to vary the overall inductance? > > Kevin Foster
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.
On Tue, 12 Jul 2016 09:45:18 +0300, Tauno Voipio wrote:

> On 12.7.16 08:47, Kevin Foster wrote: >> Would it be possible to build a single transistor Hartley oscillator by >> substituting a pancake (spiral) coil for each leg of the center-tapped >> inductor, and then placing one coil flat on top of the other so the >> insulation between them takes the place of the cap? >> >> If the current in each coil is made to flow opposite to the other, >> could their proximity to each other also be used to vary the overall >> inductance? >> >> Kevin Foster > > > 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.
(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. Version 4 SHEET 1 880 680 WIRE 304 0 -64 0 WIRE -64 16 -64 0 WIRE 304 16 304 0 WIRE 112 64 16 64 WIRE 240 64 112 64 WIRE 112 80 112 64 WIRE -64 128 -64 96 WIRE 16 144 16 64 WIRE 112 176 112 160 WIRE 176 176 112 176 WIRE 304 176 304 112 WIRE 304 176 240 176 WIRE 112 192 112 176 WIRE 304 192 304 176 WIRE 16 240 16 208 WIRE 112 304 112 272 WIRE 304 304 304 272 WIRE 112 416 112 384 FLAG 304 304 0 FLAG -64 128 0 FLAG 112 416 0 FLAG 16 240 0 SYMBOL ind 96 64 R0 SYMATTR InstName L1 SYMATTR Value 22ṁ SYMATTR SpiceLine Rser=2 SYMBOL ind 96 176 R0 SYMATTR InstName L2 SYMATTR Value 22ṁ SYMATTR SpiceLine Rser=2 SYMBOL cap 240 160 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 330p SYMBOL npn 240 16 R0 SYMATTR InstName Q1 SYMBOL res 288 176 R0 SYMATTR InstName R1 SYMATTR Value 100k SYMBOL voltage 112 288 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL voltage -64 0 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL cap 0 144 R0 SYMATTR InstName C2 SYMATTR Value 100p TEXT -208 408 Left 2 !.tran 100u startup -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!