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Hartley Oscillator: Two Coils, No Cap

Started by Kevin Foster July 12, 2016
On Tue, 12 Jul 2016 11:56:29 -0400, Phil Hobbs wrote:

> 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.
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). -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
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. >
Never heard of using a shorted or open stub (of a transmssion line) in oscillators?
Phil Hobbs wrote:
> 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 >
Are you positive?
Phil Hobbs wrote:
> 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 >
I IMAGINE you are REFLECTING a POSITIVE observation.
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes:

> 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.
This one is my favorite <http://ee.devereux.me.uk/uhf-vfo.png> ...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... -- John Devereux
On 07/13/2016 01:47 PM, John Devereux wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> writes: > >> 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. > > > This one is my favorite > > <http://ee.devereux.me.uk/uhf-vfo.png> > > ....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 -- 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 Tue, 12 Jul 2016 15:47:07 +1000, 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
If you let the headset bearings on your hog get really loose, is that a Harley oscillator? -- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
On Tuesday, July 12, 2016 at 1:47:56 AM UTC-4, 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
You'll end up with fairly large amount of indeterminate capacitance and lower-Q coils and tank circuit. The lower Q means less frequency stability, drift 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.