# Hartley Oscillator: Two Coils, No Cap

Started by 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.

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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!
```