Temperature variance … It all depends really on whether you're interested in a very precise particular frequency, or a precisely stable (but adjustable) frequency, or a very sinusoidal output, or what. .. …
Little coils have fairly good temperature stability. Overall inductances are a function of (if there is one) core material, number of turns, physical size (“involuted area”) and all that.
Little variable capacitors however are far from ideal, temperature wise. Even “varactors” are vexingly wobbly with changes in temperature.
It is for that reason, that — oh since the 1970s — that designers have tended to deploy phase-locked loops, crystal reference oscillators and digital divider chains to really 'set and forget' variable frequency oscillators. The idiosyncrasies of the varactor diodes, temperature are wiped out entirely by the PLL-and-divider chain business.
The “other problem” with the original diagram is that variable capacitors (and varactors) are notoriously small-value devices. A few to couple-of-thousand picofarads. Not very much capacitance.
This in turn, especially with “right sized” autotransformers, yields rather high frequency operation. Way up there in the kilohertz band, and very easily well into the MHz domain.
If you are (as per Eli's diagram) building an FM or AM receiver, having these 'restrictions' isn't a bug, but is a feature … the subharmonic heterodyne band shifting of the receiver's front end needs beat-generating master frequencies fairly close to the broadcast bands themselves. High kilohertz to mid megahertz.
So, again, what's the application you had in mind?
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
Little coils have fairly good temperature stability. Overall inductances are a function of (if there is one) core material, number of turns, physical size (“involuted area”) and all that.
Little variable capacitors however are far from ideal, temperature wise. Even “varactors” are vexingly wobbly with changes in temperature.
It is for that reason, that — oh since the 1970s — that designers have tended to deploy phase-locked loops, crystal reference oscillators and digital divider chains to really 'set and forget' variable frequency oscillators. The idiosyncrasies of the varactor diodes, temperature are wiped out entirely by the PLL-and-divider chain business.
The “other problem” with the original diagram is that variable capacitors (and varactors) are notoriously small-value devices. A few to couple-of-thousand picofarads. Not very much capacitance.
This in turn, especially with “right sized” autotransformers, yields rather high frequency operation. Way up there in the kilohertz band, and very easily well into the MHz domain.
If you are (as per Eli's diagram) building an FM or AM receiver, having these 'restrictions' isn't a bug, but is a feature … the subharmonic heterodyne band shifting of the receiver's front end needs beat-generating master frequencies fairly close to the broadcast bands themselves. High kilohertz to mid megahertz.
So, again, what's the application you had in mind?
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
I re-designed some 10MHz JFET Colpitts oscillators for Medical UltraSound Dopplers.
The problem was that the JFET capacitance shifts with battery voltage, temperature, and pulling of the oscillator from the loading of the single bipolar output transistor (no buffer stage).
My re-design of the oscillator used much less inductance, and much more capacitance (different LC ratio).
That swamped out most of the other changing capacitive effects.
This was a minimum parts count oscillator: 1 JFET, 1 resistor, 2 mica capacitors, and a slug tuned inductor.
What do you get when you cross an Armstrong oscillator with a Colpitts oscillator?
An ArmPitts oscillator.
The problem was that the JFET capacitance shifts with battery voltage, temperature, and pulling of the oscillator from the loading of the single bipolar output transistor (no buffer stage).
My re-design of the oscillator used much less inductance, and much more capacitance (different LC ratio).
That swamped out most of the other changing capacitive effects.
This was a minimum parts count oscillator: 1 JFET, 1 resistor, 2 mica capacitors, and a slug tuned inductor.
What do you get when you cross an Armstrong oscillator with a Colpitts oscillator?
An ArmPitts oscillator.
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A variant Hartley is THE local osc for AM tube radios.
If it isn't for FM (I forget), there's probably a reason.
I am sure there is a form which allows vari-caps (we used to do it with tube Miller Effect).
Lawrence Arguimbau was an opinionated guy but wrote very clearly on the subject when FM was young. My copy is a 1966 Modern Asia (cut-price) printing of the 1956 edition.
https://www.amazon.com/Vacuum-tube-circuits-transistors-Lawrence-Arguimbau/dp/B0006PBMLU
Vacuum Tube Circuits by Lawrence Baker Arguimbau - AbeBooks
If it isn't for FM (I forget), there's probably a reason.
I am sure there is a form which allows vari-caps (we used to do it with tube Miller Effect).
Lawrence Arguimbau was an opinionated guy but wrote very clearly on the subject when FM was young. My copy is a 1966 Modern Asia (cut-price) printing of the 1956 edition.
https://www.amazon.com/Vacuum-tube-circuits-transistors-Lawrence-Arguimbau/dp/B0006PBMLU
Vacuum Tube Circuits by Lawrence Baker Arguimbau - AbeBooks
Armstrong oscillators are commonplace in AM receivers, both tubed and BJT. The powdered iron (not ferrite) core FM local oscillator coil I possessed, as a youngster in the 1950s, had but 1 tapped winding. Hartley oscillators are definitely found in FM receivers.
I'd LOVE to have that coil on my current parts pile.
I'd LOVE to have that coil on my current parts pile.