Very stable single frequency audio oscillator (discrete components)

[c_mitra]

Much can be gained by using air core capacitor and inductors but your real problem will be the semiconductor devices that are notoriously difficult to maintain at a constant temperature (because they produce heat when they work)- tube designs are far more (about 10 times) more stable with respect to parameters.

 

[Warpspeed]

Much can be gained by using air core capacitor and inductors but your real problem will be the semiconductor devices that are notoriously difficult to maintain at a constant temperature (because they produce heat when they work)- tube designs are far more (about 10 times) more stable with respect to parameters.

Indeed this is very true !

Even Jfets running at miliwatt levels take several minutes to "warm up" and the initial frequency drift can be surprisingly high. And an oven will do nothing to help with that problem, because the junction temperature rise is relative.

Semiconductor internal capacitances will change with voltages and amplitudes and temperatures.
Its all a lot more difficult than it at first appears.

 

[neazoi]

Indeed this is very true !

Even Jfets running at miliwatt levels take several minutes to "warm up" and the initial frequency drift can be surprisingly high. And an oven will do nothing to help with that problem, because the junction temperature rise is relative.

Semiconductor internal capacitances will change with voltages and amplitudes and temperatures.
Its all a lot more difficult than it at first appears.

That is why buffered oscillators are left on, running all the time and only the next stages are switched, if it is required.
Almost every component is affected by temperature (even NP0 caps to some small extent, although theoretically not). Even crystals will do so, althought due to their high Q the problem is not that big for practical low cost circuits, unless high long term stability is needed.

The confusion seems to be caused by my definition of "stable" in the initial question. I guess a +/-5Hz of stability would be adequate for this application, since it will make the locked RF oscillator drifting by only 10Hz, which is very adequate for general SSB HF use. Even more could be accepted, but not more than 15-20Hz or so, since there might be a danger for the oscillator to lock to the next locking point.

Is it really that difficult to build a 5000ppm (post #10) multivibrator for such low audio frequencies?

 

[c_mitra]

Do you really mean that you want 30+/-5Hz oscillator?

 

[neazoi]

Do you really mean that you want 30+/-5Hz oscillator?

Yes, 30Hz +/-5Hz would be adequate to this application I believe.
BTW, I wonder about the stability of figure 3 in this page https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_6

 

[Warpspeed]

Yes, 30Hz +/-5Hz would be adequate to this application I believe.

But that equates to 30 Mhz +/- 5Mhz of corrected drift in your VFO ?

To stay within 30Hz drift at 30 MHZ requires 1ppm accuracy.
If your uncorrected VFO cannot drift more than 30Hz in 33mS (one 30Hz period) if it does, then it does not drift it JUMPS to the next locking step.

That is not much uncorrected drift 900Hz/second, and you have to do a lot better than that to stay in reliable lock.

1ppm is +/- 0.0000003 Hz required stability not +/- 5Hz in your audio oscillator.

The usual huff and puff uses a divided down crystal oscillator as the reference, and the VFO is conventional, usually running at a very few Mhz.
The VFO stability is then about the same as the crystal, which is usually judged adequate.

But the VFO needs to be a pretty good one to start with, the huff and puff is only to correct very long term slow rates of drift, which it will do.

At 30 Mhz just mechanical shock will very likely knock it out of lock pretty easily. If it jumps more than 30 Hz your frequency lock will be toast.

 

[neazoi]

But that equates to 30 Mhz +/- 5Mhz of corrected drift in your VFO ?

To stay within 30Hz drift at 30 MHZ requires 1ppm accuracy.
If your uncorrected VFO cannot drift more than 30Hz in 33mS (one 30Hz period) if it does, then it does not drift it JUMPS to the next locking step.

That is not much uncorrected drift 900Hz/second, and you have to do a lot better than that to stay in reliable lock.

1ppm is +/- 0.0000003 Hz required stability not +/- 5Hz in your audio oscillator.

The usual huff and puff uses a divided down crystal oscillator as the reference, and the VFO is conventional, usually running at a very few Mhz.
The VFO stability is then about the same as the crystal, which is usually judged adequate.

But the VFO needs to be a pretty good one to start with, the huff and puff is only to correct very long term slow rates of drift, which it will do.

At 30 Mhz just mechanical shock will very likely knock it out of lock pretty easily. If it jumps more than 30 Hz your frequency lock will be toast.

I see...
Neither an ovenized audio multivibrator would help on this?