LC Oscillator Questions

Oscillators have to have gain greater than one at the frequency of oscillation.

When turned on, the amplitude builds up until something in the circuit cuts back the gain. In simple oscillators, that "something that cuts back the gain" is almost always the active device saturating and distorting its output.

The higher your gain, the more reliable the oscillator starting up, but also the higher the distortion.

If you take the output not from the output of the active device, but from a lightly-coupled tank, then you'll see something much more like the sine wave you were expecting. This is what you see at the L1/C3 junction. But still you'll get lower distortion there if the active device isn't driven so far into saturation/distortion. And by definition you cannot suck much power out of the L1/C3 junction without decreasing the Q of the tank and making distortion there too.

You can add a few more active devices and not only buffer things but also put a fairly linear AGC in the loop. This still has distortion, but this is done intentionally in a rectifier to derive the AGC control voltage, which is then filtered. The intentional distortion does not have to appear in the output!

Clever use of devices can make the AGC loop quite beautiful. Look at the Wien Bridge or Meacham Bridge oscillators that use a light bulb in the bridge to not only be the loop-control device but also do filtering (thermal time constant of the filament).

Tim.

Thanks for the reply. Sounds pretty slick. Way out of my league though. ;-)

The "conversation" that L1 and C3 sure looks nice on the scope. :-)

Well there sure isn't much talk about it out there. Material I find is like, "here's a schematic, pick a coil and cap and your done. No one seems to care what the result looks like. Seems like you can make a reasonably decent wave _and_ still have the oscillator start reliably.

Seems to be the way people like to do it. ;-)

It seams reasonable that if I can look at the junction with a scope and the wave looks good, I should be able to tap it with a secondary JFET without destroying it. Yet I see no examples of that being done. I guess it's just easier to accomplish the waveform repair by using a tank on the output of the oscillator and not loading down the primary tank circuit.

This sounds like what Chris Jones was talking about. Do you have a link so I could check it out?

Clever stuff. :-)

In message , Anthony Fremont writes [snip]

Symmetry is a nice word to use where LC resonance is concerned. Symmetry for the drive and clipping mechanisms.

Have a look at a long-tail transistor pair with cross-coupled collector-base feedback resistors, and collectors driving a centre-tapped LC resonant circuit.

--
Tony Williams

A "classic" oscillator with AGC is the Sulzer Oscillator. Very nice pics and schematics at

For really nitty-gritty stuff about low-distortion oscillators, see Jim Williams' examples in Linear Technologies appnote AN-43. (Go to

and do a search for "AN43" without the hyphen). Most of the examples there are bridges, and distortions in the sub-0.1% category are achieved AND THEN IMPROVED UPON BY FACTORS OF HUNDREDS! Also google "Meacham Bridge" and "Wien Bridge".

Tim.

In addition to the low-noise-low-distortion-high-stability URL's I pointed you towards in my other followup, recent ARRL Handbooks have some really clever low-noise VFO circuits using a multitude of approaches, including explicit AGC circuitry. I have been slowly working my way through the cookbook examples and every example has its merits.

In typical ham use, for better or worse, stability and reliability to start-up are often the most important criteria. What you are complaining about when you see a distorted output, is something that is actually a design goal of oscillators that are followed by multipliers.

One very common method over the years of decoupling the frequency- determining tuned circuits from other frequencies generated in a radio is to run the oscillator grid tank at half the output frequency and depend on distorition to make the desired output frequency. In the simplest case a balanced or push-pull oscillator is a "No-No" because you WANT the second harmonic. The electron-coupled oscillator that was in the 50's/60's/early 70's handbooks is a classic design.

Tim.

Some people worry a whole lot about it. What they worry about, typically, is phase noise first and stability second. Lack of harmonics in the waveform are generally lower on the list. That's because filtering out harmonics is relatively easy, compared with cleaning up phase noise and stability problems. You can find lots of articles on minimizing phase noise, but there's also quite a bit of trade-secret sorts of knowledge whose owners aren't particularly interested in sharing, understandably. It's far from trivial to get the phase noise and spurious performance you'll find in the good commercial signal generators. Good as they are, though, I know of none that's good enough to be used without harmonic filtering to do low-level harmonic distortion measurements.

Cheers, Tom