Butler Oscillator Design

[Nigel Goodwin]

There is nothing wrong with my class C scematic a diode is often used, it does help to control the duty cyle.

A resistor should still be connected between base and emitter, the diode is more likely there more as a protection measure than anything else? - in case the stage is fed with a high signal level (to prevent reverse Vbe breakdown - which occurs about -7V to -9V). If anything it will reduce the generation of harmonics, as it will tend to slow down the switch off speed of the transistor.

 

[Hero999]

The base-emitter junction forms a diode meaning that uless it's either biased or has a reisitor in parallel it will act as a rectifier and C1 will just charge up. The diode in reverse parallel discharges the capacitor on negitive cycles, yes it does protect against reverse base voltage but that's incidental. With the diode present you don't require a resistor in parallel with the base-emitter junction.

 

[RadioRon]

I did look at this book but didn't calculate any values yet. I'd really appreciate it if we could work on this thing together. I was also thinking of the frequency tripler, can I do it by simply operating a transistor as a switch and then using a tank circuit to get the third harmonic from the collector?

I apologize for not responding sooner, but I was having a wonderful time visiting the Monterey Historics auto races at Laguna Seca racetrack in California. Great Fun! Anyways, I'm back now and getting busy but would be happy to correspond on this oscillator design. I'm not very far ahead of you but at least I've thought through my transistor selection and picked a favorite part and have estimated some bias and LC values. However, as in all VHF/UHF circuits, the values I start with are usually not the ones I finish with as there is some on-board optimization required in all cases. Let's get in contact via private email for further exchanges.

 

[Sig239]

You adjust the drive level by either amplifying or attenuating the input to the tripler. The easiest way is just measure the tripler output and find the max. Just guessing, but 30% duty cycle would be about right. 50% duty cycle is not good, it has only even harmonics.

You adjust the drive level by either amplifying or attenuating the input to the tripler. The easiest way is just measure the tripler output and find the max. Just guessing, but 30% duty cycle would be about right. 50% duty cycle is not good, it has only even harmonics.

A square wave is the fundumental frequency and an infinate number of odd harmonics. The sharp transitions (ie. the corners) contain all of the harmonics,(right?) so what does duty cycle have to do with it? Furthermore, how do you get even harmonics out of a signal that doesn't contain them?
https://en.wikipedia.org/wiki/Square_wave

 

[audioguru]

If the duty cycle of a square-wave isn't exactly 50-50, then it will have some even harmonics.

 

[Nigel Goodwin]

I can't believe we're still discussing duty cycle and square waves? - this is an analogue RF multiplier stage!. It accepts a sinewave input to a class C amplifier, which 'roughly' works as a switch - this doesn't mean it outputs a beautiful square wave, of any duty cycle - there's no need for it.

Essentially you're not looking for harmonics, although they may assist a little as well, you're 'ringing' the tuned circuit in the collector - so it will multiply quite happily by two or by three, and even higher if required (although two or three are by far the most common).

For anyone at all interested?, and not confused by irrelevant duty cycles?, 144MHz (2M) amateur radio transmitters commonly used 12MHz crystals so required multiplying twelve times. This was done with two doublers and a tripler - all identical looking class C stages with tuned collectors (to different frequencies obviously!).

Another (less common) crystal was 8MHz, so times eighteen (x3, x3, x2).

 

[Sig239]

If the duty cycle of a square-wave isn't exactly 50-50, then it will have some even harmonics.

Interesting, thanks. I guess this is one of those things where the "technical definition" exists mainly for the purposes of explaining it mathmaticaly. Since ideal components don't exist, I suppose you couldn't make a perfect square wave anyways. I wonder why the different duty cycle creates more even harmonics? Would the differing peaks (ie. narrow positive, wide negative) represent two different fundamentals with two different sets of harmonics?

Wow thanks guys! I love this forum! If anyone can confirm, deny, or further explain it would be greatly appreciated. If this is innapropriate I can start a new thread. I know this is venturing beyond the scope of the original thread.

 

[audioguru]

The harmonic distortion from a single vacuum tube (valve) sounds musical because it is mostly even harmonics. One polarity of the output is squashed, something like a square-wave with its duty-cycle shifted away from 50-50.