Butler Oscillator Design

[Quasar8000]

I need to design a 315MHz crystal oscillator. I'll go for a Butler Oscillator with a crystal running at 105MHz then use a tank circuit on the collector to get the 3rd harmonic thus achieving the needed 315MHz. My problem is that I don't know the exact math I have to use to calculate the component values of the oscillator. I know the circuit model and the only thing I found online was general tips on the design. I'd appreciate any advice on how to find out the exact values I need for this circuit.

 

[Hero999]

I've never heard of a butler oscillator before but Google seems to know quite a lot about them. Generally I would build a 105MHz oscillator that outputs a squarewave and add an LC filter to it tuned to 315MHz.

 

[Nigel Goodwin]

I would suggest a multiplier stage?, a 105MHz crystal is certainly likely to be an overtone crystal, so you will need the circuit tuning to 105MHz, then add a seperate multiplier stage to 315MHz.

 

[RadioRon]

There are several configurations that are referred to as Butler oscillators and some are good while others are not-so-good. For example, I see in my text that there are really two groups, one is Butler Common Base while the other is Butler Emitter Follower. The common base circuit is in the not-so-good category and may be what you are planning, so I recommend you reconsider. It is a tricky oscillator that exhibits parastic and spurious oscillation at various frequencies that make it a pain.

The emitter follower configurations are all excellent and very good choices for your application. But they put out a fairly clean signal with low harmonics so you should strongly consider a second stage to do the x3 multiplication up to 315 MHz. Building a VHF oscillator is not easy at the best of times and you will find it helpful to decouple the design challenges of the oscillator from those of trying to triple the output. A tripler stage is relatively easy to get to work, and allows you to concentrate on the oscillator at 105 MHz.

One of the things that I like about the butler emitter follower configuration is that the amplifier need only have a gain of one to function. Loop gain is provided by the voltage boosting action of the LC tank circuit in the base of the transistor.

I strongly recommend the book described below to help you. It provides enough detail on this and many other configurations so that you could probably design your oscillator, but its not for beginners. Alternately, email me and perhaps I can translate the book's guidance into useful component values. I've been working on my own circuit for a short while but am no expert so we would be exploring this together somewhat.

"Crystal Oscillator Circuits" Robert J Matthys, Kreiger Publishing, 1992.

 

[k7elp60]

This is a formula that calculates the values of the capacitor and the inductor for RF circuits.
LC=25330/f^2
LC=LC ratio, L in uH and C in pf, f is in Mhz.
The LC ratio for 315Mhz =0.255
So for example if you are using 2pf then the L=.255/2 or 0.1275 uH.

At 315Mhz very small values of stray capacitance and lead inductance can
change the calculated values.

The above formula is for total values and stry values have to be figured into
the final circuit for operation

 

[Nigel Goodwin]

At 315Mhz very small values of stray capacitance and lead inductance can
change the calculated values.

At 315Mhz very small values of stray capacitance and lead inductance WILL change the calculated values.

 

[Quasar8000]

I strongly recommend the book described below to help you. It provides enough detail on this and many other configurations so that you could probably design your oscillator, but its not for beginners. Alternately, email me and perhaps I can translate the book's guidance into useful component values. I've been working on my own circuit for a short while but am no expert so we would be exploring this together somewhat.

"Crystal Oscillator Circuits" Robert J Matthys, Kreiger Publishing, 1992.

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?

 

[Hero999]

Yes, a frequency tripler is as simple as that, build your 105MHz oscillator and add a common emitter amplifier with an LC tank circuit as the collector load.

 

[Russlk]

You want to maximize the third harmonic in the tripler, that means adjusting the drive level to get the desired pulse width.

 

[Quasar8000]

You want to maximize the third harmonic in the tripler, that means adjusting the drive level to get the desired pulse width.

How can I do that? I mean, how do I know what's the desired pulse width and how to obtain it?

 

[Nigel Goodwin]

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?

That's basically what you do, you use a transistor in class C, with a tuned load in the collector - it's a VERY, VERY common circuit - check all the VHF/UHF amateur radio transmitters (and commercial ones) for the last 60+ years.

As with any radio related query, I would HIGHLY suggest you buy either the ARRL or RSGB handbooks - you don't need current ones, any old one will do. The RSGB used to do one called "The VHF/UHF Manual" that would be fine as well.

 

[Russlk]

You want to maximize the third harmonic in the tripler, that means adjusting the drive level to get the desired pulse width.

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.

 

[Nigel Goodwin]

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're missing the point, a class C amplifier is basically a switch, this produces plenty of odd harmonics - you don't play with 'duty cycles', how would you do that with a tuned sinewave oscillator?.

 

[Hero999]

A 50% duty cycle squarewave doesn't produce any even harmonics, just odd harmonics. Use a duty cycle of less than 50%, go for 25%-40% it really isn't important.

 

[Nigel Goodwin]

A 50% duty cycle squarewave doesn't produce any even harmonics, just odd harmonics. Use a duty cycle of less than 50%, go for 25%-40% it really isn't important.

What's with all the 'duty cycle' and 'squarewave' crap?. This is a tuned sinewave oscillator - it has to be tuned because it's an overtone crystal!.

So NO squarewave, NO duty cycle - it's simple analogue RF electronics, bog standard basic stuff!.

 

[Hero999]

What's with all the 'duty cycle' and 'squarewave' crap?.

A class C amplifier is a switch which an LC filter as the load, it is effectively outputting a squarewave into a filter which removes the harmonics.

This is a tuned sinewave oscillator - it has to be tuned because it's an overtone crystal!.

I doubt you can buld an overtone 315MHz oscillator, it's better to build one for 105MHz then use a trippler which can be as simple as a common emmitter amplifier operating in class C with an LC tank as the collector load.

So NO squarewave,

Read the above.

NO duty cycle

The duty cycle is important with a class C amplifier, it needs to be below 50% but you don't have to mess around to do this, just putting a sinewave into the base through an AC coupling capacitor and a diode in reverse paralell will do this for you.

- it's simple analogue RF electronics, bog standard basic stuff!.

Indeed it is but building it might be a challange to some, don't use a breadboard or even veroboard use a proper PCB and surface mount parts if possible.

 

[Nigel Goodwin]

A class C amplifier is a switch which an LC filter as the load, it is effectively outputting a squarewave into a filter which removes the harmonics.

Which is what I said in the post immediately before your previous one - 54 minutes before!.

The tuned circuit doesn't 'filter' the input, it resonates - just like striking a bell, it's essentially an oscillator without feedback.

I doubt you can buld an overtone 315MHz oscillator, it's better to build one for 105MHz then use a trippler which can be as simple as a common emmitter amplifier operating in class C with an LC tank as the collector load.

I've also said that all along - BUT, the 105MHz will be an overtone crystal in the first place, so requires a tuned overtone oscillator followed by a 3x multiplier stage.

The duty cycle is important with a class C amplifier, it needs to be below 50% but you don't have to mess around to do this, just putting a sinewave into the base through an AC coupling capacitor and a diode in reverse paralell will do this for you.

You don't need a diode?, just feed the oscillator into a class C amplifier, it's just that simple - and so common!.

 

[Hero999]

A diode isn't essential but it is often used in RF class C amplifiers to make them easier to bias to class C.

Anyway it doesn't have to be a class C amplifier, it could easilly be a tuned class A amplifier.

 

[Nigel Goodwin]

A diode isn't essential but it is often used in RF class C amplifiers to make them easier to bias to class C.

Anyway it doesn't have to be a class C amplifier, it could easilly be a tuned class A amplifier.

Well neither of those two diagrams were correct, as they didn't include any biasing components - the 'class C' one should have a single resistor where the diode is, and the 'class A' one should have either a single resistor from base to positive supply, or (preferably) a potential divider feeding the base. The emitter resistor should also have a decoupling capacitor across it to prevent heavy negative feedback.

But in any case, this is a tripler, you don't want class A, you want class C - easier, simpler, cheaper, and FAR more effective. Where you would use class A is in a linear amplifier (no multiplication) such as an SSB transmitter.

 

[Hero999]

I admit my class A amplifier schematic is wrong, you are right. There does need to a biasing potential divider at the base, and an emitter resistor bypass capacitor is used for some applications but not always it depends on the bandwidth and in this case it would be recommended too.

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

**broken link removed**