Larkin's Oscillator Explained

=A0 =A0 ...Jim Thompson

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We're referring to an oscillator that oscillates in bursts, rather than continuously. A resonator connected to a negative resistance doesn't squegg, but on the other hand its amplitude builds to infinite (for certain values of infinite) at a rate determined by Q. You can think of an LC resonator as having a 1st order response, measuring oscillation envelope versus drive. Oscillators squegg when some amplitude limiting mechanism adds additional time delays or memory, more poles in the control loop, like the capacitor in the base of my circuit. A bigger cap reduces the tendency to squegg.

A superregen receiver is a squegging-on-purpose RF oscillator. That's a fascinating circuit.

John

The sguegging also lets you receive CW without a local beat oscillator.

Here's a cute one:

I built one of these to receive FM using a couple of the proximity fuse vacuum tubes. The whole thing would fit in a match box.

All those big R-L-C parts add the loop time lag to make it squegg.

I've always wanted to score some prox fuze tubes. Something like 100 million were made during WWII.

If you haven't seen them, here are some schematics:

ftp://jjlarkin.lmi.net/Prox.zip

And a good book:

John

Hmm, OK something like motorboating gone to the extreme.. enough amplitude modultaion to shut the oscillator off for some time. (?) Does the squegg rate stay constant in a sugerregen receiver? I'm not real sure what a superregen reciever is, anything like a marginal oscillator?

George H.

Using squegging to quench a superregen is a way to save tubes, rather than anything inherent to the superregen principle.

You can find a copy, there's a really beautiful little book by J. R. Whitehead entitled "Superregenerative Receivers", part of the "Modern Radio Techniques" series edited by J. R. Ratcliffe. Highly recommended. (Ratcliffe was an amazing character too, btw.)

Cheers

Phil Hobbs

(Currently wrestling gila monsters in Albuquerque NM. [Just kidding about the gila monsters.])

Yes, there are externally quenched superregens, but they're not as common, or as elegant.

Sounds good. I'll look for it.

It's an oscillator that is periodically quenched and allowed to restart. Ideally, when it's off, it's allowed to crawl back up towards oscillation through an exponentially decaying bias voltage, so it's creeping up on oscillation. The slightest bit of ambient RF, as from an antenna, makes the oscillation start sooner. The result is that the duty cycle of oscillation, hence the supply current, depends on the external RF.

A single-transistor or tube superregen can drive headphones from a microvolt RF signal, can detect CW or AM or FM, and can have pretty good selectivity. It's an amazing circuit.

John

George Herold Inscribed thus:

Its a technique by which an amplifier is held on the verge of oscillation by positive feedback, at which point the gain is very high. By causing the oscillation to be quenched at the instant it starts is where the "Super" bit comes from in "Superregenerative". Commonly used in radio receivers where a single active device is used to both amplify the incoming RF, demodulate it and amplify the resultant AF, often driving headphones directly. Often frowned upon because of the interference that can be caused by radiated RF.

On a sunny day (Tue, 15 Jun 2010 12:16:05 +0100) it happened baron wrote in :

In my view the gain is not 'very high', but just enough to compensate for any losses to make the overall gain '1', as needed for oscillation. The effect it has, is increasing the Q factor, so also reducing the bandwidth, by reducing the loading on the tuned ciruit. Think negative resistance. That means you get a lot of signal on that tuned ciruit. Think about it, if you really HAD that much gain available then you would not need the super-reg method.

Yep.

As in...

But I can't show that... can't give away _real_ circuit design :-) ...Jim Thompson

I think you're confusing it with a Q-multiplier, which is a marginally-stable positive feedback gizmo used to sharpen up crummy RF tank circuits in HF radios. (They're noisy as can be, but you don't care at HF.)

A superregen is an oscillator that gets quenched (i.e. turned on and off) at some more or less fixed ultrasonic frequency like 50 kHz. Quench can be internal (due to squegging or blocking), or can be applied externally.

Since oscillation has to build up from noise, even a small input signal changes the average output level dramatically. The build-up is exponential, so an input signal e times the thermal noise speeds up the build-up by one whole time constant. If you make the excess gain big enough that the oscillation achieves its full amplitude before the next quench, the average output level goes as the logarithm of the input level.

Another way to see this is that making the input signal e times bigger chops off one TC worth of infinitesimal output and adds one TC of full amplitude output, i.e. the duty cycle increases by TC*f_quench per factor of e at the input. With a class-C oscillator, the cathode current demodulates the envelope, which is where the detector aspect comes in.

There are a lot of other interesting aspects too--for instance, the carrier of each tone burst is at the free-running frequency of the oscillator, but if you do the Fourier transform, it consists only of the input frequency plus sidebands at multiples of f_quench, and the input frequency component is phase coherent across quench cycles.

There are also lots of interesting subtleties depending on whether the quench is sinusoidal or square-wave.

Really a pretty circuit. I've been wanting to do an optical version for years, but so far it's never been quite the right hammer for the nail.

Cheers

Phil Hobbs

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Excellent! I was reading a few web things about it and this is the critical point that I did not see mentioned.

Hmm, seems like you might be able to measure changes in noise with a super-regen too. (I'm not sure how you would calibrate it.)

George H.

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The rushing sound from the speaker is amplified thermal noise.

Cheers

Phil Hobbs

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I've never heard it... sigh. The noise only 'sets' the size of the first oscillation (or so). Ever 50kHz 'squegg' gets a different seed, when listening to noise. (I'm probabbly using squeeg wrong.) The noise in the noise will go like the squegg freq over the carrier. Seems like I'm going to have to try and build one. My son and I can listen to AM radio.

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I've never heard it... sigh. The noise only 'sets' the size of the first oscillation (or so). Ever 50kHz 'squegg' gets a different seed. (I'm probably using squeeg wrong.) The noise in the noise will go like the squegg freq over the carrier. Seems like I'm going to have to try and build one. My son and I can listen to AM radio.

George H.

Or even FM. Of course due to the logarithmic response the AM is sort of distorted. You can also run a superregen in linear mode, by speeding up the quench so that the oscillation never quite reaches its final amplitude. That's quite a bit tweakier, though.

Cheers

Phil Hobbs

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I don't quite 'see' how it works with FM, but that's OK. I guess AM at 1MHz just sounded easier than FM at 100MHz. Oh I was thinking of a linear mode... tweaky, gives you something to do. It's really pretty neat that with one amp you can from noise levels up to volts.

George H.

A superregen makes a decent noise generator.

John