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Old 31st Jul 2015, 10:24 am   #21
G8HQP Dave
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Default Re: Valve Anode Grid Capacitance

I suspected that might be the case, but I thought it was worth mentioning.

I find oscillator theory quite fascinating.
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Old 31st Jul 2015, 1:01 pm   #22
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Default Re: Valve Anode Grid Capacitance

Some of the mathematical analyses are related to the equations used to generate those 'Mandelbrot' images which were fashionable 20-odd years ago. Oscillators can easily show chaotic behaviour, particularly unintentional oscillators!

For a bit of fun, I unrolled a few classic oscillator designs into a very long string of tuned amplifier stages, then gave each stage a realistic level of noise. Each stage also gave realistic compression. Modern computers have so much power (carefully hidden from unsuspecting owners by the never ending bloating of everyday software) that hundred stage strings simulate wihout any delay and people can see how noise builds up, successive filtering makes the noise look sinusoidal, and the compression stabilises the level. It makes the assumption that 360 degree phase coincides with the filter peak.

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Old 31st Jul 2015, 2:38 pm   #23
G8HQP Dave
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Default Re: Valve Anode Grid Capacitance

People don't tend to think of an oscillator as being nothing more than a very sharp noise filter. The elementary radio books often hide this from the reader.
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Old 31st Jul 2015, 7:03 pm   #24
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Default Re: Valve Anode Grid Capacitance

Well, since those books to which you refer are 'elementary', then they wouldn't would they?

And I, for one, don't think of an oscillator as a filter in any shape or form. Moreover, I don't think that I am in a minority of one when I think of an oscillator as a 'producer' of a signal(s) and a filter as something that 'stops' a signal - or signals.

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Old 31st Jul 2015, 7:31 pm   #25
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Default Re: Valve Anode Grid Capacitance

It's interesting to look at oscillators in various different ways; too many people assume an oscillator produces just one frequency. In truth, it produces many frequencies. Apart from the fundamental-frequency and its harmonics, it also modulates each of these frequencies with a range of noise-sidebands, which are created by things such as thermal, shot and [in the case of pentodes] partition-noise, coupled with ripple-noise from the power supply.

In any oscillator, visualise the current flowing through every resistor [including the active device's equivalent resistance] as a thermal-noise-source which seeks to both frequency- and amplitude-modulate the oscillator's fundamental frequency. And it modulates the oscillator's harmonics. Which the non-linearity of the oscillator then re-mixes with the fundamental!

When you're looking at historic radar VHF and low-UHF-superhet stuff it was quite an issue because the noise sidebands from the free-running local-oscillator were still significant at the rather non-selective front-end's image-frequency.

Even [some would say moreso] in these days of digital synthesis, getting a truly 'clean' receiver L.O. is tricky: there's still a lot to be said for using a crystal oscillator as the first L.O. in a receiver [coupled with some seriously tight interstage filtering if you need to multiply it].
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Old 1st Aug 2015, 5:47 am   #26
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Default Re: Valve Anode Grid Capacitance

Quote:
Originally Posted by G8HQP Dave View Post
People don't tend to think of an oscillator as being nothing more than a very sharp noise filter. The elementary radio books often hide this from the reader.
That prompted me to look at a couple of relatively elementary books, to see how they handled the issue. And in particular, if noise is not mentioned, then how did they handle the question of how the oscillations started, which might be phrased otherwise as: “where do baby oscillations come from”.

Gordon King, in “Radio Circuits Explained” said this:

“The circuit of an oscillator stage used with a separately excited mixer in an a.m. receiver is given in Fig. 3.6. A sine-wave oscillator is in essence a tuned amplifier with part of the output signal coupled back to the input so that the output signal is maintained. For oscillation to occur, however, the phase of the amplified signal coupled back must correlate exactly with that of the output from whence it was considered to be derived, and the new output must be at least equal to the original output. This means that the loop gain of the amplifier must be equal to or greater than unity. For oscillation to build up the circuit is designed for a loop gain greater than unity, though this drops to unity as the signal develops. The latter is achieved either by the amplifier bottoming, which reduces the load resistance, or by the creation of a bias from the oscillator signal which changes the operation to class B (as in Fig. 3.5, for example) so that the amplifier is cut off for a part of each oscillatory cycle.”

Quite nicely put overall, but he avoids the origin question.

H. Henderson, in “Radio Reception”, said, in describing a tuned oscillator:

“Any random fluctuation at the frequency of maximum amplification causes oscillations to appear, and the amplitude of oscillation rises until non-linear operation of the valve restricts further increase. A common automatic amplitude controlling arrangement, consisting of C and R in the grid circuit, is shown in Fig. 2.28. As the oscillations build up, grid current flows on the positive excursions and C charges, making the grid negative. The grid continues to go more and more negative until the amplification of the valve adjusts itself to the attenuation introduced between output and input. Any increase in oscillation amplitude increases the negative grid bias and reduces the amplification, so reducing the oscillation. A fall in oscillation amplitude allows C to discharge through R, and the grid bias to become more positive with a resultant increase in oscillation ampli¬tude. As no bias exists when the oscillator is first switched on, the valve has maximum amplification and this assists the commencement of oscillation.

“With this type of arrangement the valve current is reduced as the oscillation builds up. Touching the oscillatory circuit or shorting out the feedback winding stops oscillation and the valve current rises. This is a useful indication that the circuit is functioning.”

Here noise does get mentioned, but as “random fluctuation”, and the filtering action is implied by “at the frequency of maximum amplification”. So the reader is still left to make the final step to “very sharp noise filter”.

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Old 1st Aug 2015, 8:43 am   #27
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Default Re: Valve Anode Grid Capacitance

Of course, the noise which kicks off operation of an oscillator doesn't cooperatively stop once oscillation starts, it keeps on contributing.

At any instant, the output of the oscillator contains a contribution from noise delayed, filtered and amplified by one pass through the circuit, plus that from 2 passes, 3 passes, and so on.

Even if you think of an oscillator kick started by a turn-on transient, that is a one-off event and the continual addition of noise will eventually overrule its influence.

This means we can never make real sine waves, we can only ever produce filtered and compressed noise. This sounds pretty disappointing and is up there with the feeling you get when you first come across the speed of light limitation.

For years people have spoken of 'phase noise' when they see random noise sidebands on the output of an oscillator. Let's get the word 'phase' out of the way. There's amplitude varying noise too on the oscillator, but many (not all) of the things we shove oscillators into tend to clip the signal and so act to strip off AM components, so it's really the phase components which get up to most mischief.

So you feed your oscillator into the world's most wonderful spectrum analyser and the picture you get shows the wanted signal rising out of a pair of noise-like sidebands, like a tree out of a clump of undergrowth.

It turns out that the 'tree trunk' is pretty much the shape of the analyser's resolution filter. No problem with such a wonderful instrument, switch it to a narrower resolution bandwidth. The 'trunk' gets narrower... but the space liberated shows more and higher noise sideband, closer to the centre. go narrower and narrower and you soon see the pattern. It's noise all the way to the top. The existence of a real spot frequency sine was only an illusion fostered by looking at it with a filter of limited resolution, or of only having a limited time to look for.

So we now have a linkage from entropy in thermodynamics to entropy in signal theory.

Yipes!

David
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Old 1st Aug 2015, 8:46 am   #28
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Default Re: Valve Anode Grid Capacitance

I saved it for a second post...

Of course, if we design a good oscillator, one with sharp enough filtering, we can make signals close enough to pure to do most things we need.

All signals are muck heaps. The art is to pile the muck up more vertically!

David
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Old 1st Aug 2015, 12:21 pm   #29
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Default Re: Valve Anode Grid Capacitance

Good analogy!
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