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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Programming Languages
Date: Sat, 02 Nov 2024 21:12:03 +0000
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On Sat, 2 Nov 2024 15:57:39 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:

>"Cursitor Doom" <cd@notformail.com> wrote in message news:ahucij5dt50fihbuenl766e80isr227gqa@4ax.com...
>> On Sat, 2 Nov 2024 13:07:32 -0400, "Edward Rawde"
>> <invalid@invalid.invalid> wrote:
>>
>>>"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vg4fff$3lok1$1@dont-email.me...
>>>> On 2/11/2024 12:01 pm, Edward Rawde wrote:
>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message news:vg3575$3bio0$1@dont-email.me...
>>>>>> You can call me old fashioned, but I still believe there's never been a
>>>>>> more elegant computer language than the original K&R C. You can keep the
>>>>>> rest; I'll stick with that.
>>>>>
>>>>> Having just got back from a vacation I thought I'd give my input to this before looking into whether it's worthwhile getting 
>>>>> back
>>>>> into sinewave oscillators.
>>>>
>>>> John May has come up with a much better sine wave oscillator than yours.
>>>
>>>That's no surprise. I can remember one or two other occasions when I thought I had a brilliant way to do it but someone else came 
>>>up
>>>with a better way.
>>>I don't specifically mean sinewave oscillators.
>>>
>>>> It also has more components, and I'm not sure that all of them are strictly necessary. Getting deep enough into the design to be
>>>> sure where the harmonics are coming from is going to be difficult. I think I'm getting there, but I'm not all that motivated to
>>>> put in the rest of the
>>>> work.
>>>>
>>>> One obvious point is that a FET channel isn't a perfect resistor - as the voltage across it rises above zero it starts looking
>>>> more like a constant current diode (and you can buy FET-based constant current diodes).
>>>>
>>>> In theory, if you added a second harmonic component to the FET gate drive you could make it look like a resistor over a wider
>>>> range of voltage, if the phasing was close enough to right.
>>>>
>>>> You've also got the point that when there's a voltage drop across the FET channel, it adds to the gate-to-channel voltage (as 
>>>> has
>>>> been mentioned here) and you can cancel that with an in-phase fundamental component
>>>
>>>The last circuit of my own had both an n fet and a p fet.
>>>I found that by adding a capacitor from one gate to the other (to try to cancel the unwanted signals in opposite phase) I could 
>>>get
>>>the unwanted gate signal below 100uV. I then had harmonics approaching 60dB down except one at 50dB (I think 2KHz). Not brilliant
>>>but not bad.
>>>
>>>There are some useful pointers here:
>>>https://sound-au.com/articles/sinewave.htm
>>>In particular where it says "Done properly, a JFET can provide distortion performance that is as good or better than a lamp or
>>>thermistor."
>>>
>>>Perhaps I'll concentrate on how to make the FET behave as a voltage variable resistor over the widest possible range.
>>>
>>>I also what to look into what I meant by crud and non crud mode in LTSpice.
>>>This mysterious effect can depend on things such as which specific computer is used and how long is specified before collecting
>>>simulation data.
>>>You can see it in the gate voltage after startup. It looks a bit like a PLL hunting and eventually locking but it doesn't happen 
>>>at
>>>startup, it happens after seconds.
>>>So I'll need to be able to post some pictures to show that. I'll get to that.
>>
>> Hi Edward,
>> I've been messing around with a real-life WB oscillator - the busted
>> one I originally posted about. AFAIK, everyone else here has just been
>> simming them, so I thought it might be useful if I provided some
>> detail that others may have overlooked.
>> The thing that stands out in my experiments in replacing the broken
>> thermistor with a pot and attempting to twiddle for the optimum sine
>> wave is just how close the waveform has to get to collapsing from
>> insufficient feedback in order to get a nice sine wave. It's a
>> knife-edge adjustment to get it right and then of course, with
>> constantly shifting temperatures it goes out of adjustment again
>> within a few seconds. But the sweet spot for the best waveshape is
>> *just* a whisker above collapse.
>> HTH.
>
>Well it's all exactly what you would expect because as was previously pointed out, a gain of 1.0000000001 (with as many 0s as you 
>want) will grow to limiting but a gain of 0.99999999999 (with as many 9s as you want) will die to nothing. So in any real circuit 
>you must constantly control the gain so that never goes into limiting and never dies. Yes this will usually be a knife-edge thing.
>
>If you try to do this manually then the slightest change in anything (supply voltage, temperature, humidity, wind direction, 
>Halloween) will tip it one way or another.
>
>Pure sine waves are surprisingly difficult to generate for this reason. Real analogue hardware just doesn't want to do it naturally 
>(unless the frequency is so high that the next harmonic is barely noticed or easy to filter). It would much rather produce a square 
>(limited) wave or nothing at all.

Indeed. And that's why it's really quite remarkable that this tiny
passive device enables the generation of such spectrally pure
sinewaves; an 'ancient' technology that still holds a candle in the
digital world of today.

>>
>>
>>>
>>>Is there an easy way to remove a DC offset from a simulation trace so that my n and p gate signals can be superimposed after
>>>startup?
>>>
>>>>
>>>> -- 
>>>> Bill Sloman, Sydney
>>>>
>>> 
>