Re: Low distortion sinewave oscillator without big capacitor.

On 7/04/2025 12:39 am, Edward Rawde wrote:

"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsu092$14oc7$1@dont-email.me...

On 6/04/2025 2:12 am, JM wrote:

On Fri, 4 Apr 2025 23:55:11 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
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"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:uop0vjp3d13t441ujfboi5aeeg08anm1je@4ax.com...

On Fri, 4 Apr 2025 16:29:27 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
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"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:add0vjdh2gcma0n9pfunq76n04cfbkhtnj@4ax.com...

On Fri, 4 Apr 2025 14:25:29 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsnufh$2ou7j$1@dont-email.me...

On 4/04/2025 11:33 am, JM wrote:

On Thu, 3 Apr 2025 19:25:33 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
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"JM" <sunaecoNoChoppedPork@gmail.com> wrote in message news:qq8tujlpciqc2jrd0ibljmjr9pd37ip6hi@4ax.com...

On Sun, 30 Mar 2025 14:54:56 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
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Not long ago JM posted a 1KHz sinewave oscillator with very low distortion.
It used a 470uF non polarized capacitor which in practice would probably be made from two 1000uF capacitors.
There's nothing wrong with that but I wanted to see whether I could make a working circuit without needing such a large
capacitor.

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You will need to adjust the feedback to suit.  Start with a -5 or -6
gain block after the integrator and adjust it's gain until the startup
is clean (no saturation).

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Here's my version of John May's variation.
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...
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Yes, that works but only 120dB down at 4KHz.
I put the damping resistor back to 47k since I don't care what happens during the first few seonds as long as it happens.
If it's necessary to wait one minute for the purest tone, that's fine with me.
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Linewraps are going to be a problem - delete all "\n" from the last few lines

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You don't want to remove \n just remove the wraps.
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BCM61B does not have two independent transistors.
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So you probably want BCM847BS which has two independent matched transistors and a very low price at digikey, so may as well
use
two
of them as shown below.
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I took the model from
https://github.com/peteut/spice-models/blob/master/nxp/complex_discretes/complex_discretes.txt
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It should only be necessary to unwrap the last line of the following.
Don't remove \n just remove the wraps, you may need to use a horizontal scroll bar.
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Version 4.1

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Best to just offset the integrator output so the amplitude is brought
under control sooner.
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Wow. That has much lower distortion too.
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If you just replace Q1,2 duals with a simple long tailed pair I think
you will get better performance.

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I'm not sure I understand how the multiplier could be implemented with just a long tailed pair.
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Just modulate the tail current and select how much to steer to the
output by directly driving the bases rather than indirectly as in your
circuit.  The following link shows one example topology, and a four
quadrant differential I/O version.  Compare the linearity of each of
them.
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https://1drv.ms/u/c/1af24d72a509cd48/EWVCUG7-jFJMu7-01VczCRcBzEC9JPHrV45x7TOunN90Gg?e=GXbvX5
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It could be used as shown here.
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https://1drv.ms/u/c/1af24d72a509cd48/EVmMVrvUD15GutoR5nCJ7QEBSeZsHWpHudqR0b8XtTLMLw?e=HIV74I

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As I've already said, I like it. I've played with it a bit.
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The ON-Semiconductor NSS40301MDR2G NPN dual comes with a 2mV guaranteed maximum difference in base-emitter voltages (at the same
emitter current). Edward Rawde's Nexperia BCM61B dual part has matched current gain, but no guarantee on the Vbe. The Nexperia
BCM847BS does offer 2mV base-emitter matching, and would presumably work just as well.
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Putting in the ON-Semiconductor dual means that your gain control circuit doesn't have to waste output swing coping with
part-to-part variation.
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I've added a cascode transistor (Q1, it should be Q4) to minimise any Early effect distortion.
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I've snipped out the op amp driving the base of Q3. Once you'd gone AC-coupled, it wasn't doing anything useful.
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And I've put a string of eight diodes in series with R10. They nominally compensate for the temperature dependence introduced by
the four rectifier diodes D2, D12, D13 and D14. In this version of the circuit the rectifiers knock about 0.6 volts off a sine
wave that peaks at 3.8V, about a quarter of the 15V rail. I haven't run the numbers to fix the best number of diodes, but
something between six and eight looks okay.
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The harmonics aren't great - most of them are about 90dB below the fundamental, but the seventh is only 85dB down.
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 Not sure I see the point if it's only 80dB down Bill.
@ 7Khz in LTSPice 24.1.5

LTSpice isn't all that reliable as predictor of low level distortion. Having an armoury of different circuits to try when you finally get around to building  and testing something real may be useful.

I can almost get that from a simple phase shift oscillator and a 1KHz tuned circuit.

I'm sure that you think so. John Larkin thought that a bang-bang amplitude control was worth suggesting...

The last time I included a diode string like that in one of my circuits (which I seem to recall had better than 80dB performance)
you told me it was nuts.

It probably was. In this case there are better ways of getting a rectified output than a simple series diode - I've posted circuits which incorporate precision rectifiers which get rid of the forward drop through the diode, and I've built circuits that used synchronous rectifiers built around transmission gates where the output isn't shifted by a temperature dependent diode drop. It went into a GaAs single crystal puller as a retrofit.
The main point of the diode string was as a satirical comment on that aspect of the design you posted, which probably counts as being hostile, but I am hostile to ill-thought out designs, hard though it is to get the design time to sort them out. I've got stuck with sorting out other peoples half-baked designs often enough, but only after my bosses had had their noses rubbed in the unfortunate consequences.
--
Bill Sloman, Sydney