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Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman <bill.sloman@ieee.org> Newsgroups: sci.electronics.design Subject: Re: Random thoughts on sinewave oscillators Date: Sat, 19 Oct 2024 19:27:52 +1100 Organization: A noiseless patient Spider Lines: 101 Message-ID: <vevqip$3q3dn$1@dont-email.me> References: <vetde5$38sbk$1@dont-email.me> <vetukv$1ici$1@nnrp.usenet.blueworldhosting.com> <veu45s$3cmo3$5@dont-email.me> <veu6um$2c1s$1@nnrp.usenet.blueworldhosting.com> <veu7kt$3cmo3$8@dont-email.me> <veu8u0$17fq$1@nnrp.usenet.blueworldhosting.com> <veucs2$3cmo3$9@dont-email.me> <veueme$3no$1@nnrp.usenet.blueworldhosting.com> <veuirv$3cmo3$10@dont-email.me> <veujd8$10dm$1@nnrp.usenet.blueworldhosting.com> <veumn5$3fbqu$1@dont-email.me> <veup74$2qhn$1@nnrp.usenet.blueworldhosting.com> <j656hjp1rq659uh61k3q75bipaf386qqh1@4ax.com> <vev7ho$2of1$1@nnrp.usenet.blueworldhosting.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sat, 19 Oct 2024 10:28:10 +0200 (CEST) Injection-Info: dont-email.me; posting-host="2e8ed10202c51ba77170da41ac256836"; logging-data="4001207"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19ci+UvpOhpZFkGvba/bU4CxxMLnLWgtAI=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:BI/zvMZNU3uZBAdRsii6+ij3iIc= X-Antivirus-Status: Clean X-Antivirus: Norton (VPS 241018-10, 19/10/2024), Outbound message Content-Language: en-US In-Reply-To: <vev7ho$2of1$1@nnrp.usenet.blueworldhosting.com> Bytes: 6223 On 19/10/2024 2:03 pm, Edward Rawde wrote: > "john larkin" <JL@gct.com> wrote in message news:j656hjp1rq659uh61k3q75bipaf386qqh1@4ax.com... >> On Fri, 18 Oct 2024 18:58:43 -0400, "Edward Rawde" >> <invalid@invalid.invalid> wrote: >> >>> "Jeroen Belleman" <jeroen@nospam.please> wrote in message news:veumn5$3fbqu$1@dont-email.me... >>>> On 10/18/24 23:19, Edward Rawde wrote: >>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message news:veuirv$3cmo3$10@dont-email.me... >>>>>> On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote: >>>>>> >>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message >>>>>>> news:veucs2$3cmo3$9@dont-email.me... >>>>>>>> On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote: >>>>>>>> >>>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message >>>>>>>>> news:veu7kt$3cmo3$8@dont-email.me... >>>>>>>>>> On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote: >>>>>>>>>> >>>>>>>>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message >>>>>>>>>>> news:veu45s$3cmo3$5@dont-email.me... >>>>>>>>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote: >>>>>>>>>>>> >>>>>>>>>>>>> "piglet" <erichpwagner@hotmail.com> wrote in message >>>>>>>>>>>>> news:vetde5$38sbk$1@dont-email.me... >>>>>>>>>>>>>> Edward Rawde <invalid@invalid.invalid> wrote: >>>>>>>>>>>>>>> >>>>> ... >>>>>> >>>>>> Without doubt, it's the trickiest aspect of the design. Definitely do-able >>>>>> though. Let us know how you get on. >>>>>> >>>>> >>>>> Ok. This simple circuit is based on the circuit you can find here. >>>>> >>>>> https://electronics.stackexchange.com/questions/697687/how-to-control-the-amplitude-of-a-wien-bridge-oscillator >>>>> >>>>> It produces a something wave. >>>>> I wouldn't call it sine but at least it's not clipping. >>>>> What's going on here? >>>>> >>>>> Version 4 >>>> [Snip...] >>>> >>>> You're hitting the flat portion of the Id vs. Vds curve around the >>>> top of the wave. In that region the dynamic resistance of the FET >>>> is very large, and therefore the gain of the opamp drops to about >>>> one. As a result, the positive tip of the output gets sort-of >>>> squashed. >>>> >>>> There are probably ways to fix this, for example by feeding a >>>> portion of the output signal to the FET gate, but a quick >>>> attempt I made didn't work very well. This is why FETs aren't so >>>> great as gain setting elements. >>>> >>>> Using a lightbulb --or more generally a PTC resistor-- for R7 is >>>> really hard to beat. >>> >>> Ok thanks Jeroen. >>> >>> It looks like the best approach for the gain control is either a filament or something like that used in the document Bill Sloman >>> posted. >>> >>> https://www.analog.com/media/en/technical-documentation/application-notes/AN132f.pdf >> >> Note that the LDR has a very small influence range on the loop gain. >> > > That's why I added R3 in this circuit. > It does not seem to be safe to reduce R3 below 1k. > > R4 helps a lot too for reasons I don't fully understand. > It may be moving the FET to a better part of its operating characteristics. > > A single rail version also works with another op amp producing 6V for R4 and two 20k resistors for R2 between 12V and 0V. > As expected, this produces twice the output voltage and I've not found a way to reduce it. > > This will probably be my final offering for a 1KHz sinewave oscillator unless anyone can suggest improvements without using light > dependent resistors. > From the LTSpice plot, I can't discern any impurity in the signal this circuit produces. > It would be interesting to see what a real circuit and a spectrum analyzer says but I probably won't be building it. > > I haven't used an LDR since playing with an ORP12 around age 10. > I seem to remember that they can degrade over time but maybe that only happens in sunlight. I got your earlier circuit to work a lot better simply by increasing R7 to 5.6k. If you use the View option on the trace viewing panel to pull out an FFT of the output (I use Blackmann-Harris windowing) from 10sec to 20 sec, you can see that second harmonic distortion is about 20dB below the primary - not great but better than it was. And the waveform looks like a sine wave. The less influence the FET has on the gain of the circuit, the better the sine wave. The AD734 would do a lot better - or at least it does in my simulations - but it isn't cheap. -- Bill Sloman, Sydney