Path: ...!weretis.net!feeder8.news.weretis.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: Random thoughts on sinewave oscillators Date: Sun, 20 Oct 2024 15:27:59 +1100 Organization: A noiseless patient Spider Lines: 130 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sun, 20 Oct 2024 06:28:03 +0200 (CEST) Injection-Info: dont-email.me; posting-host="9829e61973788f70fd1b52332324599a"; logging-data="302024"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+o72x16CkECg30eBR/qsU/pADAnBKNdYI=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:DocUbaCK7Vytr5glPOqjoohJFkU= X-Antivirus: Norton (VPS 241019-8, 20/10/2024), Outbound message Content-Language: en-US In-Reply-To: X-Antivirus-Status: Clean Bytes: 7830 On 20/10/2024 3:36 am, Edward Rawde wrote: > "Bill Sloman" wrote in message news:vevqip$3q3dn$1@dont-email.me... >> On 19/10/2024 2:03 pm, Edward Rawde wrote: >>> "john larkin" wrote in message news:j656hjp1rq659uh61k3q75bipaf386qqh1@4ax.com... >>>> On Fri, 18 Oct 2024 18:58:43 -0400, "Edward Rawde" >>>> wrote: >>>> >>>>> "Jeroen Belleman" wrote in message news:veumn5$3fbqu$1@dont-email.me... >>>>>> On 10/18/24 23:19, Edward Rawde wrote: >>>>>>> "Cursitor Doom" wrote in message news:veuirv$3cmo3$10@dont-email.me... >>>>>>>> On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote: >>>>>>>> >>>>>>>>> "Cursitor Doom" wrote in message >>>>>>>>> news:veucs2$3cmo3$9@dont-email.me... >>>>>>>>>> On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote: >>>>>>>>>> >>>>>>>>>>> "Cursitor Doom" wrote in message >>>>>>>>>>> news:veu7kt$3cmo3$8@dont-email.me... >>>>>>>>>>>> On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote: >>>>>>>>>>>> >>>>>>>>>>>>> "Cursitor Doom" wrote in message >>>>>>>>>>>>> news:veu45s$3cmo3$5@dont-email.me... >>>>>>>>>>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote: >>>>>>>>>>>>>> >>>>>>>>>>>>>>> "piglet" wrote in message >>>>>>>>>>>>>>> news:vetde5$38sbk$1@dont-email.me... >>>>>>>>>>>>>>>> Edward Rawde 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. > > If you run this circuit then View, FFT, Use current zoom extent, Ok > It implies that unwanted harmonics are 40dB down. > I'm not sure I believe that but if true then it's not bad for a very low cost circuit. The revised .asc file is a bit of a mess. You've added R11 to get the output frequency close to 1KHz. What you should have done is to have used 0.1% 10.5k resistors -it's an E96 value and you can buy them off the shelf - at R1 and R2. That got me to 1.001kHz. Since the capacitors at C1 and C2 can at best only be +/-1% tolerance parts - you can't buy anything better off the shelf - this is quite close enough. You can use a trimming potentiometer to get closer to the target frequency, but that does have its downsides. R4 certainly does make the circuit settle faster - how is a bit of a mystery - but you've created a total mess with R5,R7, R8, and R10. It's not clear what you were trying to do. I found that I could get by without R4 provided that I stuck with sensible resistance values at R7 and R8 - R7 went up to 6.2k and I put 330R across J1. The cheapest plastic +/-1% film capacitors I could buy from Element-14 in Australia cost $A1.53 each so it isn't a particularly low cost circuit. -- Bill Sloman. Sydney