Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: Low distortion sinewave oscillator without big capacitor. Date: Wed, 9 Apr 2025 14:18:05 +1000 Organization: A noiseless patient Spider Lines: 320 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Wed, 09 Apr 2025 06:18:12 +0200 (CEST) Injection-Info: dont-email.me; posting-host="c386dc0c7f6401b591b43cb67cca4871"; logging-data="4026923"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+R5Ces22aevEYrJt6svZCo2AZEektbKAY=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:T/YkxtskrL2oVqKvkgD0PpMkAgY= X-Antivirus-Status: Clean Content-Language: en-US In-Reply-To: X-Antivirus: Norton (VPS 250408-4, 8/4/2025), Outbound message On 9/04/2025 1:56 am, Edward Rawde wrote: > "Bill Sloman" wrote in message news:vt2bck$1gmbm$1@dont-email.me... >> On 7/04/2025 11:10 pm, Edward Rawde wrote: >>> "Bill Sloman" wrote in message news:vsvspr$33gdd$1@dont-email.me... >>>> On 7/04/2025 3:25 am, Edward Rawde wrote: >>>>> "Bill Sloman" wrote in message news:vsuacb$1d4ec$1@dont-email.me... >>>>>> On 7/04/2025 12:39 am, Edward Rawde wrote: >>>>>>> "Bill Sloman" 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" >>>>>>>>> wrote: >>>>>>>>> >>>>>>>>>> "JM" wrote in message news:uop0vjp3d13t441ujfboi5aeeg08anm1je@4ax.com... >>>>>>>>>>> On Fri, 4 Apr 2025 16:29:27 -0400, "Edward Rawde" >>>>>>>>>>> wrote: >>>>>>>>>>> >>>>>>>>>>>> "JM" wrote in message news:add0vjdh2gcma0n9pfunq76n04cfbkhtnj@4ax.com... >>>>>>>>>>>>> On Fri, 4 Apr 2025 14:25:29 -0400, "Edward Rawde" >>>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>>> "Bill Sloman" 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" >>>>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> "JM" wrote in message news:qq8tujlpciqc2jrd0ibljmjr9pd37ip6hi@4ax.com... >>>>>>>>>>>>>>>>>> On Sun, 30 Mar 2025 14:54:56 -0400, "Edward Rawde" >>>>>>>>>>>>>>>>>> wrote: >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> 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. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> 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). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Here's my version of John May's variation. >>>>>>>>>>>>>>> >>>>>>>>>>>>>> ... >>>>>>>>>>>>>> >>>>>>>>>>>>>> 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. >>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Linewraps are going to be a problem - delete all "\n" from the last few lines >>>>>>>>>>>>>> >>>>>>>>>>>>>> You don't want to remove \n just remove the wraps. >>>>>>>>>>>>>> >>>>>>>>>>>>>> BCM61B does not have two independent transistors. >>>>>>>>>>>>>> >>>>>>>>>>>>>> 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. >>>>>>>>>>>>>> >>>>>>>>>>>>>> I took the model from >>>>>>>>>>>>>> https://github.com/peteut/spice-models/blob/master/nxp/complex_discretes/complex_discretes.txt >>>>>>>>>>>>>> >>>>>>>>>>>>>> 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. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Version 4.1 >>>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>> Best to just offset the integrator output so the amplitude is brought >>>>>>>>>>>>> under control sooner. >>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> Wow. That has much lower distortion too. >>>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> If you just replace Q1,2 duals with a simple long tailed pair I think >>>>>>>>>>> you will get better performance. >>>>>>>>>> >>>>>>>>>> I'm not sure I understand how the multiplier could be implemented with just a long tailed pair. >>>>>>>>>> >>>>>>>>> >>>>>>>>> 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. >>>>>>>>> >>>>>>>>> https://1drv.ms/u/c/1af24d72a509cd48/EWVCUG7-jFJMu7-01VczCRcBzEC9JPHrV45x7TOunN90Gg?e=GXbvX5 >>>>>>>>> >>>>>>>>> It could be used as shown here. >>>>>>>>> >>>>>>>>> https://1drv.ms/u/c/1af24d72a509cd48/EVmMVrvUD15GutoR5nCJ7QEBSeZsHWpHudqR0b8XtTLMLw?e=HIV74I >>>>>>>> >>>>>>>> As I've already said, I like it. I've played with it a bit. >>>>>>>> >>>>>>>> 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. >>>>>>>> >>>>>>>> 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. >>>>>>>> >>>>>>>> I've added a cascode transistor (Q1, it should be Q4) to minimise any Early effect distortion. >>>>>>>> >>>>>>>> I've snipped out the op amp driving the base of Q3. Once you'd gone AC-coupled, it wasn't doing anything useful. >>>>>>>> >>>>>>>> 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. >>>>>>>> >>>>>>>> The harmonics aren't great - most of them are about 90dB below the fundamental, but the seventh is only 85dB down. >>>>>>>> >>>>>>> >>>>>>> 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 don't see anything particularly hostile there Bill. Just different points of view. >>>>> >>>>>> 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. >>>>> >>>>> When I started work as a fresh graduate (but one who also had practical experience with everything from TV antenna systems in >>>>> hospitals to AY-3-8500 based games) I was amazed at some of the analogue circuit design blunders I encountered. >>>>> In one case I built a piece of custom test equipment which needed +15V and -15V. An available transformer had two suitable >>>>> isolated >>>>> secondary windings so I just used two 7815 devices. >>>>> Only to be told by a more experienced "Designer" that connecting the output of a 7815 to ground would short it out and I had to >>>>> use >>>>> 7915 for that..... >>>> >>>> What he should have said was the you were messing up the ground returns by hooking up the +15V output of the second 7815 to the ========== REMAINDER OF ARTICLE TRUNCATED ==========