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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Low distortion sinewave oscillator without big capacitor.
Date: Wed, 9 Apr 2025 14:18:05 +1000
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On 9/04/2025 1:56 am, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vt2bck$1gmbm$1@dont-email.me...
>> On 7/04/2025 11:10 pm, Edward Rawde wrote:
>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsvspr$33gdd$1@dont-email.me...
>>>> On 7/04/2025 3:25 am, Edward Rawde wrote:
>>>>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vsuacb$1d4ec$1@dont-email.me...
>>>>>> 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:
>>>>>>>>>
>>>>>>>>>> "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:
>>>>>>>>>>>
>>>>>>>>>>>> "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:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> "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:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> "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:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> 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
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