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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: Thu, 10 Apr 2025 00:38:27 +1000
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On 9/04/2025 11:57 pm, Edward Rawde wrote:
> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vt4se2$3qshb$1@dont-email.me...
>> 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.
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