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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Current mirror version of the lowish distortion 1kHz sine wave osillator
Date: Thu, 16 Jan 2025 09:49:39 -0500
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vmb18t$3h0dq$1@dont-email.me...
> On 16/01/2025 2:57 pm, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vm9tod$37i55$1@dont-email.me...
>>> On 16/01/2025 7:23 am, Edward Rawde wrote:
>>>> "Don" <g@crcomp.net> wrote in message news:20250115a@crcomp.net...
>>>>> Edward Rawde wrote:
>>>>>> Bill Sloman wrote:
>>>>>>> This just reworks my circuit to use a controllable asymmetric current mirror instead of the FET for gain control. I take the
>>>>>>> feedback from the full wave rectifier and switch every half-cycle to reconstruct a variable amplitude sine wave to control 
>>>>>>> the
>>>>>>> output amplitude. It does use a lot of components, but it strikes me as fairly comprehensible.
>>>>>>>
>>>>>>
>>>>>> First I corrected the usual line wrap issues.
>>>>>>
>>>>>> In the latest LTSpice (24.1.0) it took me a good hour or two to find out why I was getting strange netlist errors for all the
>>>>>> opamps
>>>>>> in the circuit.
>>>>>>
>>>>>> This turned out to be .ENDS in the BAS70L model. Remove .ENDS and the issues go away.
>>>>>>
>>>>>> So this is the circuit I'm simulating in 24.1.0 with no component updates available.
>>>>>> I'm expecting it to take 2 hours to complete.
>>>>>
>>>>> Thank you for your tip to wait 2 hours for results. For what it's worth,
>>>>> Bill's original LTSpice source worked for me "as is."
>>>>
>>>> It did for me too, but because it looked like simulation time would be long I moved it to another machine I use over remote
>>>> desktop.
>>>> On that machine LTSpice asked if I'd like the latest version so I upgraded to 24.1.0.
>>>> After the upgrade I got very strange netlist errors which didn't mention BAS70L and it took a while to figure out the cause.
>>>>
>>>> I'm now revising the simulation time to a minimum of 4 days for 10 seconds.
>>>> 24.1.0 seems to be faster.
>>>
>>> My computer seems to be faster. It mostly simulates at about 25msec/sec for me so I get my 10 seconds in about seven minutes of
>>> real time. There are spots early in the process where it slows down, but not for long.
>>>
>>> I pushed up R27 from 9.1R to 13R to make the Ic(Q6) and Ic(Q9) peak currents more nearly equal - you lose some base current in 
>>> the
>>> second current mirror and R7 corrects for that.
>>>
>>> It doesn't make much difference to the third harmonic content in  the output which is still only 59 dB below the fundamental. I
>>> suppose I ought to low pass filter the current through R30 with a filter with a 1msec propagation delay, but that would be even
>>> more components.
>>
>> With an FFT from 525-536ms with Blackman-Harris window I'm seeing nearly 100dB down at 1kHz and 80dB at 2kHz.
>> But there's also crud only 60dB down at 600kHz where crud means an almost contiuous spectrum between 500kHz and 600kHz.
>
> This sounds like a typo.

Ah yes, I meant 2kHz and 3kHz. 100dB down at 2kHz and 80dB down at 3 kHz from 754 to 778 ms, then it ran out of drive space.
Crud has now moved to 1.5 MHz.

> The fundamental is at 1kHz, and I've set up the circuit so that it is just above 0dB. The second harmonic at 2kHz is about 
> at -80dB, but the third harmonic at 3kHz is at about -60dB. Numerical integration does produce high frequency crud - also known as 
> rounding error and quantisation error. We'd have to build a real circuit to get a credible idea of how it performed at high 
> frequencies, and you have to build it fairly carefully to avoid lots of crud on the supply rails, which gets everywhere.
>
> My full-wave rectifier does do some fast switching, which could make it a noise source.
>
> -- 
> Bill Sloman, Sydney
>