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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Final final 1kHz oscillator
Date: Wed, 20 Nov 2024 13:55:22 +1100
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On 20/11/2024 1:29 pm, Bill Sloman wrote:
> On 20/11/2024 12:59 pm, Edward Rawde wrote:
>> "Bill Sloman" <bill.sloman@ieee.org> wrote in message 
>> news:vhibce$1t7v2$1@dont-email.me...
>>> On 18/11/2024 2:58 pm, Edward Rawde wrote:
>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message 
>>>> news:n7iijjdeqecl0kmub0bq5in0dbm60m7qam@4ax.com...
>>>>> On Thu, 14 Nov 2024 11:14:28 -0500, "Edward Rawde"
>>>>> <invalid@invalid.invalid> wrote:
>>>>>
>>>>>> "JM" <sunaecoNoChoppedPork@gmail.com> wrote in message 
>>>>>> news:t5fajjdteskfftvkf84iqsp2vc4b9ta5kj@4ax.com...
>>>>>>> On Fri, 8 Nov 2024 15:43:41 -0500, "Edward Rawde"
>>>>>>> <invalid@invalid.invalid> wrote:
>>>
>>> I've taken John May seriously, and reworked my phase shift oscillator 
>>> to use three op amps, all with their non-inverting inputs
>>> tied to ground.
>>>
>>> The results aren't all that impressive, the lower harmonics are about 
>>> 65dB below the fundamental. Start-up is slow - it takes
>>> about five seconds of simulation before the circuit settles down into 
>>> steady state operation, and that after I dropped the
>>> integrating capacitor from 6.8uF to 680nF. The nice thing about the 
>>> circuit is that I can explain what each bit is doing. I can
>>> see a few tweaks that might be worth trying.
>>>
>>
>> The simulation died after 3 seconds, as if insufficient gain to 
>> sustain oscillation.
> 
> The circuit starts off by reducing gain. I think I can do something 
> about that.
> 
>> You seem to have a lot of components.
> 
> But I know what they all do.
> 
>> Are you sure you really need all those ferrites?
> 
> No, but they are cheap, and op amps injecting hash into the power rails 
> are never a good idea. Power supply rejection drops with frequency, and 
> U1 in particular pulls spikes of current from the power rails twice per 
> cycle.
> 
>> Here's my really definitely final final offering.
>>
>> A comfortable 90dB down at 2kHz and > 100 dB elsewhere.
>> Also looks clean either side of 1kHz.
>> Whether or not this has anything to do with reality I do not know.
>> The diode stabilizer string will be temperature dependent,
>> requiring readjustment of R4 and a total of three adjustments are needed.
>> Only three op amps (two packages) are needed.
> 
> I'll take a look at it.

Simulate for 350 seconds? Don't collect data for the first 50 seconds?

I've no idea why you are using the LT1994. The circuit doesn't have a 
common mode problem, so why are you using an op amp designed to deal 
with one?

This is sci.electronics.design not sci.electronics.incremental-development.

And the six diode "stabiliser string" is nuts. If you need a 3.6V 
reference voltage there are lots of ways to do it with more precision 
and better temperature stability.

__
Bill Sloman, Sydney