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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 13:29:37 +1100
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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.

-- 
Bill Sloman, Sydney