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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: 80dB now but still needs improvement at 1KHz
Date: Tue, 19 Nov 2024 11:47:00 -0500
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"Liz Tuddenham" <liz@poppyrecords.invalid.invalid> wrote in message news:1r3a1m9.1lg1mngftnegwN%liz@poppyrecords.invalid.invalid...
> Edward Rawde <invalid@invalid.invalid> wrote:
>
>> "Liz Tuddenham" <liz@poppyrecords.invalid.invalid> wrote in message
>> news:1r37pyc.1bodve2fz5t4wN%liz@poppyrecords.invalid.invalid...
>> Edward
>> Rawde <invalid@invalid.invalid> wrote: > > [...] >
>> >> Now approacing 90dB if you believe the simulation with with this
>> >>experimental circuit. > You'll want to find something else to do while
>> >>it simulates.
>> >> How to inprove it further?
>> >
>> > At 1 Kc/s, a distortion figure of 90dB represents one cycle in 10^9, so
>> > you would have to run it for 1000 seconds before the startup transient
>> > became insignificant.
>>
>> The startup transient was not included in the FFT but in any case the
>> circuit has other issues so here is a circuit which works as intended.
>> Harmonics are approaching 100dB down and 1k c/s is below 90dB. Simulate
>> overnight then stop. It will be at about 250 seconds. Select roughly the
>> last 50 seconds and FFT on current zoom extent. Number of data points may
>> need increasing.
>
> The startup transient of the FFT would have to be included, so that may
> set a limit to the accuracy of the simulation measurements.  Even if you
> start on a zero-axis crossing, you will get spurious results because of
> the finite length of the sample.

Yes I understand that. It's actually a sinewave multiplied by a rectangular function.
That's why I try to use a sample length of many tens of seconds.
A window function might help but then I'd have to decide which window to use.

>
> This can be demonstrated by doing FFT on samples of fewer and fewer
> cycles from the middle of an apparently pure sinewave.  As the sample
> length decreases, spurious harmonics begin to appear, even though they
> don't exist in reality.
>
> [...]
>> We could probably have a discussion here about why c/s is better than Hz
>
> I don't know if it is 'better' but it suits me  - and nobody seems to
> have any difficulty understanding it.  By using a descriptive term,
> rather than a commemorative one, beginners find it easier to understand
> as it sounds less like jargon intended to exclude the uninitiated.

Yes that's why I sometimes prefer to use it.

>
> (I have nothing against H.Hertz, it's just the principle that worries
> me).

I think it goes back to a time when some people thought they had more influence than they had, so we all had to use rectangular 
boxes for logic gates etc.

>
>
>> Version 4
>> SHEET 1 2196 932
>> WIRE -64 -576 -96 -576
>> WIRE 48 -576 16 -576
>>
> [...]
>
> I don't use Spice, so the rest doesn't convey anything to me.

I don't currently have a lab to play in so LTSpice is useful but there's no knowing how close a model of reality it is.

>
>
> -- 
> ~ Liz Tuddenham ~
> (Remove the ".invalid"s and add ".co.uk" to reply)
> www.poppyrecords.co.uk