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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: 80dB now but still needs improvement at 1KHz
Date: Mon, 4 Nov 2024 21:10:56 +1100
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On 4/11/2024 5:31 pm, Jan Panteltje wrote:
> On a sunny day (Sun, 3 Nov 2024 23:06:04 -0500) it happened "Edward Rawde"
> <invalid@invalid.invalid> wrote in
> <vg9h7b$277o$1@nnrp.usenet.blueworldhosting.com>:
> 
>> Simulation isn't fast but if you let it complete and do an FFT on the last 30 seconds, it's 80dB down at all unwanted
>> harmonics.
>> There do however seem to be unwanted sidebands close in either side of 1KHz
>> Any suggestions for improvement?
> 
> Publish a circuit diagram.
> Not that hard to make !

He did. The .asc file is a circuit diagram. I've looked at it and run 
the simulation - if only for 200 seconds rather than 300 seconds.

The FFT says that sine wave is as good as he claims.

The circuit diagram  doesn't make it clear what it's various parts are 
there to do - it makes sense to group the components in a way that lets 
somebody looking at the circuit diagram get some feel for what the 
components are doing.

And it you are using the LT1994 it makes sense to read the data sheet 
carefully enough to notice that the Icom pin 2 should be bypassed with 
at least 100nF to ground.

https://www.analog.com/media/en/technical-documentation/data-sheets/1994fb.pdf

John May's circuit makes it clear that the second FET isn't strictly 
necessary. There are cheaper ways of getting rid of the even-order 
harmonics.

I've managed to dig out the precision, full wave rectifier that I used 
in my circuit, which is a half-wave rectifier to which you add just 
enough of the full sine wave to deliver both halves of the sine wave 
(one of them inverted) at it's output, which calls for a couple 10k 0.1% 
thin-film precision resistors on a common substrate, which you can buy 
off the shelf.

-- 
Bill Sloman, Sydney