[sdiy] Seeking a very old & specific message from Dec 2006

Mike Bryant mbryant at futurehorizons.com
Mon Sep 13 17:56:21 CEST 2021


Or a milking machine !  Those seem to raise ground by 30-40V each time they suck.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
Sent: 13 September 2021 15:36
To: Neil Johnson
Cc: synth-diy
Subject: Re: [sdiy] Seeking a very old & specific message from Dec 2006

You can lock to mains, and it'll filter everything out nicely, assuming your mains feed and earth potential aren't affected by: a blender, an oven, a cnc mill, a mogen, a tig welder, a relay computer, an aluminium plant, ......................

:-)


On Mon, Sep 13, 2021 at 1:20 PM Neil Johnson <neil.johnson71 at gmail.com> wrote:
>
> Hi Tim,
>
> > The notch filter concerned was this one:
> >
> > https://www.ti.com/lit/an/snoa680/snoa680.pdf
> >
> > The rather-too-perfect matching of component values in SPICE can 
> > make the simulated notch response really, really narrow, so it can 
> > be difficult to see that it is there at all. My suggestion was to 
> > replace the op amp follower in the simulation with a 
> > voltage-controlled voltage source where the gain can be set 
> > directly, to 0.9 say - this should make the notch much easier to 
> > see. (If the follower/buffer has gain 'k', the Q of the filter is 
> > apparently Q = 1/(4*(1-k)), so it blows up with k exactly 1.)
> >
> > (And from what I could gather from quick Googling last night, the 
> > filter tends to be rather tricky to use in practice, because of its 
> > use of such big resistor values and the necessity of getting 
> > component values 'just right' in order to have the notch exactly at 
> > 60Hz.)
>
> Very high Q notch filters in reality will be far less than the ideal, 
> both due to component tolerances and also component drift over time.
> One way round this is done in the HP 339A where they wrap the notch 
> filter inside a couple of servo loops locked to the main oscillator.
> If you really wanted to notch out mains noise then you would get the 
> best result somehow locking the notch filter to the local mains 
> frequency anyway as that is likely to vary during the day.
>
> Cheers,
> Neil

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