[sdiy] mixer

[Magnus Danielson]

From: jhaible at t-online.de (jh.)
Subject: Re: [sdiy] mixer
Date: Sat, 3 Nov 2001 14:56:15 +0100

> > Also, tricks used for error cancellation in precision circuits, like
> having a
> > resistor in series with the + input that is the same as the equivalent
> resistance
> > seen by the - input (eg, Rf || Rin) is actually detrimental, because there
> is no
> > phase cancellation of noise between the + and - inputs - everything adds
> as RMS
> > (the square root of the sum of the squares of all the individual noise
> sources).
> 
> This "trick" is for reducing offset voltage comming from input bias current,
> not for noise reduction. I think it even *adds* noise, so better bypass it
> with a capacitor.

Bypass with a capacitor? Electrolyt or tantal cap? Noise sources right there.
Especially as the electrolyt cap ages.

Actually, most parts of a curcuit is not hurt by a little offset,
except where you

* Modulate levels (VCA/OTAs and thus in VCFs and naturally over
  switches and pots)

* Integrate (such as VCOs, LFOs, Expos)

In static gain and filter stages for instance it is usually not that
critical. For such stages you can make a single offset correction for
the whole chain, and thus add the minimal noise just for the offset
correction side.

But if you really want to look at noise issues, you need to look at
all the components (including op-amps etc), see how values affect
noise and how it shapes frequency wise. The better noise levels you
want to acheive the better models you have to do and the more effort
needs to go into balance the noise sources for the intended
properties. Also, there are other constraints on values than noise
itself.

Just discussing the resistors is not sufficient and can be quite
missleading.

Cheers,
Magnus