Re: The Larkin front end

On Wed, 08 Nov 2006 16:58:10 +0000, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:



John Larkin wrote:

Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:
SioL wrote:
"Eeyore" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote
Consider the inherent rejection of common mode signals in a differential front end. It
doesn't work so well if the signals are different sizes ( rather obviously ).

I'll use 1% caps here too.
Graham

I think I understand John.
If the first stage is a balanced opamp input, its a biq question how "balanced" its function
is as a rf detector, the interference signals are likely way out of the operational range
of that opamp. Thermocouple sounds like a very slow device. Pretty much killing
anything even remotely RF should work pretty well.

A ferrite bead won't do that.

Graham

The only complication being that it does.

In the AM band where most of the problems lie ? Around 1MHz ?

Graham


The guts of most ICs still work fine at 1 MHz, so little rectification
takes place. And wiring and pcb traces don't resonate at low
frequencies like this. High-Q resonances often make EMI sensitivity at
some frequencies tens of times worse than the broadband sensitivity.

Our company is in a wooden building in sight of the Twin Peaks
broadcast tower and not far from the Mt San Bruno antenna farm. If we
scope things, we see all sorts of stuff, the biggest usually around 1
MHz, a big AM station. But if we look at things like diffamp outputs
using a bandlimited scope - the rectified stuff - we don't usually see
audio, which is what we'd get from detecting AM... we see what's
obviously video! And when hand-waving or grabbing cables shifts DC
offsets, ferrite beads will usually help a lot.

http://www.chamoismoon.com/Resources/Murals/SF-Sunset-TP.jpg


Beads are low-inductance, very lossy inductors. They come as close to
the magical human finger damping effect as you can get in a
surface-mount part.

John

.

Relevant Pages

  • Re: The Larkin front end
    ... John Larkin wrote: ... doesn't work so well if the signals are different sizes. ... A ferrite bead won't do that. ... using a bandlimited scope - the rectified stuff - we don't usually see ...
    (sci.electronics.design)
  • Re: The Larkin front end
    ... John Larkin wrote: ... they pretty much never resonate. ... the sensitivity was hugely enhanced by internal wiring and PCB ... ferrite bead, we got another 10:1, presumable because it blocked entry ...
    (sci.electronics.design)
  • Re: The Larkin front end
    ... John Larkin wrote: ... doesn't work so well if the signals are different sizes. ... I'll use 1% caps here too. ... If the first stage is a balanced opamp input, its a biq question how "balanced" its function ...
    (sci.electronics.design)
  • Re: Need VCO for 14 MHz PLL
    ... John Larkin wrote: ... looking for VCO that will run at 14.4 MHz. ... do not need to lock output signals "in phase" to input ...
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  • Re: The Larkin front end
    ... doesn't work so well if the signals are different sizes. ... A ferrite bead won't do that. ... It sounds to me like we're comparing lossy low-pass filters to common-mode ... There's no reason the ferrite in a wound choke can't be lossy too. ...
    (sci.electronics.design)