Re: Where is the RFI detected?

On Tue, 03 Oct 2006 22:53:05 GMT, the renowned "PN2222A"
<pn2222a@xxxxxxxxxxxxx> wrote:

Greetings:

I'm working on remediating RFI for a lab instrument.
The detector end picks up small (microamp) signals from
a resistive transducer, intended bandwidth is in the hundreds of KHz.

The input is sent thru a DC-blocking capacitor, 0.1uF, past two clamping
diodes (silicon diodes back-to-back to ground), then into the - input of an
OP-27. Feedback resistor is 82K shunted by 10pf.

There's another gainstage with voltage gain of 50.

My RF input is swept from appx. 100MHz thru 500MHZ, radiated, 3V/M strength.
It's amplitude modulated at 1KHz.

The RF is probably being coupled into the cables (from the sensor and power
supply), and the cable design isn't perfect: the shield on the cable is
tied directly to the negative lead from the sensor and to ground on the PCB.

The RF shows up as the 1KHz signal at the output of the second stage: it's
being detected somewhere in the amplifier chain, it seems.

We're working on standard spells and incantations for RFI: ferrites on the
cabling, improved shielding for the circuits, animal sacrifices and so
forth.

My question: How is the 100MHZ RF being detected? I think it isn't the
diodes, but can check that. If I have RF coupled into the - input of the Op
Amp, does it get rectified? What if I'm pushing the ground node around?

Thanks for any help!
PN2222A

Biased? Of course I'm biased!

Almost surely nonlinearity in the op-amp inputs. Keep it out of there.
You have a couple of orders of magnitude in frequency and a fairly
healthy tranducer signal to play with.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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