Re: Where is the RFI detected?

PN2222A 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!


i can't imagine 100Mhz--500Mhz ever being detected and past via a resistive transducer how ever, i can see how the detector it self
would react to the signal and use it's own structural capacitance
to filter the high frequency and thus produce a low frequency that
is a variation of amplitude of the original frequency.

also, taking in light of things, you have miller effects in
components (op-amps, transistor amps etc.) that in it self, can produce
the same effect and only show you the end results of the variant levels
of amplitude. fairly a common practice for cheap detection and also an
hindrance in other cases.


just my 2 cents worth.


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

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