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RF Susceptibility & Operational Amplifiers

I was personally exposed to the effects of radio frequency interference (RFI) on analog components when I had my first VHF ham transceiver. I attempted to power it off of an inexpensive power supply. It worked fine in receive, and it worked fine running from the battery in transmit mode at 2W. However, it failed miserably in transmit mode at 5W on the power supply. It did work OK at 5W in my car.

I began to suspect it was the transmitter in close proximity to the power supply. Thinking about what might be wrong, RFI crossed my mind.

Operational amplifiers are a workhorse of analog design. However, they can give faulty output or even be damaged when exposed to radio frequency energy. This can occur in the control electronics for MRI machines, cellphones, public service radios (used by police, firefighters, and emergency medical technicians), and wireless modems. The power supply circuitry for these control electronics can also be affected. Even the lowly transistor can suffer from RFI.

I looked at the construction of the power supply and noticed many things about it that were suspect. It was apparent that RF could leak in through the loose-fitting painted case. This looked hard to solve. The op-amp and the power transistor that formed the voltage regulator for the supply had long leads — in effect little antennas. Knowing that both the op-amp input and the transistor base-emitter junction would form diodes, I saw that I had a couple of radio receivers sitting there. The rectified RF was changing the bias points of these two components. What was I to do?

I needed the circuits to work at DC, of course, and to regulate out the low-frequency ripple. At higher frequencies (like 144MHz), I needed to shunt the RF energy to ground, so the diodes would no longer have effect. Getting some 100pf capacitors from the junk bin, I soldered these in to shunt the RF such that it took a path other than through the diodes. This cured the issue, and I was happily on the air at 5W hitting the mountain top repeater. There were no more issues with the power supply from then on.

This article from Analog Devices explains RF rectification in op-amps and gives circuits to mitigate the effect.

Analog Devices also offers an excellent reference on instrumentation amplifiers. It contains many circuits and guidelines for preventing RFI rectification in them.

Have you encountered RF susceptibility in any electronics over the years? If so, what were some of the solutions?

4 comments on “RF Susceptibility & Operational Amplifiers

  1. John Dunn
    March 11, 2013

    I had a brief moment about this with the late Bob Pease at one of his lectures. Please see this URL:

    http://licn.typepad.com/my_weblog/2011/09/beware-of-loop-gain-john-dunn-consultant-ambertec-pe-pc.html 

     

     

  2. DEREK.KOONCE
    March 12, 2013

    Good lesson to learn. Need the rib-punch sometimes to look at the real-world details to work with.

  3. WKetel
    March 13, 2013

    Understanding the reason that all of those “extra” capacitors and choke coils are needed is indeed a valuable education. You are fortunate that the ouput dropped instead of going up to the maximum. Those extra parts are what to look for when purchasing a power supply for radio equipment. 

  4. Brad Albing
    March 14, 2013

    Certainly care must be exercised when adding caps at or near the inputs of the op-amps. Sometimes, making the leads/foil traces on the PC board as short as possible; and then adding caps (or inductors/ferrite beads) before the input resistors is better. That way, you can avoid introducing an extra pole into the feedback loop. Of course, on an existing piece of bench equipment, it's not practical to redesign the PCB.

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