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desing for high strength RF immunity

I have a design for a simple addressable temperature probe (PIC and dallas semi "1wire temperature probe") that will be placed near a high strength RF transmiter. Transmitter is for long range coms spanning several miles in the 500Mhz to 1Ghz frequency range (5 - 20 watts?).

The unit cannot be placed inside a shielded enclosure and the electonics will recieve direct exposure to the RF.

What are some circuit design practices to harden the circuit form the effects of the RF field.

Are there any components that are particularly sensitive to RF.

Mosfets (enhancement or delpetion) that may turn on unexpectadly?

BJTs?

PIC micros with internal RC oscillator?

opamps?

The board does have individual wires that extend off the board and I'm sure would pickup significant RF.

ferrite beads?

diodes?

thanks

Reply to
love2tha9s
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Lots. For example a metal shield box inside (tuner shell). Good layout, full ground plane, trace sandwiching, PCB inductors, caps, common mode chokes, series resistors. Etc.

Nearly all semiconductor parts are.

That'll take a lot of RF. I'd be more worried about the substrate diode, depending on what the FET drives.

Big time. They have an undesired built-in rectifier in the form of a base-emitter junction.

Depends on how much you let get into it via ports and supplies.

Yep :-(

They help if placed strategically.

They can protect against exceeding electric pain thresholds but can make susceptibility worse because they act as rectifiers.

Seriously, if this is a critical project or one with a tough deadline get professional help from an RF expert before the boss is breathing down your neck or nervously pacing the hallways.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

5-20W direct RF exposure with no shield... you may have a big problem on that one. Sounds to me like your signal level will be quite low... making the situation that much worse. Why is shielding out of the question???

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Reply to
Telstar Electronics

I presume you don't mean 'near a transmitter' but rather 'near an antenna'? The transmitter, while undoubtedly a source that can cause interference, is not going to be radiating lots of RF. That's the antenna's job.

If you really want a thermometer insensitive to near-field antenna placement, look at fiber optic solutions

Shielding can be a simple matter; have you considered the placement of that one-wire probe inside a copper tube? Regular plumbing fittings are very good shielding for the MHz frequency range. The only alternative to shielding is to keep the circuit very compact, then bypass (with capacitors) and choke (with ferrite beads) all the circuit paths sensitive to RF. If you can make the circuit slow enough with filtering, the (slow) temperature signal will dominate the total (signal + noise + RFartifacts).

Reply to
whit3rd

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