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>> 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?).
>>
5 - 20 W is not remotely high power. Bypass all the low frequency leads from the temperature probe to ground with .001 uF capacitors and short leads. Assuming everything is CMOS, you could also put ferrite beads or 100 Ohm resistors in series with the PIC leads that are exposed to the RF. This is probably overkill, though. The main thing is to keep the leads short, and use a PC board with a ground plane; this can even be done with a 2 layer board. I am also assuming that the antenna is outdoors, and not connected directly to the RF box.
Tam
> 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.
>>
>
> 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.
>
>> Are there any components that are particularly sensitive to RF.
>>
>
> Nearly all semiconductor parts are.
>
>
>> Mosfets (enhancement or delpetion) that may turn on unexpectadly?
>>
>
> That'll take a lot of RF. I'd be more worried about the substrate diode,
> depending on what the FET drives.
>
>
>> BJTs?
>>
>
> Big time. They have an undesired built-in rectifier in the form of a
> base-emitter junction.
>
>
>> PIC micros with internal RC oscillator?
>>
>
> Depends on how much you let get into it via ports and supplies.
>
>
>> opamps?
>>
>
> Yep :-(
>
>
>> The board does have individual wires that extend off the board and I'm
>> sure would pickup significant RF.
>>
>> ferrite beads?
>>
>
> They help if placed strategically.
>
>
>> diodes?
>>
>
> 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
>
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