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Re: 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?). >>
***************************************** 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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Reply to
Tam/WB2TT
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It can be helpful to select capacitors with a self resonant frequency which is close to the transmitter frequency. They will then have the lowest possible impedance.

Many manufacturers supply selection tools which allow the impedance to be plotted as a function of frequency. I have used the Murata tool "Murata Chip S-Parameter & Impedance Library" which despite the name is such a graphical application.

John

Reply to
jrwalliker

OK, Lets back up. I assumed you were going to put your thing inside the transmitter box. Perhaps not. Here is an example. I have an AC powered CO detector that went off when a 1200W transmitter whose antenna is about 50 feet away is active. To fix it, I had to do two things.

To prevent the house AC wiring from acting as an antenna, I connected a .01 uFd 1000V capacitor across the AC line inside the CO detector. This almost fixed the problem, but not quite.

An examination of the CO detecto's PC board showed no VCC bypassing other than a 470 uFd capacitor at the rectifier. Adding a second bypass cap (also .01) from VCC to ground fixed the problem.

Tam

Reply to
Tam/WB2TT

On 3 Jan 2007 13:09:44 -0800, snipped-for-privacy@yahoo.com Gave us:

The shielded enclosure you mentioned. Don't place it in one. Place one ON the board. If a quarter inch or half inch thick can is too much, you have bigger problems.

The other way is to place only the transducer where the reading are to be taken, and port the data/signal over to the electronics at a distance.

You could make a little bluetooth (I'm sure they are out there) transmitter for the transducer, and read the data anywhere within 100 feet or such.

Reply to
JoeBloe

That will entirely depend on what the OP means by "near". I have had to debug cases where the RF went smack dab into the bond wires and metal layers of chips and nothing short of changing to metal shielding helped.

--
Regards, Joerg

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

not at all! You absolutley need a differential signal path for your littel temperature signal riding awash in a sea of common mode rf. just encode temp differentially, run the wired pair through the hell of noise and subtract, in a sheilded and qiet space!

ask me how @@

snipped-for-privacy@att.net

Reply to
LVMarc

Yes, differential will help with the RF portion that doesn't hit the sensor directly. Best is both, shiedling plus differential like it's done on aircraft (Twinax etc.).

--
Regards, Joerg

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

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