'How noisy is a PIC'
This is in response to Martin's method for calibrating a C508. I'm
curious as to whether its possible to screen a PIC so that it cannot
be "heard" by a short wave radio receiver. Is an aluminium casing
sufficient to block the PIC's RC oscillator signal?
Geoff Wooton asked:
>This is in response to Martin's method for calibrating a C508. I'm
>curious as to whether its possible to screen a PIC so that it cannot
>be "heard" by a short wave radio receiver. Is an aluminium casing
>sufficient to block the PIC's RC oscillator signal?
Yes, that will probably help a lot. If RF silence is something that
concerns you, you should also use the lowest oscillator frequency
consistent with your application, as high frequencies will be much more
difficult to keep bottled up.
Reginald Neale wrote:
> Geoff Wooton asked:
> >This is in response to Martin's method for calibrating a C508. I'm
> >curious as to whether its possible to screen a PIC so that it cannot
> >be "heard" by a short wave radio receiver. Is an aluminium casing
> >sufficient to block the PIC's RC oscillator signal?
> Yes, that will probably help a lot. If RF silence is something that
> concerns you, you should also use the lowest oscillator frequency
> consistent with your application, as high frequencies will be much more
> difficult to keep bottled up.
....and, use the lowest possible supply voltage....
NOTE: remove asterisks from email address to reply directly
Paul Mathews, consulting engineer
non-contact sensing and optoelectronics specialists
PIC's have the potential of being one of the quieter CPU's around
since they do not tend to have physically large busses carrying their clock
signal to long runs of wire. There is a saying in the amateur radio
community that goes like: "components don't radiate. Wires do." The
traces on a circuit board, peripheral cables, and even the proximity of
cables to oscillators or clock devices all effect how much radio frequency
noise a particular device throws out.
The DC power to a digital system must be isolated against radio
frequencies by filter chokes and or feed-through capacitors or it will become
an unwitting antenna for both broadcasting and receiving radio signals.
Think of any long wire to the circuit that is not shielded or
protected with a feed-through capacitor as a broadcast antenna.
The 12C series with their internal oscillators should be very quiet
as long as the power supply or other lines going to the device are blocked
for RF per poses.
If one needs to use a PIC's oscillator as the master clock for
a larger system, then expect somewhat more noise.
As someone already mentioned, the higher the frequency, the worse
the problem. This is because it takes shorter runs of wire to make good
antennas at higher and higher frequencies. The square wave signals produced
by clocks are also very rich in odd-order harmonics. This means that the
third harmonic of a 20 MHZ clock crystal is 60 MHZ which is right in the
spectrum of low-band television channels in many parts of the world.
Depending upon the exact frequency involved, the strength of the interfering
signal and the strength of the desired television signal, this could create
serious reception problems.
With a working computer, the potential problems are actually
worse since the processing of the program being executed creates all kinds
of momentary or sustained frequencies that leak in to the air. While a
square wave creates third-order harmonics, there is enough asymmetry around to
make pretty good harmonics at all the even frequencies also so there are all
kinds of birdies floating around in a system that is not RF-tight.
This is by no means a complete discussion of radio frequency
interference. The American Radio Relay League has excellent literature
devoted specifically to this topic. It is probably good reading for all on
this list whether they find amateur radio interesting or not.
Martin McCormick WB5AGZ Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group
Lifetime opportunists Bulky Mailings Candy Company. We specialize in suckers.
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