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'[EE] ISM band xtal for lower RFI noise emissions?'
I asked here before but my email went to the bit bucket somehow. Do people consider using an ISM band clock xtal in an attempt to move all/most self generated harmonics out of normal radio bands? This is more for keeping radiated noise out of one's own project than for passing RFI emissions conformance testing
On Sun, July 8, 2012 1:02 am, Peter P. wrote:
> I asked here before but my email went to the bit bucket somehow. Do people
> consider using an ISM band clock xtal in an attempt to move all/most self
> generated harmonics out of normal radio bands? This is more for keeping
> radiated noise out of one's own project than for passing RFI emissions
> conformance testing.
I really doubt it would make much difference- your effort is better spent
doing good layout- watching where the return current flows, short current
paths, etc. Self noise and harmonics are really hard to predict- while
*most* are at at a small integral multiple of the fundamental, they can
interact, and go all over the place. If you are doing a radio where the
fundamental is right on the crystal's frequency, it might make a bit of a
difference, but no matter what, good signal integrity will be the key to
the best performance.
Just outside of Austin, TX
The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with
I agree, good layout and bypassing IS the answer.
We just put the system clock(s) where they need to be, and try not to
We routinely put two layer boards without much ground plane through
part 15, Industry Canada, and CE with emissions that are barely
detectable, with processors running from 4 - 120 MHz, and a fair
amount of peripherals and cables. We don't spend anything extra on
shielding, and many of our products are in plastic enclosures.
Route power so that both sides go into one side of the bypass cap
pads, and out of the other side, go to the chip that you're bypassing.
Don't drive signals faster than you need to. You can add series
resistors at the driver to slow down the edges, but you'll have to
experiment for best compromise.
If you can get access to any sort of a spectrum analyzer, and make up
a small shielded loop probe, it will help you isolate troublesome
points before spending lots of money on testing. RF-Bay amplifiers
are a great value for a front end amp. Relatively low cost, and good
The number one rule is that it's far easier not to make the noise,
than to shield it once you have made it
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