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Christopher Cole
science forum beginner
Joined: 08 Jun 2005
Posts: 3
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 Posted: Fri Apr 21, 2006 1:50 pm Post subject:
400MHz signal swamped out by 80kHz
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I have a 1mW 400MHz transmitter that needs to operate inside of a very
strong electromagnetic field operating at 80kHz. The 80kHz field is
emitted from a large (1.5' dia) coil with 2000V P-P across it. When I
place the 400MHz transmitter (doing a CW) inside the center of the
energized large coil, its signal disappears (according to my spectrum
analyzer, looking at a span of 395-405MHz).
Why isn't the 400MHz signal discernable through the 80kHz EM field? Is
it possible that the whole area becomes saturated to the point where any
EM signal at any frequency is affected or attenuated due to the presence
of the stronger field at 80kHz?
Thanks for any input,
-Chris
--
/> Christopher Cole <\ <\
<< Cole Design and Development \\ email: cole@coledd.com \\
\\ Computer Networking & Embedded Electronics \\ web: http://coledd.com >>
\> \> </ |
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Tom Ring
science forum addict
Joined: 06 Jan 2006
Posts: 57
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| Posted: Fri Apr 21, 2006 7:04 pm Post subject:
Re: 400MHz signal swamped out by 80kHz
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Christopher Cole wrote:
| Quote: | I have a 1mW 400MHz transmitter that needs to operate inside of a very
strong electromagnetic field operating at 80kHz. The 80kHz field is
emitted from a large (1.5' dia) coil with 2000V P-P across it. When I
place the 400MHz transmitter (doing a CW) inside the center of the
energized large coil, its signal disappears (according to my spectrum
analyzer, looking at a span of 395-405MHz).
Why isn't the 400MHz signal discernable through the 80kHz EM field? Is
it possible that the whole area becomes saturated to the point where any
EM signal at any frequency is affected or attenuated due to the presence
of the stronger field at 80kHz?
Thanks for any input,
-Chris
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Well, I would need more information to do a good guess, but here are a
couple.
1) Your spectrum analyzer front end is overloaded by the 80KHz signal.
Try putting a high pass filter or bandpass filter in front of it. A
helical or cavity filter is easy to make for 400Megs.
2) The one I think more likely is that there is something nonlinear in
the main circuit. Is it a sinusoidal waveform? If not, and if it's
square or triangular, etc., then you are making lots of harmonics.
Also, if there is any metal to metal contact in the area that has
corrosion on it, you will get harmonics from rectification. It is
likely one of those two or something similar.
3) Are you sure that the transmitter will operate correctly in a high
EM field?
tom
K0TAR |
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P.
science forum beginner
Joined: 25 Apr 2006
Posts: 3
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| Posted: Tue Apr 25, 2006 8:03 pm Post subject:
Re: 400MHz signal swamped out by 80kHz
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On Fri, 21 Apr 2006 14:04:23 -0500, Tom Ring <news0002@taring.org>
wrote:
| Quote: | Christopher Cole wrote:
I have a 1mW 400MHz transmitter that needs to operate inside of a very
strong electromagnetic field operating at 80kHz. The 80kHz field is
emitted from a large (1.5' dia) coil with 2000V P-P across it. When I
place the 400MHz transmitter (doing a CW) inside the center of the
energized large coil, its signal disappears (according to my spectrum
analyzer, looking at a span of 395-405MHz).
Why isn't the 400MHz signal discernable through the 80kHz EM field? Is
it possible that the whole area becomes saturated to the point where any
EM signal at any frequency is affected or attenuated due to the presence
of the stronger field at 80kHz?
Thanks for any input,
-Chris
Well, I would need more information to do a good guess, but here are a
couple.
1) Your spectrum analyzer front end is overloaded by the 80KHz signal.
Try putting a high pass filter or bandpass filter in front of it. A
helical or cavity filter is easy to make for 400Megs.
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I would filter out everything below 1 MHz. That should be easy.
| Quote: | 2) The one I think more likely is that there is something nonlinear in
the main circuit. Is it a sinusoidal waveform? If not, and if it's
square or triangular, etc., then you are making lots of harmonics.
Also, if there is any metal to metal contact in the area that has
corrosion on it, you will get harmonics from rectification. It is
likely one of those two or something similar.
3) Are you sure that the transmitter will operate correctly in a high
EM field?
tom
K0TAR
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4) It may also be an impedance problem. When one of the systems has a
much lower impedance than the other, signals may get low. Depends on
what and where you measure.
Pieter |
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