Author Topic: Reducing Interference Part 1  (Read 623 times)

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Offline Nova

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Reducing Interference Part 1
« on: November 01, 2014, 05:28:08 PM »
I have an intermittent interference problem that appears to be coming from the electrical distribution system.  Image 1562 shows the spikes with a lowered amplifier gain of 10*2.  The interference persists in both amplifier channels despite a grounded aluminum foil shield around the channel 2 antenna, which is the lower of the two oscilloscope traces.  Apparently shielding does not reduce this type of noise.  The noise spike is very obvious when holding an oscilloscope probe up to an extension cord; the noise interferes mildly with amateur radio communications.

When the pulse is present, the system enters interference mode with a gain more than 10*2.  Using lower gain limits system detection to stronger and/or nearer lightning strokes.  Without the interference and with the amplifier gain at 10*10, the system detects lightning up to 2500 miles distant.

Has anyone had better or different results using shielding around the ferrite antennas?

Thanks,
Joe

Offline W3DRM

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Re: Reducing Interference Part 1
« Reply #1 on: November 01, 2014, 05:59:34 PM »
Before I would worry about attempting to shield the noise, I would suggest trying to identify where it is coming from by shutting off various circuit breakers in the house. Once you identify a specific circuit, then start the process of eliminating appliances, lamps, wall-warts, etc until you find the problem. You may also be able to find the noise using a portable AM radio tuned to a station-free spot on the dial to search for the general location of the noise generator.

You mention it is intermittent. Is there any specific time of day when it comes on or goes off? Also, looking at your waveform image, it appears the pulse-rate is around 8.2ms. If my calculations are correct, that give you a frequency of about 120 hertz. It could be a full-wave rectifier diode somewhere that is producing the spikes when the ac signal switches. Of course this is assuming the values and settings shown on the scope are calibrated and accurate. Once you isolate the house power circuit producing the noise, you should begin looking for any wall-warts in it that may noisy.
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Offline Nova

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Re: Reducing Interference Part 1
« Reply #2 on: November 01, 2014, 10:31:03 PM »
As I mentioned, the noise appears to be coming from the electrical utility distribution system.   The 240 volt line to the house has small notches near the positive and negative peaks of the AC waveform where something is arcing.  These notches coincide with the noise pulses from the lightning detector amplifier.  And yes, you are correct the noise pulses are 8.3 mSec apart which is 1/2 of the 60 cycle line frequency.

We had rain mid afternoon; after 10 or 15 minutes the noise stopped.  The rain later stopped, and the noise returned about a half hour later.  Unfortunately, shortly afterward  an additional set of pulses appeared, one of which is stronger than the original noise.

So, my original question: Has anyone had better or different results using shielding around the ferrite antennas?

Thanks,
Joe

 

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