For some background, we recently took a product to the EMI lab for EMI Pre-compliance testing and came close to passing, but we have some issues in the 30 to 100MHz range that just put us out of compliance. Unfortunately, as a small shop, we have been unable to see the same noise source in near field testing with our meters and probes, making it guesswork to try and resolve the issue.
We attempted to create a biconical antenna and it is picking up an amplified version of the noise that our probes see, but not the noise seen in the test lab. We have a 100MHz scope with FFT capability and a 1-3300 MHz spectrum analyser, but are still blind.
Is anyone able to offer suggestions on approaches that we might take or something we could build, in order to get that view of the noise that is eluding us?
The product has several cables which are the likely cause, but without some means to see the noise and then try to reduce it, we are stuck going to the test lab which is very pricey.
The following are the results of the test from the test lab. We are trying to get Class A certification but ensure some margin:
The test is the FCC 30MHz to 1GHz test for emissions.
The following is the spectrum analyzer output from our cobbled antenna at a distance of about 1-2 meters from the device, as shown in the subsequent image.
The device shown is a surrogate, as we don't wish to publish the product image just yet.
This next image is our oscilloscope and probe. The spectrum is pretty much the floor baseline at every point except above the power supply, where it is slightly raised but at a different frequency than what is seen in the official EMI lab.
We are using the following spectrum analyser, which worked well for us in reducing some spikes caused by the CPU and harmonics and helped get us to this point.
The antenna was roughly based on information from the following links without the machining involved:
We could not find a plan for an antenna that was suited exactly for our frequency, so we hoped it might work despite the antenna factors not being a good match. We observed the WiFi quite clearly, but lack a good source in the 1-100MHz range to test against.