Electronic – How to measure EMI without special equipment

emcmeasurementRF

I have bought a cheap dashcam to play with, only to discover that my GPS receiver stops working when it's on. After I ruled out power supply issues (both devices have batteries), I'm left with a hypothesis that the dashcam produces EMI interfering with GPS carries frequencies (1.2-1.5 GHz).

Before I start wrapping my head the dashcam in tinfoil, I want to have an actual measurement of the interference I'm facing. Unfortunately, all I have at home is a multimeter and a toy oscilloscope, in no way capable of GHz measurements. I'm thinking about making some sort of antenna probe for my measurements:

schematic

simulate this circuit – Schematic created using CircuitLab

Is this approach realistic? Is there a way to reliably tune it to the frequency range of interest?

Best Answer

The best receiver you have is your GPS (or a friend's). Without locking to the spread spectrum code to reduce the effective measurement bandwidth, the signal level is waaay below the ambient thermal noise.

What facilities does your GPS have for showing the strength and quality of the satellite signals? My car Tomtom only tells me how many satellites it has acquired, when it's locked to them. My walking Garmin OTOH has a page that gives me strength and quality of signals for all satellites it can see, whether they are fully locked or not.

On some open ground, run a suitable GPS receiver, look at the satellite quality, and then bring your dashcam up to 3m, 2m, 1m, you get the idea. This will enable you to get a qualitative measurement of whether it is indeed the dashcam, and then how well anything you do to it works.

It might be that energy is being emitted in all direction through the plastic case, in which case you're lost. It might be that most of it is conducted down the supply cable, and a suitably sized ferrite 'stopper' on the cable will improve things no end.