Noise and Interference basics (Raymond Schouten) txtpage5 --------------------------------------------------------- NOISEFLOOR & INTERFERENCE MEASUREMENT Once the equivalent noisefloor is estimated (see previous page) the expected output noise value can be compared to a measured value. When starting to measure: EXPECT TO HAVE INTERFERENCE SIGNALS ! The first step is to find and solve the major interference problems. What to use for monitoring interference: A voltmeter will NOT show all interference signals A digital scope COULD show all interference signals (depends on settings) An analog scope WILL show all interference signals (but you can have trouble seeing them: dim lights) A spectrum analyzer can be usefull too but it has to span a MHz range and can be missing transients. How to handle: 1-Replace the measured device(sample) by a resistor Rs in a metal box (=Sample-simulator) 2-Connect a scope to the output amplifier of the setup (no filters) 3-Look for line interference by setting the scope trigger to line sync (timebase 10ms/div) If there are no periodic signals then: 4-Set the scope trigger to the input channel Turn the timebase faster while trying different trigger levels to find periodic signals. If there are no periodic signals then: 5-Estimate output noise by adding a lowpass filter(1) to the scope input and dividing top-top value by 6 to get rms value. If this is within 1 to 3 times the estimated noise (2) then: 6-Connect the real device and do 2-3-4-5 again (1) Filterfrequency Fm ,see previous page (2) If it's even lower: switch ON the amplifier