The outputs of the power supply aren’t perfectly continuous. There is a small oscillation called ripple and on top of this oscillation we have some spikes, called noise. We need to see if ripple and noise are within specs: maximum of 120 mV for the 12V outputs and maximum of 50 mV for the 5V and 3.3 V outputs. These numbers are peak-to-peak voltages and the lower, the better. In fact we always like to see noise and ripple below half of these numbers. This is something that multimeters can’t detect and that is one of the several reasons why reviews based solely on multimeters are flawed. To measure ripple and noise we will use an oscilloscope.
Since ripple and noise aren’t in the range of MHz we can use a cheap PC-based oscilloscope, and in our case we bought a Stingray DS1M12 from USB Instruments. This equipment is simply an analog-to-digital converter (ADC) with a program that collects data sent by the ADC and plots a chart on the screen.
Our load tester has a BNC connector for installing an oscilloscope, allowing us to monitor any one of the power supply outputs (there is a switch where we can choose which output we will monitor). During our tests we will monitor each power supply output for each load pattern. Whenever possible we will try to bring details of the noise level when the power supply was delivering its maximum power because it is usually under this scenario that we usually find the highest noise level.
In Figure 5, you can see an example of the output presented by the Stingray scope. Here we were monitoring noise from the +12 V output of a power supply and since we were using the 0.01 V/div scale (i.e., the distance between each horizontal line represents 0.01 V or 10 mV) the peak-to-peak voltage is a little bit above 20 mV, well under the maximum admissible noise – which is great, by the way. If you can’t see this the program tells you the peak-to-peak voltage, the RMS voltage and the frequency of the noise in a human-readable format (Figure 6).
One final note. The ATX12V specification states that ripple and noise should be measured with a 0.1 µF ceramic capacitor and a 10 µF electrolytic capacitor attached to the oscilloscope probe. Our load tester has these capacitors behind its panel, so we don’t need to add them. This is another advantage of having a professional load tester.