Pard Testing Configuration | Agilent Technologies AN 372-1 guide

Test Overview/Procedures

To make PARD measurements, the electronic load used should operate in CR mode for constant volt- age and constant current power supplies. The load should also have lower PARD than the power sup- ply being tested. This is especially important when measuring the PARD of linear power supplies, since they typically have excellent PARD specifications.

A regulated AC source should be applied to the input of the power supply under test. PARD meas- urements are made at the lowest and highest speci- fied values of AC input to the power supply, and at the lowest and highest specified source frequencies.

Proper connections between the instruments and power supply under test are essential when making these measurements. Since PARD consists of low level, broadband signals, major test set concerns are ground loops, proper shielding, and impedance matching. A digitizing oscilloscope can be used for peak-to-peak measurements (see Figure 8). High frequency noise spikes need to be measured, and therefore the digitizing rate of the oscilloscope must be at least five times the maximum PARD frequen- cy for proper sampling. To eliminate cable ringing and standing waves, the typical configuration includes coaxial cabling with 50 Ohm terminations at both ends. Capacitors should be connected in series with the signal path to block the DC current.

A true rms RF voltmeter should be used to meas- ure the rms specification. Precautions similar to those for the peak-to-peak measurements should be considered. For both measurements, care should be taken to prevent ground loops. Since most oscilloscopes and true rms voltmeters have ground referenced inputs, testing a power supply with grounded outputs may create such a ground loop. In this case, it may be necessary to use instru- ments with floating (differential amplifier) inputs to eliminate this problem.

The first set of PARD measurements should be made with the AC source voltage and frequency set at the lowest specified values, and with the power supply under test at its minimum and then maximum rated load value. A second set of meas- urements should be made with the AC source set at the highest specified values of amplitude and frequency, and with the power supply minimally loaded and then maximally loaded. To test multiple output power supplies, PARD measurements for each output should be made with all other outputs set initially to minimum load, and then to maxi- mum load.

Figure 8. PARD Testing Configuration

AN 372-1 specifications

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