i.Reading on differential voltmeter should not vary from reading recorded in Step g by m o r e than 8mVdc for Models 6204B and 6205B or 10mVdc for Model 6206B.
5-21 Ripple and Noise.
Definition: The residual ac voltage which is superimposed on the dc output of a regulated power supply. Ripple and noise may be specified and measured in terms of its RMS or (preferably) peak-to-peak value.
Ripple and noise measurement can be made at any input ac line voltage combined with any dc output voltage and load current within rating.
5-22 The amount of ripple and noise that is present on the power supply output is measured either in terms of the RMS or (preferably) peak-to-peak value. The peak-to-peak measurement is particularly important for applications where noise spikes could be detrimental to a sensitive load, such as logic circuitry. The RMS measurement is not an ideal representation of the noise, since fairly high output noise spikes of short duration could be present in the ripple and not appreciably increase the RMS value.
5-23 The technique used to measure high frequency noise or “ spikes” on the output of a power supply is more critical than the low frequency ripple and noise measurement technique; therefore the former is discussed separately in Paragraph 5-31,
5-24 Ripple and Noise Measurements. Figure
5-6A shows an incorrect method of measuring p-p ripple. Note that a continuous ground loop exists from the third wire of the input power cord of the supply to the third wire of the input power cord of the oscilloscope via the grounded power supply case, the wire between the negative output terminal of the power supply and the vertical input of the scope, and the grounded scope case. Any
ground | current circulating in | this loop as | a | result |
of the | difference in potential | EG between | the | two |
ground points causes an IR drop which is in series with the scope input. This IR drop, normally having a 60HZ line frequency fundamental, plus any pickup on the unshielded leads interconnecting the power supply and scope, appears on the face of the CRT. The magnitude of this resulting noise signal can easily be much greater than the true ripple developed between the plus and minus output terminals of the power supply, and can completely invalidate the measurement.
5-25 The same ground current and pickup problems
TM 11-6625-2965-14&P
Figure 5-6. CV Ripple and Noise, Test Setup
can exist if an RMS voltmeter is substituted in place of the oscilloscope in Figure 5-6.However, the oscilloscope display, unlike the true RMS meter reading, tells the observer immediately whether the fundamental period of the signal displayed is 8.3 milliseconds (1/120 Hz) or 16.7 milliseconds (1/60Hz). Since the fundamental ripple
frequency present on the output of an
supply is 120Hz (due to full-wave rectification), an oscilloscope display showing a 120Hz fundamental component is indicative of a “clean” measurement set- up, while the presence of a 60HZ fundamental usually means that an improved setup will result in a more accurate (and lower) value of measured ripple.
5-26 Although the method shown in Figure 5-6Ais not recommended for ripple measurements, it may prove satisfactory in some instances provided certain precautionary measures are taken. One method of minimizing the effects of ground current flow (IG) is to ensure that both the supply and the test instrument are plugged into the same ac power buss.
