HP HP-Bench-Briefs-1980-01-02 free download

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rise in the wire. Obviously, a wire is Average responding ac meters are insensitive t o the ac signal's inexpensive and are fine for measur- geometry. ing sine waves where high accuracy is not required. But for non- One limitation to this technique is sinusoids, only a true rms respond- that ambient temperature adds to ing ac meter will do. J J""" - the heat produced by the ac signal. To overcome this, a second, carefully @o@l*o a!"? 001'. !Os I BOdB) t 60 dB1 I 40dBl 10"" ( 2048) matched thermocouple is placed in AMOUNT OF HARMONICDISTORTION series with the measuring ther- Figure 3. Error produced when a mocouple (Figure 5 ) . The two ther- average responding ac meter is used mocouples a r e connected with to read the fundamental. lses originating at THERMOCOUPLE the zero base line contain a dc Trying to measure a square wave component. Only a true rms with a n average responding ac responding ac meter capable of meter is next to hopeless. For exam- + measuring ac dc can determine the ple, the square wave in Figure 4 has true rms value o such a signal. f a peak-to-peak amplitude of 2V.The heating or rms value is 1V. This voltage as measured by an average responding voltmeter would be 1.1V. Average responding ac meters