|
When measuring the value of an alternating current signal it is often necessary to convert the signal into a direct current signal of equivalent value which is known as the root mean square (RMS) value. Most low-cost instrumentation and signal converters carry out this conversion by filtering the signal into an average rectified value and applying a correction factor. The value of the correction factor applied is only correct if the input signal is sinusoidal. The true RMS value is actually proportional to the square root of the average of the square of the curve, and not to the average of the absolute value of the curve. For any given waveform, the ratio of these two averages is constant and, as most measurements are made on what are (nominally) sine waves, the correction factor assumes this waveform; but any distortion or offsets will lead to errors. Although in most cases this produces adequate results, a correct conversion or the measurement of non-sinusoidal values requires a more complex and costly converter, known as a ''true RMS converter''. ==Thermal converters== The RMS value of an alternating current is also known as its ''heating value'', as it is a voltage which is equivalent to the direct current value that would be required to get the same heating effect. For example, if we applied RMS to a resistive heating element it would heat up by exactly the same amount as if we had applied . This principle was exploited in early thermal converters. The AC signal would be applied to a small heating element which was twinned with a thermistor which could be used in a DC measuring circuit. The technique is not very precise but it will measure any waveform at any frequency (except for extremely low frequencies). A big drawback is that it is low-impedance: that is, the power used to heat the thermistor comes from the circuit being measured. If the circuit being measured can support the heating current, then it is possible to make a post-measurement calculation to correct the effect, as the impedance of the heating element is known. If the signal is small then a pre-amplifier is necessary, and the measuring capabilities of the instrument will be limited by this pre-amplifier. In radio frequency (RF) work, the low impedance is not necessarily a drawback since 50 ohm driving and terminating impedances are widely used. Thermal converters have become quite rare, but as they are inherently simple and cheap they are still used by radio hams and hobbyists, who may remove the thermal element of an old unreliable instrument and incorporate it into a modern design of their own construction. Additionally, at very high frequencies (microwave), RF power meters still use thermal techniques to convert the RF energy to a voltage. Thermal-based power meters are the norm for millimeter wave (MMW) RF work. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「True RMS converter」の詳細全文を読む スポンサード リンク
|