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electra 2008:
An understanding of instruments is very important.
Here are some experiments you might consider, and it would be important to describe the theory as to why.
Note: the ratio of RMS to average of a full wave rectified sine wave is 0.70711/0.63662 = 1.1107 . 0.63662 derives from the exact value of 2/Pi, 0.707 from sq-root 2/2 .
Consider 5 different waveforms and 5 different instruments. These were generated from a Tektronix function generator.
The waveforms are:
Sq-wave +/-10 peak, sine peak 10 V, triangular peak 10 V, and rectangular +10 V for 8 MS and -10 V for 2 MS, and offset the rectangular waveform to + 20 V and 0 V.
The instruments are:
Simpson 260 (circa 1948, an antique), Simpson 270 (circa 1970, tautband), Fluke 27, Fluke 87, and Oscilloscope.
Results:
Code:
The following are measured in the AC position.
Sq-wave ( Adjusted to read 10.0 on Fluke 87, and verified on the scope)
Measured Theoretical
10.0 10.00 Fluke 87
11.4 11.11 Fluke 27 ( 1.0 * 1.111 = 1.111 )
11.0 11.11 Simpson 260
11.0 11.11 Simpson 270
10 Scope peak above and below 0
Triangular-wave ( Adjusted to read +/- 10.0 peak-to-peak on scope)
( The error on the 87 might be from my setting of the peak on the scope.)
(This is where an accurately created waveform would be a better source.)
Measured Theoretical
5.94 5.77 Fluke 87
5.67 5.56 Fluke 27 ( 0.5 * 1.111 = 0.5555 )
5.2 5.56 Simpson 260
5.55 5.56 Simpson 270
10 Scope peak above and below 0
Sine-wave ( Adjusted to read +/- 10.0 peak-to-peak on scope)
Measured Theoretical
7.09 7.07 Fluke 87
7.03 7.07 Fluke 27
6.9 7.07 Simpson 260
6.9 7.07 Simpson 270
10 Scope peak above and below 0
Code:
Rectangular-wave ( Adjusted on scope +10 for 8 MS, -10 for 2 MS)
Measured Theoretical
7.92 7.86 Fluke 87
7.15 7.04 Fluke 27
16.7 DK Simpson 260
11.0 DK Simpson 270
10 Scope peak above and below 0
Rectangular-wave ( Adjusted on scope +10 for 8 MS, -10 for 2 MS)
( The same as the preceding except in DC position. )
Measured Theoretical
6.00 6.00 Fluke 87
6.00 6.00 Fluke 27
6.0 6.00 Simpson 260
6.00 6.00 Simpson 270
10 Scope peak above and below 0
Code:
Rectangular-wave ( Adjusted on scope +10 for 8 MS, -10 for 2 MS)
( Function generator was not modified, but 10 V was added )
( from an HP DC supply)
( AC position, AC and Output includes a series capacitor. )
( The actual RMS value of this waveform is 17.89 )
Measured Theoretical
7.90 8.00 Fluke 87 ( This result is because of the input capacitor.)
7.14 7.11 Fluke 27 ( (4*0.8 + 16*.2)*1.11
38. DK Simpson 260
7.0 7.11 Simpson 260 ( AC and Output )
18. DK Simpson 270
7.0 7.11 Simpson 270 ( AC and Output )
10 Scope peak above and below 0
Rectangular-wave ( Adjusted on scope +10 for 8 MS, -10 for 2 MS)
( Function generator was not modified, but 10 V was added )
( from an HP DC supply)
( DC position, )
Measured Theoretical
16.00 16.00 Fluke 87
15.99 16.00 Fluke 27
15.7 16.00 Simpson 260
15.9 16.00 Simpson 270
+20 and 0 Scope peak above and below 0
The Fluke 87 is classified as a true RMS reading meter. The Fluke 27 and the Simpsons are full wave rectified average measuring and scaled to read RMS on a sine wave. Both the Flukes have a series input capacitor in the AC voltage position and therefore strip the DC component from the input. The Simpsons have no input capacitor in AC unless you change to the Output terminal which adds a series capacitor in the AC mode. The 1948 260 had a 1000 ohms/volt sensitivity on AC, newer ones and the 270 have 5000 ohms/volt on AC. On DC all the Simpsons have 20,000 ohms/volt.
The DK above means I do not know and I really have no idea for the strange results. But note these are for a signal with a non-zero DC component to an AC waveform.
Note: The measured results for the Flukes on the rectangular waveform were the same with and without the added DC component. However, note real RMS value is much higher because of the DC component.
The 1948 Simpson probably used a copper oxide rectifier, and newer ones probably use silicon or germanium diodes.
I have only glanced at the following reference but it may be useful to you:
http://www.allaboutcircuits.com/vol_2/chpt_1/3.html
A better and more accurately controlled way to generate the test waveforms would be a computer program controlling a precision DAC.
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