What is the crest factor of an oscilloscope?
The crest factor of the oscilloscope is often asked. If you check the specifications, the only explanation for the crest factor you can find is "This reading does not depend on any signal, ie the crest factor." Here we will explain what kind of value the crest factor is and what the value can be used for. For multimeters, the crest factor should be taken into account when measuring non-sinusoidal AC voltages. Crest factor is defined as the ratio of peak voltage to rms voltage.
The typical crest factor is:
Sine wave: 1.414
Square wave: 1
25% duty cycle pulse: 2
For multimeters, (it can handle very high crest factors), but because you ca n’t really handle the shape of the waveform you are measuring, your readings may be wrong because your signal peak is too high. The peak value can overload the circuit, and the reading may be too low and not displayed on the measurement problem. A multimeter with a true rms reading can specify the value of the largest crest factor that it can truly handle accurately. Due to the saturation characteristic of the true effective value conversion chip, this value is usually near the lowest point of the full scale reading. There will be better accuracy near the half scale. Therefore it is a good habit to check the reading of your multimeter in the second highest range and verify that the reading is in the same order of magnitude. The crest factor of Fluke 87 can reach 3. The crest factor of the Agilent 33401A and 33970A is "a crest factor of up to 5 near the full scale" (the scale has an additional 0.40% error)
For ScopeMeter products, the situation is different. First of all, there is always a waveform on the screen to indicate you to tell whether the signal is overloaded. In addition, ScopeMeter calculates the true effective value measurement value from the sampled waveform data. As long as the sampled value is outside the range of the A / D converter, the reading is no longer displayed, and OL is indicated. For example, Fluke 123 will display OL in the range of 5V when the peak value is greater than 8.5V. This will result in a full scale crest factor of 8.5 / 5 = 1.7. Although this value is not very significant, it is not important for ScopeMeter because the device does not display incorrect readings. In addition, when used in automatic mode, it will automatically change to the next higher range. (ScopeMeter can measure peak value and effective value at the same time)
ScopeMeter can therefore accurately measure the true RMS value of the signal without relying on the crest factor. The limitation is that it requires a higher range to correctly capture the peak signal. Due to the higher range, you can only measure the limit value of the lower range for the effective value. If the effective value of the signal is too small, you can stop using the low-range values ​​with less accuracy. For the oscilloscope, the useful range is the range with a crest factor of up to 10. For example, a 100Hz 5V pulse with a 1% duty cycle, measuring the same signal with Fluke 87 (I) will give a reading of 174.1mV, which will be given when manually switching to the next higher range A better measurement effect: 0.483mV.
Make sure your measurement equipment can handle the shape of the signal you are measuring. If you are not sure, then do a second check at a higher range, or choose equipment that will never show incorrect readings.
Why are the readings obtained on the Fluke 123 different from those displayed on the Fluke 190-series or other oscilloscopes?
As you know, we have very good specifications for the accuracy of ScopeMeters. But sometimes you will find different readings between Fluke 123 ScopeMeter and Fluke 190-Series ScopeMeter (for example for high frequency amplitude measurements). This difference is too large to be explained by the error of the equipment. Recently we found that sometimes the reading of Fluke 123 is no longer, and Fluke 190 has a good reading. The reason for the analysis is due to different loading circuits.
The Fluke 123 (STL 120) test probe is a 1: 1 probe, which tests the circuit in megaohms / 225pF. The Fluke 190-series probe (VPS200) is a 10: 1 probe, which is 10 Megaohms unload the impedance of F to test the circuit.
Unlike the Fluke 190-series probe that uses the VPS200, the Fluke 123 ScopeMeter uses an STL 120 shielded test head, which will load the circuit at approximately 20 times the frequency (say> 1MHz). When the internal resistance of the circuit is small, it will not affect the reading, but when the resistance is large, you need to pay attention. The test will cause a decrease in the voltage on the circuit under test, resulting in unrealistic test results.
Use a 100: 1 probe at high frequencies
At high frequencies (> 1MHz), the resistance of the capacitive components of the probe is large. A standard 10: 1 probe, such as VPS 200, has a capacitive resistance of 14pF. The 100: 1 probe, such as the new VPS 201, has a capacitance resistance of 6.5pF. Using a 100: 1 probe will greatly reduce the load on the test circuit.
(The maximum sensitivity of the Fluke 190 Series ScopeMeter with a 100: 1 probe is 500mV / div).
After 47 hours, Fluke190 stopped working, why?
When the Fluke 190 series works with the Fluke 123 or ScopeMeter B series power adapter, instead of the BC190, each function will work very normally, but after some time, the Fluke 190 series will be The battery shuts down when it runs out. Because when you use a recording function of the Fluke 190 series, other power adapters will not be able to capture the important information you are trying to capture.
The PM8907 / 80x cannot provide enough current to the Fluke 190 series. The battery of the Fluke 190 series may be depleted when using the PM8907 / 80x instead of the BC190 / 80x. This is mainly due to the limited output power of PM8907 / 80x. PM8907 / 80x provides 300mA output current, while BC190 / 80x provides 840mA output current.
operating:
Use only those power adapters provided by the device. Because the device has been tested and certified by the adapter.
When sending demo equipment to customers, make sure that the power adapter is included so you can get a good first impression.
Suggest:
Do not use the PM9651 / 00x adapter from the Fluke 160 series.
This adapter is not suitable for measuring higher than 30Vrms
â— How to display Lissajous graphics?
In order to quickly display Lissajous graphics (or XY display), use the following method.
1 Connect probe A to signal A and input B to signal B.
2 Connect all ground probes to the ground.
3 Convert all inputs to AC coupling to subtract the effect of DC offset.
4 Select "A vs B" in the calculation menu.
5 Lissajous graphics will be displayed on the screen.
Note: Using mobile keys A and B can change the position or you can also use the cursor to position.
Optimize your Lissajous graphic display
For the waveform display of your optimized Lissajous graphic mode, it is best that you have at least one but no less than 3 cycles of signal in the record. With the analog range, the time base is not used, and the electron beam is directly controlled by the x and Y inputs A and B. In a digital oscilloscope, such as ScopeMeter, the time base setting can affect the Lissajous graphic display. In the digital range, the waveform is obtained first, and then the micro processor is used to create the Lissajous graphic image.
Changing the time base setting can improve the signal display.
The graph shows the display when the period is too small, only part of the signal is displayed !.
If the time base is set too slowly, too many signal periods will be obtained and not all signals will be evenly distributed!
For mechanical or low frequency applications, it is recommended to use a bandwidth limiter to reduce unwanted noise. Used under normal conditions with the afterglow function turned off.
How to use a two-channel Fluke-123 oscilloscope to record three-phase voltage?
According to Murphy's law, when you measure phases 1 and 2, the third phase of the three-phase system will be deposited. A simple method allows you to observe three phases simultaneously on the two-channel trend drawing screen.
Set up:
Connect channel 1 to phase A and the ground of channel 1 to phase C. Connect the channel 2 you connect to phase B, and its ground wire to phase C. (Note: As you know, Com A and Com B are internally connected in Fluke 123!). Now select the menu "VAC" (or "VAC + DC") and you will see two sine waves from the input. Make sure that both channels have the same input sensitivity. Now select the "Trend Plot" in channel A and you will see two graphs with almost the same voltage!
Interrupt:
If the voltage in phase A decreases, you will see this change in the channel (trend graph), (above), if the voltage in phase B decreases, you will see this in 2 Variety (trend graph), (bottom graph). But when phase C changes, you will see two pictures
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