Yes.The first measurement is done on a high impedance source.
Does the displayed signal level on the screen of the scope displayed in the correct level?
Hi,
* "50 Ohm input setup" is for direct measurement of 50 Ohms signals, connected with 50 Ohm impedance cables...and when you want the signal to be terminated with 50 Ohms inside the scope.
* "FFT": no need to change setup when doing mathematical functions.
Yes.
BUT... it shows the signal level at the probe input. It can´t show the unloaded source signal level.
Example: signal source: 5V with internal impedance of 4M. Probe 1M.
--> 4V will be dropped within the signal source. 1V will be at the probe input --> the scope will show this 1V.
The oscilloscope should always display the correct input level, no matter if 1 MOhm or 50 ohm input termination is selected. Scaling for probe attenuation may be applied automatically or need manual setting.
What however varies with oscilloscope input impedance (= load impedance) is the output voltage the 50 ohm source.
Hi,
The scope will allways show the signal at the probe input. (provided correct gain setup).
Now it depends on your load .. how the ouput value will vary with 10M load, 1M load or 50 Ohms load.
Klaus
Nobody can tell how your oscillator behaves.So my considerations on the example above are correct?
Hi,
Nobody can tell how your oscillator behaves.
It´s not unusual that the oscillator stops completely when (over) loaded with 50 Ohms.
Klaus
Some probes 'tell' the scope whether they are set to x1 or x10 mode, they have extra pins as well as the BNC plug to tell it to change the scale on the screen. If the probe or scope do not support automatic scaling, you have to take it into account yourself. Using x10 will make the trace 10 times smaller.
Don't forget to set the compensation on a x10 probe or your readings could be wildly out.
Brian.
I'm not familiar with that scope but I would guess the option is the one to use. It almost certainly does nothing except change the graticule scale, not the waveform height but the number of volts per division the markings represent. Using a x10 probe actually reduces the signal by ten times but the scope can compensate by magnifying the scale ten times. The advantage of a x10 probe is it also isolates the scope's cable and input impedance from the circuit you are investigating, thereby minimizing it's influence on that circuit. The drawback is that for very tiny signals (probably in the uV region) the result is too small compared to system noise to be usefully measured.
Compensation is absolutely essential. Inside the x10 probe there will be a low inductance resistor in series with the tip to drop the signal level but in doing so it makes a low pass filter from the cable and scope input capacitance to ground. It follows that the probe will have a frequency response that rapidly attenuates higher frequencies - not very useful! The fix is to add a small variable capacitor across the resistor such that it compensates for the HF roll-off. If you look at any x10 probe you will see it has an adjustment slot on it, usually the shaft of a tiny pre-set variable capacitor. It needs adjusting to match the probe to the scope while keeping a flat frequency response.
Thankfully, adjusting the compensation is very easy and I think there is a built-in tool in the scope to help you. From photographs I can find, in the bottom right corner of the front panel it looks like there is a ground point and a calibrator output. What you do is this:
1. connect the probe in x10 mode (there is nothing to do in x1 mode if the probe has a switch)
2. connect the probe ground clip to the calibrator ground post
3. hook the probe tip on the calibrator output
4. Adjust the Y gain and sweep speed until you see a few cycles of waveform.
5. CAREFULLY, with a non-metalic screwdriver, adjust the compensation until you get a square wave.
If the waveform has rounded edges, it is under-compensated, if you see pointed overshoot on the edges, it is over compensated. You should try to get the top and bottom flat with square corners. Once you have done it, there is no need to adjust it again for that oscilloscope.
In step 5, I can't stress enough how careful you have to be, the capacitors are usually very fragile so make sure the screwdriver fits the slot well and never force it. If you break the capacitor the whole probe is trashed!
Brian.
So if I read 10vpp ona circuit at 1Meg and then I read 10vpp on another circuit when loaded at 50R, what does it mean, that the two circuits have different powers or the same? This is tricky.
FFT is just a mathematical way of converting time domain signals (normal X-Y scope display) into frequency domain (frequency spectrum) display by sampling the waveform and analyzing it's periodic contents. It doesn't change the input scaling at all.
Brian.
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