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Siglent SDG2042x high amplitude issue

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W4PJB:

--- Quote from: Berni on May 13, 2024, 09:37:14 am ---The whole reason why specifying impedance is important is because dBm stands for "dB miliwatt"

Both the AWG and DMM operate using voltage and so they must do math conversions to translate between volts and watts. So in order to do that the impedance must be known. The other thing is that AWGs always have 50 Ohm output impedance even if you you are not turning on 50 Ohm mode. So when presented with 10Meg impedance they assume the voltage drop over its own 50 Ohm impedance is zero and so show a 2x larger voltage. If you want a signal source that looks like 0 Ohm impedance you typically need to add a high power buffer amplifier on the output.

In general dBm only makes sense when used in the context of RF circuits where everything has matched impedance (so it is known and constant). This is why all RF gear (synthesizers,spectrum analyzers, network analyzers..etc) works by default in dBm. When you try using dBm for anything else it just makes life harder and introduces extra math for conversions, hence why most DMMs don't even have the ability to display dBm.


If you really want to use dB while operating with voltage you actually want to be using dBµ that stands for "dB microvolt" a voltmeter can measure that without needing to worry about impedance.

--- End quote ---


--- Quote from: 2N3055 on May 13, 2024, 11:05:01 am ---
--- Quote from: ozkarah on May 13, 2024, 09:10:01 am ---
50 Ohm load is not mandatory.
You can set the function generator output to Hi-Z or set the reference impedance to 50 Ohms on your multimeter:

(Attachment Link)

--- End quote ---

If you set AWG to HI-Z and load is 10MΩ multimeter than voltage on meter and AWG will mostly agree. Multimeter will have large input capacitance, and as your frequency goes up there might be errors, provided your meter can measure high enough.

But if you have multimeter with dBm function, if set your multimeter to dBm and 50Ω reference, and connect meter to AWG without 50 Ω terminator,  you will have wrong measurement compared to what was set on AWG. Meter can measure dBm/50Ω but has no terminator inside. You still need external 50Ω terminator.

--- End quote ---


Thank you both! My assumption was since the meter should know it's own impedance, if tell the meter it's 50 ohms, it should be able to calculate the difference between it's own impedance, and the desired load impedance. But on further thought, the generator, also not being natively 50 ohms, is doing it's own math, and also expecting to see a 50 ohm load to get where it needs to go. So neither instrument really know what's in the circuit. Now the need for a dummy load makes perfect sense.

Just needed to get it in the right context. Thanks!

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