Products > Test Equipment
Siglent SDG2042x high amplitude issue
<< < (3/4) > >>
tautech:

--- Quote from: W4PJB on May 08, 2024, 02:25:44 pm ---Hi All, I'm hoping someone can point me in the right direction troubleshooting a high amplitude issue on my new, unmodified SDG2042x.

I've set the generator to 1khz sine wave, 0.0 dBm. On my oscilloscope, I'm reading roughly 1.26Vpp, which translates to 6 dBm @ 50 ohms. That number is backed up by my Keithley 2000, which is reading right around 6 dBm @ 50 ohms reference.

If I go to the "HighLevel" menu, there is a value of 316.3 mV, and if I zero that, my meter goes to 0.0 dBm, but switching back to "Amplitude" shows the generator also entered a value of -6.021 dBm into the "Amplitude", with an "Offset" of -158.1 mV.

What am I missing here? Why is there always a 6.0 dBm offset being added to my output at all times? Am I missing a parameter somewhere?

--- End quote ---
Yup.

In 50 Ohm output mode any waveform generator expects a 50 Ohm balanced transmission line, 50 Ohm coax and a some sort of 50 Ohm feedthrough termination.

Ideally a 50 Ohm scope input or a 50 Ohm feedthrough however a BNC Tee with a 50 Ohm termination (load) can be used as a makeshift feedthrough termination.
W4PJB:
Thank you! I need a few adapters to properly splice in a dummy load, but I got creative last night and was able to cobble together a stack of banana adapters to put the 50 ohm load from my Nano VNA into the circuit, and, viola!

It's not a perfect 0 dBm but then again, nothing I am using is calibrated, so this is to be expected. It's way close enough for the hack work that I'm doing. Also, it proves the theory, and the more I think about it, it makes sense that the load would have to be actual, and not calculated. Learning every day!




--- Quote from: tautech on May 08, 2024, 08:07:21 pm ---
--- Quote from: W4PJB on May 08, 2024, 02:25:44 pm ---Hi All, I'm hoping someone can point me in the right direction troubleshooting a high amplitude issue on my new, unmodified SDG2042x.

I've set the generator to 1khz sine wave, 0.0 dBm. On my oscilloscope, I'm reading roughly 1.26Vpp, which translates to 6 dBm @ 50 ohms. That number is backed up by my Keithley 2000, which is reading right around 6 dBm @ 50 ohms reference.

If I go to the "HighLevel" menu, there is a value of 316.3 mV, and if I zero that, my meter goes to 0.0 dBm, but switching back to "Amplitude" shows the generator also entered a value of -6.021 dBm into the "Amplitude", with an "Offset" of -158.1 mV.

What am I missing here? Why is there always a 6.0 dBm offset being added to my output at all times? Am I missing a parameter somewhere?

--- End quote ---
Yup.

In 50 Ohm output mode any waveform generator expects a 50 Ohm balanced transmission line, 50 Ohm coax and a some sort of 50 Ohm feedthrough termination.

Ideally a 50 Ohm scope input or a 50 Ohm feedthrough however a BNC Tee with a 50 Ohm termination (load) can be used as a makeshift feedthrough termination.

--- End quote ---
ozkarah:

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:



Berni:
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.
2N3055:

--- 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.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod