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Most accurate signal generator
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loop123:

--- Quote from: nctnico on March 30, 2024, 12:18:31 pm ---Aha, just do as I wrote before: get an RF step attenuator (like this one from Aliexpress: https://nl.aliexpress.com/item/32905603744.html ) and connect it between the generator output and the ADC. You'll need to terminate the step attenuator with 50 Ohm so you'll need a 50 Ohm feedthrough terminator.

--- End quote ---

I figured out the sine waves noises don't merge into one another because they don't affect the past. And at 1kHz amplifier setting. The noises of 50Hz vs 900Hz is identical because making it pulse faster (higher frequency below 1kHz) doesn't produce more noise. And I want to test this.

I'm in the process of buying a RF attenuator. Which one should I choose in the following? Supposed my signal is 1V and I want to convert it to 10uV. What would happen if I choose the 30dB model vs 90dB model? Please show what would happen to the 1V original amplitude. And in the picture of the unit at left of screenshot. What button or number should I press? What does SMA vs N mean?  And the frequency is so high at 3Ghz. My frequency only audio range at most. And lastly. Is the 50 Ohm feedthrough terminator you mentioned above just an originary 50 Ohm resistor? Thanks!

nctnico:
I'd go for the 90dB model. You can fine adjust the level on the function generator.

For a feedthrough terminator you can use something like this:
https://nl.aliexpress.com/item/32661228102.html

There are some threads on this forum where you might find better (all metal enclosure) 50 Ohm BNC feedthroughs (or SMA version). Another option is to use an SMA T-piece and SMA 50 Ohm terminator. At 1kHz it doesn't matter, good shielding is what you need.
Aldo22:
Why does he need such an expensive attenuator?
Wouldn't one of these be enough (e.g. 40dB)?

https://www.aliexpress.com/item/1005006197857889.html
nctnico:
That is an option but the step attenuator allows to make many different levels quickly which is handy while experimenting.
gf:

--- Quote from: loop123 on March 31, 2024, 10:53:58 pm ---I figured out the sine waves noises don't merge into one another because they don't affect the past. And at 1kHz amplifier setting. The noises of 50Hz vs 900Hz is identical because making it pulse faster (higher frequency below 1kHz) doesn't produce more noise. And I want to test this.

--- End quote ---

Sorry, I can't follow your thoughts. What do you mean with "noises of 50HZ vs. 900Hz"? You have noise, and you have your wanted signal. Just consider them as two independent signals. At the end, noise and wanted signal simply add up. The noise does not change if your wanted signal changes. You don't get a "different noise" if your wanted signal is 50Hz or 900Hz.

Nevertheless keep in mind that your noise is not white, but your amplifier also suffers from 1/f noise at low frequencies, and that you deal with bandwidth-limited noise (~1kHz). Both imply that your noise is not independent and identically distributed, but it is autocorrelated. So the deformation of the waveform due to noise (if you zoom-in) will definitively look different for a 50Hz sine wave signal and for a 900Hz sine wave signal. I have attached example plots for 1kHz-bandlimited (2nd order Butterworth) white noise and 12dB SNR. Note that for the 900Hz signal, the noise mostly affects the envelope. Keep in mind that these plots are still not represenative for the noise of your amplifier.

To get a more representative picture, can't you record the noise floor of your USBamp (e.g. with Audacity, as you already did it with your other amplifier) and then use Audacity to add an artificial sine wave signal to the recorded noise in order to see what you would get?


--- Quote ---Supposed my signal is 1V and I want to convert it to 10uV.

--- End quote ---

That's a factor of 100,000 or in other words 20*log10(100000) = 100dB.


--- Quote ---What button or number should I press?

--- End quote ---

The enabled stages simply add up. Example: If you turn on 20+16+2, then you get 38 dB.


--- Quote from: nctnico on April 01, 2024, 11:33:24 am ---That is an option but the step attenuator allows to make many different levels quickly which is handy while experimenting.

--- End quote ---

The OP was very much focused on accuracy. The chosen step attenuator is likely not a precision device. I wonder what total accumulated uncertainty can be achieved for the involved components (generator's no-load voltage, generator's output impedance, 8 attenuator stages, and the terminator impedance).  I have doubts that we are still within 1% at the end.
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