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Oscilloscope input noise comparison

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cruff:

--- Quote from: maxwell3e10 on October 13, 2018, 08:02:53 am ---The input is terminated into 50 Ohm.

--- End quote ---

Do you mean the input connector was shorted and you used the built-in 50 ohm termination mode of the scope?  Or did you use the scope's high value termination mode and terminated with 50 ohms at the input connecotr?

So effectively you are measuring the thermal noise of the termination resistor(s)?  Or am I missing something?

maxwell3e10:
Some scopes have a build-in 50 Ohm mode and some don't. I meant to add a 50 Ohm resistor outside to short the input. One could also use a shortening BNC. It wouldn't make much difference unless the noise is 2-3 nV/Hz^(1/2).

nctnico:
Usually the 50 Ohm and 1M Ohm paths are different so the noise is also different.

maxwell3e10:
Yes, that appears to be the case. I had always assumed in the past that a 50 Ohm input impedance setting on a scope simply switches in a 50 Ohm resistor at the input of the 1 MOhm amplifier.

rf-loop:

--- Quote from: maxwell3e10 on October 18, 2018, 03:23:08 am ---Yes, that appears to be the case. I had always assumed in the past that a 50 Ohm input impedance setting on a scope simply switches in a 50 Ohm resistor at the input of the 1 MOhm amplifier.

--- End quote ---

It depends oscilloscope. In low price and low frequency oscilloscopes this is "typical" cheap  way to do it. They need only 50 ohm "DC" load resistance. Nothing good in this method for high frequencies - except price. Also noise in these kind of front ends are not at all excellent. Also very small internal noise currents can produce high noise voltages over these 1M pathway after input 50ohm load resistor.

I have seen these many times, connecting external 50ohm feed thru or terminator to 1M scope input it can some times  show even more noise than open or LF-RF shielded input BNC using metal caps.

Time ago one perhaps most worst case  was really amazing effect in some Owon models where lot of internal SMPS circuits was made like wide band radio jammer. "Every kid can design SMPS" so they put cheap unexperienced engineer kids to design these..or for make some modifications to reduce manufacturing costs.  These noise currents flood wild inside scope every part and flow thru different parts and if connect external 50ohm displayed noise level rise a lot of. Because 50 ohm resistor make possible to internal transmitted noise have now pathway to input BNC GND and this generated  noise current produce voltage over 1M.  But if input was open (example if use BNC metal cap without center connection) , less noise. But this was special case.

If scope have this cheap 50 ohm resistor method or if it have real 50ohm input impedance they may be quite different if think analog front end noise.

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