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How much noise floor and other things matter in oscilloscope usability
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Fungus:

--- Quote from: G0HZU on December 25, 2021, 05:43:59 pm ---Here's my old HP Infinium scope set to 1mV/div with the 30MHz bandwidth limit enabled.

The Rigol scope was showing about 1mVpkpk  on a 20MHz bandwidth setting which seems really noisy in comparison to my HP scope from the 1990s. 

--- End quote ---

What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.

Does your HP have waveform averaging mode?

At least it looks like your HP can zoom out.

David Hess:

--- Quote from: G0HZU on December 25, 2021, 11:24:56 am ---I have a really old HP Infinium digital scope with 500MHz bandwidth and it has the noise performance I would expect. At full 500MHz bandwidth and with the input set to 50R termination and 1mV/div it shows about 100uV rms noise when the Vrms measurement is enabled. When fed with noise as the signal under test it can typically measure wideband or narrowband noise signals with acceptable results down to about 200uVrms. This scope is fairly limited in terms of features (and memory depth) compared to modern scopes but it does at least have the noise performance I'd expect from a scope like this.
--- End quote ---

When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.
G0HZU:

--- Quote ---What's the bandwidth/sample rate of that? The Rigol has 350MHz pathways and is sampling at 8GHz which inherently produces noise, no way around it.
Does your HP have waveform averaging mode?
--- End quote ---

I'm not quite sure what info you are asking for but this is a really old HP scope using very dated technology. The basic specs are 2GSa/s and 500MHz bandwidth across 4 channels. Yes it has an averaging mode but it isn't turned on. The 30MHz bandwidth limit is turned on for the screenshot in my previous post.
G0HZU:

--- Quote ---When a low impedance input is used, higher bandwidth oscilloscopes bypass the high impedance buffer and without that, the input noise can be much lower.  The high impedance buffer has to be bypassed because at some point it will not have enough bandwidth.

An example of this in old oscilloscopes is the venerable Tektronix 485 which was built at a time when the fastest high impedance buffers were 250 to 300 MHz.  In 50 ohm mode, instead of inserting a 50 ohm feedthrough termination before the high impedance buffer, a coaxial relay directs the signal around the high impedance buffer.

Later oscilloscopes managed high input impedance up to 500 MHz and I think some now manage 1 GHz.
--- End quote ---

If it helps, I can switch the scope to 1Meg input and attach a 50R load and it looks pretty much the same. There might be a tiny bit more noise but any change is barely perceptible.
oz2cpu:
some scopes are just badly designed, full of switchmode own noise..
the good old Rigol 1054 that we almost all owned ..
here i posted a few pictures, look and cry

https://webx.dk/rigol/

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