Noisy Channels

[Ground_Loop]

I just acquired a Tek 784D functioning perfectly except for noise that appears to be uniformly about 1.0mV p-p on all four channels.  I looked through the performance checks and specs and have not seen a figure for expected quiescent noise.  However, I also have a 584D (converted to 784D) and the channel noise is essentially zero.  Pardon the photo colors. The shutter system isn't very compatible with photography.

Of course schematics for this are non-existent; at least I haven't found them.  I'm suspecting power supply noise, but wanted to see if anyone here has tackled this before.

[David Hess]

Let's do the math:

1 millivolt peak-to-peak is roughly 170 microvolts RMS.  Noise bandwidth of 1 GHz is roughly 1.6 GHz.  So noise density is 170uV/Sqrt(1.6 GHz) yielding 4.3 nanovolts/SqrtHz which does not seem high to me.

When you made the comparison between the 784D and 584D, were they both configured for 50 ohm inputs or high impedance inputs?  They should have very different noise levels.

[Ground_Loop]

I did the comparison with all channels on both machines 50 ohms, then 1 MOhm, both of those with DC, AC, grnd coupling with results identical on all configurations.  In all cases the inputs were open. I've done factory reset and SPC a couple times and it did alleviate some minor offsets, but the noise remained.

You're right it doesn't seem high and I can live with it. I Just thought it was odd that this otherwise very well cared for scope has this anomaly. Of the dozens of used instruments I've purchased off EBay this is by far in the best physical condition and asking price was too good to pass up.

Also, the noise is not common. It's completely random across all four channels, so I'm thinking it may not be bleeding through from a common source.