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Noise problem on Agilent oscilloscopes

Question asked by Coris on Nov 4, 2011
Latest reply on Aug 26, 2012 by Coris
I own an DSOX2012A. I had as many others Agilent customers the same problems with big noise over the signal, or without input signal. The noise level in my oscilloscope was not exactly so big as described in another thread (about another type scope), but was big enough (3-4mV and also a clear frequency/pulses) to get me frustrated after I`ve paid the price for this device.
I started to take a close look and examine this problem. I found out the follow:
A big noise source in this oscilloscope is its main switching power supply. Is quite surprisingly that Agilent chosen a such PSU type for theirs products.
Another main noise source is the display and its internal power supply.
The important noise sources are the small power units too, spread on the entire main PCB and on the power PCB. All those small ferrite trafos should be shielded with appropriate magnetic shield material. Agilent used only aluminium plates in between the internal devices/PCBs in their oscilloscopes...
The input signal process/amplifier stage is very poor/inefficient shielded. The fan ( in DSOX2012) which run very near those input circuits is a quite important noise source for it.
My taken solutions: removed the main Agilent power supply, and replaced it with a serial regulator PSU (an transformer is now in its place and the oscilloscope GND is completely isolated from the main). Shielded the most of the small power supplies on the main PCB and power PCB. Efficient shield placed over the input circuits. Place a magnetic shield on the back side of the display. Ferrite beads on the all wires between the power PCB and the main PCB.
The result: 1mV noise (pure noise and no any frequencies or pulses) on the horizontal range down to 200ns. 800µV noise on 200 - 50ns range. 300 - 500µV noise in the last range of 50 - 5ns. This with a probe 1:1 on the input, shorted with spring GND. No any other interferences/frequencies from outside get in now. I do not like at all this resulted noise level, but is better than before, in my case...
I should expect that Agilent should think more professional in designing their products, specially about those quite simple solutions to minimise the noise. Should definitively not be the customer/the buyer who have to improve an Agilent product after one has already paid quite expensive price for it.