After receiving my new Rigol DS1102E scope I soon noticed the distracting noise produced by the fan. I found posts from others who had replaced the stock fan with a lower noise upgrade. Several Rigol owners noted that a larger 80mm fan would fit into the enclosure, although it couldn’t mount like the stock fan. I decided to replace my fan and did some research on specific models of low noise fans (a good source of info is silentpcreview.com). One brand that seemed to get consistently good reviews was the Nexus Real Silent 80mm Case Fan (model SP802512L-03).
I searched online for audio recordings of this fan that compared it to the stock fan, but couldn’t find anything helpful. After receiving the Nexus fan I decided to make my own recordings in case others might be interested. I used a good quality microphone placed about 4” from the fan (and perpendicular to the direction of airflow) and used a 12 volt power supply to drive the fans. I also did some experimentation with a series resistor to reduce the speed/noise of the Nexus fan – and settled on a 47-ohm ¼ watt resistor to reduce the voltage to about 10.5 volts. I uploaded recordings of the stock fan and of the Nexus running at both 12 volts and at 10.5 volts. These recordings weren’t intended to make absolute sound level measurements - only to provide relative comparisons of fan noise. Just for fun I used Amadeus Pro to generate the frequency spectrum of each recording. One thing I noticed with the stock fan was that its speed gradually increased as it was running in the enclosure. It wasn’t clear if some speed control was being used, but I found a very nicely done schematic of the Rigol scope at
http://rigol.codenaschen.de/index.php/Schematics and it appears that the fan is powered only from an unregulated 12-volt supply. Since the fan voltage didn't seem to be regulated, I thought that might the reason for a speed change, but I noticed a similar speed variation when I ran the fan from the regulated supply. You can hear some of this speed change in the recording.
I installed the Nexus fan with two-sided tape and foam insulation to hold it in place while avoiding direct contact between the fan housing and the scope enclosure. The fan is a tight fit with the case cover on, so I don’t really have any concerns about it moving. The overall noise of the Rigol is dramatically reduced with the Nexus fan, so I’d recommend this type of upgrade to others. One final thing I noticed was that the scope produced a small amount of noise due to vibration conducted through the hard plastic feet of the case. I added a couple pieces of foam rubber under the feet, which eliminated that problem. I think the Rigol is now about as quiet as it’s going to get with a modest investment in a new fan.
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