Since the relatively loud fan noises also bothered me a lot, including slight clicking noises, I initially decided to just replace the fan with a Noctua NF-A9
FLX. However, I was still not completely satisfied with the result of the FLX. So I decided to waive my warranty and converted my device with the Noctua NF-A9 PWM similarly as described here
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg5119110/?topicseen#msg5119110. After that I was satisfied.
Here are my settings for the controller:
Voltage of the base speed (~910rpm): ~5.2V (fan stops running at approx. 4V)
Controller acceleration temperature: 30°C
Temperature for maximum speed (2000rpm): 30°C + 15°C = 45°C
After switching the scope on at a room temperature of 22.5°C, the fan runs for approx. 8 minutes at the lowest speed level (912rpm @5.2V) (channels 1 and 3 on + FFT for channel 1 on).
The controller then increases the speed by 1 step (1150rpm @6.4V) and remains there with the above-mentioned measuring mode of the oscilloscope.
After 1 hour in the above-mentioned measuring mode, I was able to measure the following temperatures:
Room temperature: 22.5°C
Exhaust air temperature directly behind the fan: 32°C
CPU temperature on the base of the heat sink: 51°C
ADC temperature on the heat sink of an ADC: ~41°C
Using a calibrated measuring microphone (Beyerdynamic MM1, 50 cm away from the screen and 30 cm above the tabletop) I was able to measure the following A-weighted equivalent continuous sound levels (LAeq) in a very quiet living room:
Oscilloscope off: LAeq=25dB (probably the noise floor voltage of the microphone)
Oscilloscope switched on (Fan running with1150rpm @6.5V): LAeq=27.5dB
After my first conversion step, in which I only replaced the existing fan with an Noctua
NF-A9 FLX (without controller module, @12V operating voltage @1600rpm), I measured the following sound pressure levels:
Oscilloscope switched off: LAeq=25.5dB
Oscilloscope switched on: LAeq=36dB
After a second conversion step (Manufacturer warranty still valid), in which I used the
Noctua NF-A9 FLX with the noise reduction cable @1250 rpm I mesured:
scope off: LAeq=25dB
scope on: LAeq=31dB
With the Noctua
FLX it would have been possible to use another noise reduction cable (1050 rpm). But that was too uncertain for me with regard to higher room temperatures, and so I decided to convert my device with NF-A9 PWM as described in the link mentioned above. The FLX fans are not so well suited for DC controller operation because they stop turning at ~5V.
UPDATE:I replaced the 3-wire DC controller with a 4-wire PWM controller (see photo below).
The PWM controller regulates the fan speed continuously and allows the temperature thresholds to be set in steps of 1°C. The duty cycle for speed control can be changed in steps of 1%.
Settings for the PWM controller:
PWM duty cycle for base speed: 42% (~910rpm)
PWM accel. temperature: 31°c
PWM max. speed temp. : 48°C (2000rpm)
Turn off temp.: 0°C (no switch off function)