3.5 KiB
Low-cost USB Soundcards (CM108)
For high precision and better reproducibility of alpha particle measurements, a dedicated small USB soundcard (3 - 10 EUR, depending on the distributor) is recommended:
The ICY-BOX IB-AC527 on the left and the König Electronic CMP-SOUNDUSB12 on the right ("3D Sound Controller 5.1") use both a chip series from C-Media Electronics Inc: CM108 datasheet.
It's microphone input well specified with a flat frequency response in the required 1-5 kHz range and good signal to noise ratio. As standard USB Audio Class devices, no drivers are required and they are recognized as a regular audio input in many operating systems, including Android & iOS.
The effective input impedance of the CM108's microphone input is about 210 k Ohm according to this report. Please note that none of the modifications described in that report are necessary in order to operate the alpha-spectrometer with the shown energy resolution.
I have personally not tried any modifications to the stock CM108 USB soundcards. It would certainly impact the alpha energy calibration and require a new one.
Since the original input impedance of those soundcards is rather low compared to a standard oscilloscope input with 1 Mega Ohm input impedance (directly at the BNC socket), the measured signal pulses are quite smaller when recorded with soundcards.
Recording settings
All reference measurements have been taken with one of the blue C-Media CM108-based König Electronic USB soundcards using the following settings:
- 48 kHz sampling rate
- 16-bit sample resolution
- 100% microphone input volume level
- gain boost enabled
Operating system specific tips
The CM1018 USB devices usually appear in the system as "USB PnP Sound Device".
The CM108 chip has a gain boost setting of 22.5 dB which must be enabled when reusing the energy calibration.
On Mac OS X, there is no option for enabling the gain boost, it seems on by default. In "Audio MIDI Setup", the input device may show up as 2-channel although it is actually mono only. Adjust the resolution, sampling rate, and input level there (level of 1.0 = 100%). Then make sure it is selected as the current audio input device in "System Preferences / Sound".
On Windows, a 3-letter option called "AGC", "CGA" or "CAG" ("Adaptive/Auto Gain Control") is present in the advanced sound device settings. This is rather wrongly labelled as there is no adaptive gain feature in the chip. It represents the gain boost feature and should be therefore turned on. Also, set the input level to 100%, try to make sure that no additional/hidden software amplification is enabled.
On Linux, the graphical command-line tool alsamixer can be used to adjust the microphone input settings of all attached soundcards. Select the external USB device with the hotkey "F6" followed by "F5" to show all controls. Turn up the levels on all channels called "Mic" to 100 (there are two-channel controls labelled as "Mic" in my case). Enable the "Auto Gain Control" setting (representing a static gain boost) by hitting the "m" key for that channel (un-muted = On, muted = Off). This control setting should then show "00" instead of "MM".