Gamma Spectrometry Forum

Until a few weeks ago, I used to connect my GS-1102 Pro directly to the USB port of my mac book pro. The audio recordings always contained a periodic interference signal:
Even though the amplitude of that interference is quite low (the red line in the image above is at 0.25% of the maximum possible amplitude), a high pitched sawing is audible in room volume playback.

I also own a small, primary switched power supply with an USB socket. I believed that using a second power supply would give even more noise or create a humming loop. Nonetheless, I gave it a try. To my surprise, the baseline now looks like this:
Valentin

Valentin,

I am surprised to see that the macbook produces more noise than the adaptor, but yes, it's a good idea to try a few different power sources. One good adaptor doesn't mean that all adaptors are good. Some of them are really bad, and it's not always the cheap ones.

The noise is most likely coming via the preamp, and not so much from the stabilised HV.

If the noise is less than 0.3 arbitrary units in PRA ( 0.3%) you are not doing too bad.

Of course, that noise level depends on the input gain setting. I usually adjust the input gain such that the K40 Peak shows up between 35%-40% (or arbitrary units to use PRA terminology).

However, my recorded spectra always have a thin, nongaussian peak at the very low end. I still need to investigate if it is artefacts from the pusle detection algorithm, or random discharges inside the PMT (e.g., thermal photons or ionizing radiation hitting some dynode), or noise of some other origin...

Valentin

Valentin,

Thats correct, if you increase the gain you also increase the electronic noise, this is why the normal setting for the preamp gain is very low, in the order of 10%.

The best way to set your preamp gain is to set the voltage to the optimum for your PMT, then set your sound card input volume to 80-90% and adjust the preamp gain so K40 is around 50 arbitrary Units in PRA.

If everything is working as it should, then the gain should be quite low for NaI(Tl) detectors.

Steven

Steven,

I did not change the preamp inside the GS-Pro so far, just adjusted voltage and input level of the sound card. That is because it already took some time to determine a good input voltage for the PMT and I was to lazy to run more experiments with yet another parameter.

Many audio devices (even the built-in microphone in the mac book) do not have real, hardware volume control. The samples from the AD converter are just multiplied by the driver in software. For such devices, it makes sense to run with a high input volume (as long as the samples are not truncated due to numerical overflow) in order to benefit from full converter resolution.

However, the UR 22 has a real potentiometer to play around with, and one always gets back unscaled audio data from the AD. When fixing the voltage as well as the desired K40 peak location, there is a real degree of freedom for balancing the GS-Pro preamp against the sound card's internal amplifier. When I have time, I should try to find the optimal gain which gives the least noise.

Valentin

Thanks for explaining the sound card volume thing, that makes sense, so in most cases setting the PC line in volume to 100% and turning down the GS preamp should achieve the optimum result.

If you want to reduce USB power supply switching noise you can use a D.C. power source (battery). I use a Tekkeon variable output rechargeable D.C. power source. When charged up the Tekkeon will run the GS-Pro for 3 to 4 days and has almost no noise at 5 volts D.C. output. I use the Tekkeon and a Steinberg UR-22 and I am very happy with the results.

Bob
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