Spectrum Distortion

[James Luck]

Hello All,

I previously asked Steven this, but between asking and now I've lost my message history.

I currently use a Gs-usb-pro,
My spectrums are coming out in PRA pushed far left.
Makes it very difficult to really see what im looking at.

Any advise, or if Steven sees this, can you help me again please.

Thank you,
James Luck

[jneilson]

Could be a lot of things. The most obvious thing with a spectrum pushed far left is needing more amplifier gain.

Depending on the detector used, it may also be that an increase in bias voltage might help?

I'm sure there's plenty of other things to try - but perhaps it would help to provide more detail on the setup and what you're already tried.

[Sesselmann]

James,

You have identified the complexity of any function that has more than two variables, in this case 4 main variables that affect the spectrum shift.

1. Voltage
Set the voltage to what you think is correct for your probe - most PMT's will be fine betwen 650V and 750V

2. Shape
The SHP trimpot on earlier versions of the GS-PRO-V5 affected gain (short pulse higher peak or long pulse lower peak)
(newer versions of the GS-PRO-V5 with black circuit board have pole zero cancelling, so the peak height is more or less constant)

3. Volume
Obviously the preamplifier volume will affect the pulse height, so increasing the volume will push peaks to the right.

4. Channel Pitch
The channel pitch sometimes called Bin size is a software setting, it is usually in arbitrary units and essentially sets width of each channel.

First of all and most important, we are using a sound card chip (PCM2900) to digitize the analogue AC signal from the detector. The analogue input on the chip has a dynamic range of 1.98 Volts meaning more or less +- 1Volt. As we are only using the positive half that essentially means we mneed to keep our tallest pulses below 1V (cosmic rays occasionally exceed this).

*Note:
If your GS-PRO does not have a black circuit board, turning the SHP and VOL potentiometers fully clockwise can kill the opamp.
In any case neither of these 15 turn trimpots should need to go further than half way, but it can happen inadvertantly.

If you are using ImpulseQt the optimum setting seems to be when the mean pulse is around 10%.

Steven