Hello,
I'd like to measure the current noise of a laser diode current source. The working point of the laser diode is about 100 mA at about 1 Volt.
I've learned that to measure the current noise one drives a resistive load (instead of a laser diode) and measures the resulting voltage across the load resistor with a scope. I can also set the oscilloscope to generate a FFT of the signal to see the frequency components of the noise. At least that's the theory.
So I connected a 10 Ohm resistor to the outputs of the current source as a shunt. Also I attached a coaxial cable (RG174) to these outputs. I soldered the braided shield to the output that is connected to GND and the center wire to the other output.
This "probe" looks like this:
Now when I connect this "probe" to the current source without connecting the current source to anything else (no power supply, no serial connection) the scope shows a strange signal in the FFT window:
The top part of the scope shot shows the signal in time domain (orange), the bottom part shows the signal in the frequency domain. There, the pink line is the FFT of the signal averaged over 16 measurements, the white line shows the noise floor when the probe is not connected to anything (except the scope).
I wonder what signal I'm receiving with my probe: The orange line shows that the noise is not always present in the signal. Noise drops drastically for about 1.2 ms. A separate measurement shows that the noise drop periodically (every 100 ms).
Regarding the lower part, there's a prominent noise that covers the frequency range from about 1.04 MHz to 2.60 Mhz.
What could that be? I switched of my PC and nearby devices but the noise is still there. However, the amplitude of the noise changes when when I move the PCB (current source).
Any ideas? Thanks.
PS: edited posting to add missing text and scope shot