Strange noise/signal on scope

[DanielDaniel]

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

[radiolistener]

It's unclear what is your installation schematic, could you please provide schematic?

But in general, your "probe" looks like antenna, so if you have some noise source nearby it can be easily received on such "probe"...

[tggzzz]

The answer is in your other identical thread.

[DanielDaniel]

You are right. I can remove the PCB from my 10 Ohm shunt probe and instead attach a short piece of wire to the signal line of my probe. With this "antenna" I see the same noise on my scope as with the PCB connected.

However, If I remove the 10 Ohm resistor, the FFT signal looks completely different and the prominent 1 - 2 MHz signal is not observable any more.

So far I could not yet detect the source of this radiation.

[radiolistener]

However, If I remove the 10 Ohm resistor, the FFT signal looks completely different and the prominent 1 - 2 MHz signal is not observable any more.

There are two possible reasons for that (in practice you will have both of them in combination):
1) adding resistor affects noise power transferred from the source
2) adding resistor affects noise figure of RF frontend input (it can decrease or increase noise figure depends on exact type of LNA)

So, adding resistor affects both - your signal source and your oscilloscope input noise performance.

There is also third reason for the noise level change (which I think is dominant in your case) - adding resistor turns your installation into magnetic loop antenna which is very sensitive for near field magnetic interference. It's the same as adding short circuit, but adding resistor decrease Q factor of such magnetic loop antenna in comparison with short circuit and it makes it more wide bandwidth.

So far I could not yet detect the source of this radiation.

it can be display of your oscilloscope or any other digital circuit near your installation. You can try to find it by moving your "probe" around room and find location where noise level is increased.

[DanielDaniel]

Hey Radiolistener,

Thank you for your comprehensive response.

To make the measurement reproducible, I created a more professional "antenna": I'm using a 5 m BNC coax cable so I can mount it somewhere in the room. I attached a tee BNC adapter to its "far" end. There, I added a 50 Ohm terminator plug and another plug with a 8 cm copper wire soldered to the center pin, see photo:



Then I observed the signal on my scope (which is a Rohde & Schwarz RTB2004).

A) With the FFT set to a range of 0 - 12 Mhz, I see the prominent "noise hat" in the 1 - 2 MHz region (which is also visible in the earlier scope shot above):



B) With the frequency span slightly changed to a range of 0 - 13 MHz, the scope displays a completely different spectrum:



So I think what I see here is a glitch based on the oscilloscope's firmware or hardware limitations. 
My trust in the FFT function of this scope is now quite limited.

[radiolistener]

It may be self emission of oscilloscope. When you change frequency span, it leads to different frequencies in hardware and as result different spectrum picture.

It's better to use loop wire instead of a long wire to search noise source.

[DanielDaniel]

I don't agree. Why should the displayed spectrum suddenly change completely if you change the frequency range only slightly?  
Further, I dug out an old (rather historic) spectrum analyzer and could not detect said strange signal component with it.
For me that shows that the FFT implementation is     
At any rate, it think that the FFT results of the RTB2004 should be treated with caution.

[gf]

Your sample rate is relatively low. What you see can be higher frequencies which are folded into the first Nyquist zone by the sampling process. Since the sample rate is different in both screenshots, the folding of aliases is expected to be different as well.