If you have a single frequency spectral point at say 2kHz and it is -55 dBu then it has a voltage level of 0.001377449 Vrms (see this for the conversion). If you also had another spectral point at 2.1kHz and it was -60 dBu (0.000774597 Vrms), then you'd calculate the RMS value of these two points by this: -
RMS Value of A and B together = \$\sqrt{A^2 + B^2}\$
You'd get a value of 0.001580305. That's two points.
Now extend this method to all the points within the spectral band of interest. If it means summing 10,000 points then taking the sq root of this sum, then that's what you have to do.
The modern digital oscilloscopes are sophisticated analog beasts!
Most of the modern day high speed digital and analog equipment, such as computer interfaces (USB, SATA, Gigabit Ethernet) are tested, designed and refined using digital oscilloscopes. Even many SoCs containing complex analog and digital peripherals are validated using digital oscilloscopes. For example USB 3.0 can have speeds up to 5Gig bits per second. The interfaces are literally probed by digital oscilloscope inputs and careful test setups are built around them.
Even high speed analog blocks such as ADCs, Amplifiers, Filters and Oscillators are tested using DSOs.
However from a purchase point of view, these are very expensive oscilloscopes. For the highest analog bandwidth available, the boxes from companies such as LeCroy (part of Teledyne now), Keysight (Changed from Agilent's T&M division), Rhode & Schwartz and Tektronix, may cost a Ferrari!
But for most of hobby use, student laboratory or even a decent embedded testing there are value-for-money oscilloscopes from above companies and many other from around the world. There are also PC based USB oscilloscope products (BitScope, Picoscope or USBee).
Digital oscilloscopes exists because they work! And engineers use them! I use them!
Most of the time, we expect more from a box and potentially use an unsuitable signal for analysis. A high speed square pulse stream on a lesser bandwidth oscilloscope will look smoothed out! Or even as a sine-wave! Because all the higher frequency part of the signal is filtered out on channel.
These are few questions you may want to ask yourself before choosing an equipment.
Ideally every signal is of infinite bandwidth. Only that the higher
harmonics are very feeble. So choose the "Analog bandwidth" of the
scope based on your signal.
Try to use the full dynamic range of the scope (Full bit resolution
vs. full scale). If your interest is about superimposed parts of a
signal, like that sharp glitch on a sinewave output of a switched
power supply, go for higher ADC resolution scopes.
If the signal is small, the scope will amplify it. If the signal is
large the scope will attenuate it to suite the full swing of the
internal ADC. Some times you may want to use the auto-scale feature
of the scope.
If the signal is too small amplitude, then amplifying it will also amplify some noise. If the signal has large glitch, then attenuating it will reduce its details.
We should also look into the merits of Digital Vs. Analog scopes
- Most DSOs have sophisticated Analog Front Ends (AFE). Which is again software controlled and offers extra leverage based on signal. Signal conditioning, amplifying and even isolating are handled in digitally controlled AFE.
- Next to AFE is the heart of a digital scope, which is a high-speed
ADC. This technology has improved leaps and bounds in the last decade.
- There is a ping-pong or daisy-chained RAM buffering of ADC samples before they are pushed to a dedicated computer. If you know DSP, you will know the 'value' of digital samples!
- The raster / rendering of digital signals on a decent UI actually gives ability to have multiple cursors both horizontal & vertical, easy scale adjustments, visualization, attached measurements and mutiple channels in one go!
- I think multiple channel, channel math&logic, advanced triggering capabilities are the most useful features of a DSO.
However if you admire pure analog signals, directly imposing themselves on a phosphor screen, nothing wrong with that too!
Best Answer
Looking at the zoomed versions, and assuming the X axis is time in seconds, you can see a 10ms periodicity in the strongest component, and many of the other components show the same periodicity.
That points at full wave rectified 50Hz mains as being the underlying cause. It provides a rich source of switching spikes and current transitions at 100Hz.
If I read the first zoomed one correctly, there is something oscillating at 27MHz, frequency modulated by 100Hz spikes. If you can identify the source of that frequency, you may be a lot closer to your answer.
Where you go from here is not so clear :
there could be EM coupling from high power supplies into sensitive circuitry : screening would reduce the electric field coupling, but magnetic screening (iron or mu-metal) may be required if the coupling is magnetic. (If your sensing circuit is a high impedance node, suspect electric coupling; if low impedance, suspect magnetic). Walk round the room turning everything else off while someone watches the scope and shouts when you make a difference. This includes heating, air conditioning, computers, and especially lighting since you mention fibre optics.
there could be noise on your power supplies. If you have sensitive amplifiers powered from a switching PSU, run them from a linear regulator instead. Improve decoupling. Measure the noise on your power and ground lines. Isolate any high current circuitry from this system. Adopt star earthing and eliminate any ground loops.
There could be noise on the scope's own power supply. Amplify the signal (without introducing any of the problems above) to make the scope's own noise less significant. Or buy a better scope...