Several 8Bit Scopes: Keysight DSO-X3xxx; DSO-X4xxx; Yokogawa DLM2xxx
10Bit Scope: Rohde & Schwarz RTA4xxx
attached are three pictures, one taken with Femto Transimpedance Amplifier set to 1kOhm; Spectrumanalyzer FPC1000; the other two with same Amplifier and Scope RTA4004 set to 50Ohm at input; 15mW Laser at 1% - 0,15mW output power; DC-voltage at 50Ohm Termination measured with DMM Keysight 34461A at 20.3mV that correspons to -20dBm.
Task is to get the FFT lines up to 10MHz to 60dB below the DC-Carrier, that is below -80dBm. At 200µs/Div scope gives very good result, but at 500µs/div its result is useless.
The lines come from a DC/DC-converter operating at 2MHz and its harmonics.
The R&S surely has best sensitivity of the scopes listed above, but the Problem whatever Scope: you change time base and / or FFT Parameters and lines in FFT appear and vanish.
Lines appear, that aren´t there, if we look at same signal with spectrum analyzer.
What signal levels we are looking at?
A typical optical power of a low to mid power laser (as far as we manufacture) is 10mW to 20mW.
A biased photodiode converts that with about 0.25..0.75 mA/mW into photocurrent
However we have to specify Signal to Noise Level at say 1% power, not just at maximum power.
So we get at 1%, 20mW and 0.5 mA/mW a DC current of 10µA
We then use transimpedance amplifier (TIA) with 1kOhm and get 10mV DC Voltage; or the cheaper option biased photodiode + shunt resistor 1kOhm with the same result
We can not use higher shunt or TIA value, because this will result in unsufficient bandwidth; same is, we nannot use smaller area photodiode which could provide higher bandwidth, but we work with free beam and have to align the beam onto the diode active area.
With the TIA we can go into 50Ohm Spectrumanalyzer, with the photodiode/shunt we need 1MOhm input on Scope or Spectrumanalyzer, also if we want to search inside the circuit we need 1M or additional special probe / preamp with 50Ohm output.
With my current project it is spec, that at 0.2% of 15mW max optical power the noise is <1% of the 0.2% within 10Hz..1MHz; additionaly at 1% optical FFT lines need to be less than 0.1% form 10Hz..10MHz.
We recently bought above Femto Photo diode amplifier with switchable gain, up to 100MHz Bandwidth and 50Ohm Output, combined with lowcost spectrum analyzer R&S FPC1000 we recently got as addon as we purchased the RTA4004, but that photo amplifier alone costs 2000€ and with the FPC we don´t get low enough to make sure there is no line interference or to measure RMS Noise levels 10Hz to 1MHz..10MHz depending on spec.
The RTA4000 is really good for the RMS Noise Measurement at 0.2% Power with Photodiode + 1K Shunt, DSO-X3000, DSOX-4000 and DLM2000 where of no use because the imternal noise level was to high. Even the low cost R&S RTB2000 does beat those 3 other scopes probably because its 10Bit ADC; but for the FFT the RTA dosn´t work reliable, because I see ghost lines.
Bevore I got the RTA4000 I had to do the RMS noise Measurement using 2 different R&S URE RMS Voltmeters, but they where no longer real reliable.
Maybe, if we run into need to also specify Noise Levels 0.1Hz to 10Hz we will consider a signal analyzer, but currently our costumers don´t require this kind of spec.