Generating a signal through simulation tools

Hello,
I want to generate a signal similar to the one shown in the attachments acquired using labview which is the output of a phototransistor.The output image contains 10 signals each contaning each acquired by using 1k samples and below freq images shows freq components only 1 signal of 1k sample.
The images2,3 represent the freq component present in a single sample.From the images it can be analysed that it has DC volatge of 2.6V but I am confused as it contains different frequenices and most(very high number) of the frequencies are lying below around 1Hz and some are lying at around 135 and others at 275 to 300 and very less number of other frequency components.
What all things to be taken to generate these kind of freq's in multisim/LTspice and How to add these different kind of frequencies to the DC component so as to generate the signal that will be same as in the shown image.

I want to genearte this signal in multisim/LTspice as this will used as a input for designing the filtering stage in multisim/Ltspice.

Thanks.
Attachments:View attachment Generate_signal.pdf

Replaying a sampled signal should work in most simulators. Either as SPICE standard PWL source or e.g. a *.wav file which can be read in by LTSpice.

There may be reasons to use an artifical signal, e.g. to modify signal parameters at will. But I fear your signal specification is yet too vague to generate a respective signal model. I presume you know that a magnitude spectrum doesn't fully determine a signal because it lacks the phase information.

Hello FvM,
The signal generated from the phototransistor is of very small amplitude and is comprised of many other noise frequencies as shown in the image.
I am using the NI multisim as my simulation tool and my intention is to design a filter stage by feeding this input.I really don't have any idea of how this can be generated like what all the amplitudes corrsponding to different freq components should be considered.
Can you help me out in solving this.
Thanks.