Photonic Sampler in Analog To Digital Converter

[Jamal]

I am implementing a photonic sampler in optisystem13 for analog to digital converter.
I am using optical gaussian pulse generator, modulating an RF signal with it in an MZM modulator and getting sampled optical pulses.
Now I want to digitize this sampled signal, but there seems to be no digitizing device.
Could there be any way to implement this ?

[Damian Marek]

We have an electrical digital to analog converter, but no digital to analog converter that works directly on optical signals. Could that work?

[Jamal]

Actually I am implementing an analog to digital converter in which only sampling is done photonically.
After I sample electrical signal with an optical pulse generator using MZM modulator, I convert the optical analog signal to an electrical signal using a photodiode. Now I get sampled electrical signal and now I want to digitize its amplitude. And so I am looking for any component that takes sampled analog electrical signal and digitizes its amplitude to turn it into a digital electrical signal. Is that possible ?

[Damian Marek]

Yup, just its found in the Signal Processing -> Tools -> Electrical

[Jamal]

Yes but that Analog to Digital converter first samples and then quantizes, I was looking for a quantizer only
Thanks

[Damian Marek]

If you keep that component at the default “Sample rate” the sampling will stay as the original signal. Then you can play with just the ADC resolution to change the quantization.

[Jamal]

Thank you Damian
It seems to work, as it doesn’t sample but quantize.
Can you please also explain why the component behaves in such a way ?

[Jamal]

Can you have a look at my project file. The ADC output seems to have not changed at all. The signal is not quantized

[Damian Marek]

There are two main mistakes here:

1) This is maybe not a mistake but in the simulation your samples per bit is only 16 and the width of the gaussin pulse is 0.04 of a bit period. The sampling period is then 3.125 ps and the gaussian pulse width is 2 ps. The Gaussian pulse is much shorter than the sampling period so the signal right off the bat will not be represented very well. I would increase the sample per bit to much larger like at least 128.

2) If you check the normalize parameter in the ADC component you should see the discretization. The way it calculates the resolution is a little strange to me. Basically, the resolution of the output signal is equal to 2/(2^(n)). So in your case with 3 bits gives 0.25 A.U. Your electrical signal is very large because of the amplifier, so it is hard to make out the discrete nature. Checking the normalize option will bring the max amplitude down to 1 A.U. and it will be much more clear.

Regards

[Jamal]

Thank you Damian for pointing out such crucial mistakes

I have made a few changes and checked ‘normalization’ and then it gives a quantized output as shown in figure1. However with 3 bit resolution it only shows 5 levels instead of 8.
And without the normalize parameter checked, the ADC does not give any output maybe because of very low power output.

Also, you said that by keeping ADC parameter ‘sampling rate’ to default, we would get a quantized signal. Is there a way to get only sampled signal and not quantized ?

[Damian Marek]

The resolution includes a negative portion of the amplitude, so you would see 9 different values (including 0) for a function that is negative as well as positive.

You could use an Electrical Downsampler component to only change the sampling rate. Use the Find component function.

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