# My fantasy idea: oscilloscope from spectrum analyzer

#### Motanache

Joined Mar 2, 2015
540
For example I would like to analyze with oscilloscope the high frequency signals.
But such oscilloscope does not have any hobbist.

My idea is to realize a spectrum analyzer.

If from oscilloscope image can be obtain spectrum by Fourier transform,
Why did not think about a reverse process?: From spectrum to the shape of signal.

Normally you can not, because there is no information about the "initial phase" in spectrum.

This is the problem that I think for quite a long time and I try to put into practice.

#### MrChips

Joined Oct 2, 2009
26,479
You are correct. The frequency spectrum one observes is the power spectrum. The phase information has been lost. You cannot recover the original time signal from the frequency spectrum alone.

#### Motanache

Joined Mar 2, 2015
540
E.g:

I have this stick:

It's cheap enough that I can make changes, solder wire:
http://www.changpuak.ch/electronics/SDR.php
Spectrum:

Suppose we have a signal that through Fourier transform generates a spectrum that not includes components under 24MHz.

What other information should bring from that PCB as from its spectrum to be obtain waveform (time domain)?

DVB-T

#### Motanache

Joined Mar 2, 2015
540
You can laugh, but I wanted to measure the phase difference between every two "consecutive" frequency in the spectrum (using two receivers).

But the phase difference between two sine waves of different frequencies makes no sense.

To find somehow information lost "after Fourier transform" as I can do reverse transformation.
Since a very high frequency oscilloscope is very expensive.

I suggest we have in mind the construction of a radio receiver and give mathematical examples for trying to obtain from time domain frequency domain

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#### dannyf

Joined Sep 13, 2015
2,197
Why did not think about a reverse process?: From spectrum to the shape of signal.
mathematically possible: FFT on a FFT output from a signal is the original signal itself.

practically it is difficult as you almost surely need custom software / hardware, not to mention the update frequency being slow, etc.

People are working on acquisition hardware that coupled with software functions as a scope -> many smartphone based scopes are like that.