FFT Averager - RTL SDR IQ bin files
The applet accepts RTL SDR IQ files using the OsmoCom rtl_sdr.exe capture tool and performs a complex FFT spectrum analysis (256 points default).
Fourier transform of digitized amplitude data.
FFT lengths between 32 and 2048 are chosen from the drop down menu.
The Amplitude and Phase scrollbars can be used to modify the amplitude and phase quadrature balance of the I and Q data.
The cluster display centre/bottom reacts to this adjustment and any adjustment will alter the averaged FFT output.
To achieve maximum sensitivity in a real receiver the front-end gain should be sufficient for noise to exceed the ADC minimum threshold.
The 0-dB value on the Signal and Noise scrollbars is just sufficient to cause the ADCs to exceed the ADC upper range.
As well as overdriving the ADCs, the effects of ADC Linearity errors and Clock Jitter can also be viewed.
The latter is of importance for higher aliased frequencies, and this can be observed by noting the effects as the RF signal is increased beyond the Nyquist frequency (1024MHz for a complex FFT default example).
The effect of quantization, DFT points, window function, pulse width, rise-time, and pulse position on the signal output SNR can also be evaluated.
Signal-to-noise ratio is assessed using the cursor strobe, from measurements of the signal peak, and on the noisy baseline. Note that the input RF noise power is divided between the DFT channels (length). In the default example with the RF band = 1.024 GHz, the bin noise is down by 10log(1024) = –30.1 dB.
Both independent signals that are available may be programmed with different parameters and are selected for adjustment using the Modify Signal radio buttons at the bottom of the applet. It is instructive to view the effect of complex signal types on the detectability of weak signals.
The Save button saves the FFT bins and powers to the Data Console together with the parameter values selected for a particular design.