TECHNICAL DESCRIPTION AND OPERATING MANUAL ST131 <PIRANHA II> and ST131N
67
ANNEX 2
DIGITAL SIGNAL PROCESSING. SOME OBSERVATIONS.
Digital signal processing based on the FFT provides a better, compared with the analog reception
path, the sensitivity at the same display width of the spectrum.
We will show the following relation:
F=2B/N *
, where:
F - frequency bandwidth of one harmonic.
B - frequency bandwidth of the whole band. In our case, it ranges from 10 MHz to 500 Hz.
N - number of FFT points. For the main unit it is 512, for the software – 16384 (the FULL RANGE
mode) or 4096 (the BAND mode).
* this ratio does not consider certain parameters which do not affect the general idea.
The displayed noise DNL is calculated with the formula:
DNL = -174 + NF + 10 * log(F)
, where:
NF - noise factor.
As can be seen, the calculation considers the width of the harmonics depends on the number of
FFT points. The less the value - the lower the displayed noise level and therefore there is more sensitive.
Thus, if you change the value of the span, the sensitivity will be changed.
For example, the difference in the displayed noise level between the bands 10 MHz and 500 Hz is
40 dB (excluding the noise factor).
In the context of search activities we can say that the user has a possibility to change operatively
the sensitivity of the device from the communication receiver (tenth microvolts value) up to the
broadband field detector (tens of microvolts value).
