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Any oscillator will not only output one frequency but will show small variations around the chosen frequency. This is called close in phase noise. To understand the limitations of the tinySA I wanted to measure the phase noise of the SI4432.
First I used a RSP1 and an AD9851 to establish the ability of the RSP1 to measure close in phase noise. Apart from some spurs this measurement showed some noise starting (the bump at the bottom of the peak marked with "A" below -70dB below the signal .



Then I switched the tinySA to signal generator mode which combines the out of the two SI4432 modules at 434MHz and 444MHz into the mixer to generate a 10MHz output signal. As can be seen in the second picture the hump at the bottom of the peak market with A has increased with 10dB and also became wider. This is caused by the fluctuations of the oscillators in the two SI4432 and what is call close in phase noise.




The practical impact of this phase noise can clearly be seen in the third picture taken from the tinySA measurement of the same SA9851 signal.
At the bottom of the peak at 10MHz there is a widening visible. The difference between the second and third picture is caused by the difference in resolution bandwidth.?
The RSP1 was measuring with a 500Hz FFT bin size where the tinySA measured with a 30kHz resolution filter bandwidth.
To better understand the practical impact I added a signal at -54dBm at an offset around 60kHz. (10.058MHz). As can be seen this close by signal is almost masked by the phase noise of the -20dBm signal at 10MHz.
Luckily the SI4432 dynamic range is still very good as there is no sign of overloading. Even when both signals fall within the bandwidth of the 434MHz IF filter of the tinySA the performance of the SI4432 is still good.



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HBTE Files section:?/g/HBTE/files
Erik, PD0EK