Measuring side band suppression in a SSB transmitter using the tinySA #tinysa
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Measuring SSB modulation poses some problems for the tinySA. First the modulation bandwidth is typically 2.7kHz which is very close to the minimum BW of the tinySA. Next the phase noise of the SI4432 will have a significant impact measuring with such low BW.
So I decided to give it a try. The test setup is HDSDR for the I/Q encoding and a Softrock ensemble II RXTX as transmitter.? At first I set the LO and the TX frequencies both to 7.1MHz to simulate what you will get when you use traditional transmitter using a side band suppression filter. The test tone used was 2.5kHz to have almost the maximum distance between the LSB and USB signals. Setting the I/Q balance way of just shows the unwanted upper sideband as a right shoulder about 15dB below the main signal. Tuning the I/Q balance in the HDSDR calibration panel is easy and quickly the shoulder representing the upper side band disappears in the noise With only 30dB measurement range available due to the phase noise floor it is impossible to see of the I/Q balance side band suppression can be tuned to 40dB or better. In HDSDR you are free to choose the TX frequency versus the LO frequency as long as the delta between TX and LO is within the bandwidth of the audio channel. So I set the TX 10kHz lower to avoid the phase noise. Without tuning you get this (different vertical scale!) and after some tuning the left peak disappears in the noise and it must be at least 40dB below the wanted sideband. In conclusion the tinySA is just capable to visualize this SSB modulation and can support tuning. -- HBTE Files section:?/g/HBTE/files Erik, PD0EK |