#qcx Si5351A Phase noise measurements #qcx

Hans Summers

Hi all

I have published an article on phase noise measurement techniques and in particular, relating to the phase noise of the Si5351A in the QCX. Having developed the equipment and skill to make this measurement is also expected to be very useful for final performance testing of the forthcoming QSX transceiver. 

I am grateful to Kevin ZL1UJG and the 100+ correspondence emails we had on this topic; some information from Kevin is also presented on the page and agrees with my conclusions. I had always feared phase noise measurement and thought it out of reach of mere mortals such as myself but was pleasantly surprised; to be sure it isn't trivial but neither is it as difficult as I had feared ;-) 

The QCX phase noise performance was found to be GOOD, and my measurement matched ARRL to within 1dB in their QST August 2019 review (ARRL measured -135 dBc/Hz at the transmitter output, at 10 kHz and 50 kHz spacing). The review is reproduced with permission, at http://qrp-labs.com/qcx . Various things were tried to improve the phase noise performance (capacitor and inductor filtering of the supply to the Si5351A) - none made any difference; the conclusion is that the QCX is already as good as it gets and performance is determined by the PLL of the Si5351A chip. 

Other conclusions from the experiments are relevant to anyone using the Si5351A. Specifically, heavy loading (50-ohm impedance) damages the phase noise performance of the Si5351A quite significantly. This can be partially mitigated by using a 0.1uF DC-blocking capacitor. A clean supply is also important (Kevin's results). Previous acquired knowledge on Si5351A output loading indicates that heavy loading will also cause crosstalk between outputs, as well as deteriorate the output waveform and reduce its amplitude. 

Therefore anyone using the Si5351A in applications where high drive levels are required, such as a diode ring mixer, should make sure that they buffer the Si5351A outputs, not use them directly connected to the mixer or 50-ohm attenuator pads. I have seen some Si5351A breakout boards that use small 50-ohm transformers at each Si5351A output and this is also wrong (from the perspective of phase noise performance). 

73 Hans G0UPL

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