Measuring spectral purity

Here are 3 examples of inspecting the quality of a signal at 11MHz. For these measurements the RBW was set to 10kHz and the reflevel fixed at -20dBm to ensure the noise floor was visible. Also the SI5351 was routed through a 10dB attenuator to get all 3 signals on comparable input levels. Attenuation was kept on automatic.

First the signal from an ancient analog signal generator


The disturbance around 15MHz are caused by the signal generator.

Second the signal from a DDS


And third the signal from a SI5351


Keep in mind the baseband signal is at least 15dB above the reflevel. This shows the dynamic range of the tinySA

As you can see on the screen dump a full scan with this amount of detail (e.g. low RBW and spur reduction) does require about 30 seconds where a 250MHz wide scan with default settings requires less than 0.5 seconds. This is caused by the insertion of 13 extra scan points per display point (see ratio of RBW and VBW) and because of the time required for the RBW filter to stabilize.


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TinySA-SpectralPurity (last edited 2020-10-16 07:56:27 by RudolfReuter)