Spur Free Dynamic Range
When measuring the spectral purity of a transmitter it is important to understand the spur free dynamic range (SFDR) of the tinySA. This SFDR determines the minimum level of harmonics that can be measured with the tinySA.
Starting with a fairly clean signal around 7MHz. The 2nd harmonic is at -63dBc
A low pass filter is inserted between the transmitter and the tinySA to remove the 2nd harmonic and indeed it disappears into the noise
The RBW is reduced to lower the noise floor. The 2nd harmonic is now again visible at -71.5dBc
And a little attenuation is added to check if the remaining 2nd harmonic is internally generated.
As the level of the 2nd harmonic moved down it was internally generated. With this 4dB attenuation the 2nd harmonic is now below -78dBc which is the maximum SFDR of the tinySA.