Measuring Third Order Intermodulation
The measurement function for OIP3 can also be used to measure the IIP3 of the tinySA. This IIP3 is the lower boundary of the OIP3 that can be measured by the tinySA and it is dependent on the chosen attenuation.
As input signal we use two fairly decent sinuses at 8MHz and 9MHz from two different signal generators combined through two 6dB attenuators and a resistive combiner into the low input of the tinySA. The chosen level of -9dBm is well below the 1dB compression point of the first mixer of the tinySA. Setting the LO drive at the default level, the attenuation to 0dB and using the OIP3 measurement function we measure an IIP3 of about +17dBM
Increasing the LO drive to the maximum improves the IIP3 level to +25dBm for the left intermodulation product but the right intermodulation product remains much worse at +22dBm
When we increase the tinySA input attenuation to 5dB the left intermodulation product disappears so it was generated internally. The right intermodulation product does not change so it is present in the input to the tinySA
Now we know the IIP3 of the tinySA we can start measuring the IIP3 of an amplifier. First we use two tinySA to create a two tone generator. Here is the two tone output including the harmonics.
EVen though there are attenuators between the tinySA generators and the passive combiner there is still some 3rd order intermodulation created as can be seen when we look close in
The OIP3 of the this two tone generator is around 8dB so we have to careful to create a test setup that ensure this does not hide the amplifier IIP3.
Now we add the amplifier AND a 30dB attenuator after the amplifier in the path between the two tone generator and the tinySA and we get:
As the signal level we observe is 3.5dB lower as without the amp and attenuator we have to add the 3.5dB to the measured IIP3 of around -2dBm so we get an IIP3 of the amp of +1.5dBm. The is 6.5dB lower than what we measured without amp and attenuator so we can be sure we are measuring the IIP3 of the amp.