Some more real measurements.
Now with calibrated dBm scale
First to determine the IIP3 I'm using two clean sinus generators through two attenuators combined into the input to measure the IIP3
One is set to 6MHz, the other at 7MHz
At 5MHz and 8MHz you can just see the input third order intermodulation products (IIP3) at -79dBm and -74dBm.
Calculating the IIP3 gives +9.5dBm which is surprisingly good.
This implies that when you keep your input signal below -25dBm there will be no internally generated spurs.
It's good practice to check by adding 10dB attenuation if any harmonics you see are internally generated or already present in the input signal.
A good example of measuring IMD is this measurement of my Softrock Ensemble RXTX during transmission through 60dB of attenuation:
The fundamental is at 11.45MHz and second, third and fourth harmonic at -47dBm, -46dBm and -58dBm compared to the the fundamental.
It seems the output low-pass filter of the SoftRock is doing a good job.
