IP3 measurment (for amplifier)
I want to measure IP3 of an amplifier and i do not know what the required leve of 2 tones is. is it dependet on the acceptable spur levels of the amplifier or it has a general procedure with predefined level of signals?
many thanks in advance,
Regards,
IP3 is a "small signal" parameter. It can be measured for large signals, but it is not as meaningful when the amplifier is close to compression. A good starting point would be 20 dB below the amplifier P1dB.
First, using just one tone find the P1dB of the amplifier (compression point). From that point go down 3dB (P1dB - 3dB) and that is the higher limit of the two-tone level. The equation below is valid only if the amplifier is not in compression.
OIP3[dBm]=Pout[dBm]+(IM3[dBc] / 2)
so, you mean there is no difference between the levels below(p1dB-3dB),there is no specific point?
As a rule of thumb, each tone should have max. 20dB below of P1dB compression point.
For instance, if P1dB=20dBm, each tone can have max. 0dBm output level.
3dB below P1dB is used in Power Amplifier evaluation (various power classes), when less than this is used in LNA design (small signal).
When you back-off too much from P1dB in PA linearity evaluation, this will not give the right information about IM3 vs Zout. This happen because the PA output impedance is very dependent by the output power.
If you are in the small signal region of an amplifier, IP3 will be the same for different power levels. So for instance you can measure IP3 at -10 dBm and use that value to predict the intermods at an input power of -20 dBm, or vice versa (assuming both powers are in the small signal region). If the amplifier is in compression or near it, that is no longer true. The IP3 value measured in compression will vary with input power, so it can not be used to predict intermods at a different power level.