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Mixer 3rd order intermod measurement problems

Question asked by jamesh on Nov 18, 2008
Latest reply on Sep 12, 2010 by jamesh

I am using the two tone test to measure the 3rd order IP of a passive double balanced mixer.

Conditions as follows:

25KHz separating F1 and F2. Input power to mixer  = -15dBm (each tone)
Mixer conversion loss = 7 dB
INPUT 3rd order intercept point = 25dBm
FLO = 66MHz, 136MHz, 530MHz and 940MHz, 17dBm at the LO port.
There is a IF of 350 KHz for each LO frequency.

Equipment E4403B ESA-L SA

3 x 8642B signal generators

From theory I expect an IMR of 80dB, in practice I am measuring an IMR of 71 dB

I have noticed that the IMP's are sensitive to IF O/P impedance at 2x FLO, e.g. If I Null out the 2X FLO product (At the IF output) with a notch filter the IMP's reduce by approx 9dB.

I believe the IF port mismatch at 2XFLO is causing reflection of the 2XFLO product and further intermodulation in the mixer.

I have built a IF diplexer to dump the energy above 1MHz (IF passband) into a 50 ohm load. The circuit has a S11 of 40dB at 2xFLO, however this does not reduce the IMP level as expected.

I assume the mixer does not want to see a 50 ohm impedance at the IF output at 2xFLO.

I am also assuming that the IF port impedance at 2X FLO for each of the above LO frequencies will be different!

How can I trap the 2XFLO product at each LO frequency whilst maintaining 50ohm match in the passband and elsewhere ?

Thanks in advance for any help.