IP3 measurement setup

[luckyali]

Hi all,
I would like to measure the 3rd order intercept point for a mixer by myself...

anyone who performed the IP3 measurment by himself (or knows the approved method) and has the setup diagram (or link to any file, page) for this kind of measurment ?? please let me know
I Shall b very much thankful for this help...

 

[jiripolivka]

You need two signal generators with a coupler to get one input to mixer. Then a spectrum analyzer to see the spectrum at mixer output.
Adjust the input signal pair to get equal power into the mixer, then see the spectrum. In addition to the output signal pair at appropriate frequencies you will see more pairs under and above the "nominal" pair.
IP3 is usually defined for 20 or 30 dB ratio between the "nominal" output power and the first "other" side bands.

When you vary mixer input level(s) smoothly, you will see that the above ratio varies by more dB than the dB change of the input level. This means that mixer element is being saturated, and generates more harmonics and combination frequencies in the output spectrum.

Find Agilent White Paper on Intermodulation Measurement to find more details and instrumentation.

 

[ninju]

Do as above. Give two input signals, from the same source, of the same amplitude and frequency. This simplifies things greatly.

 

[luckyali]

You need two signal generators with a coupler to get one input to mixer. Then a spectrum analyzer to see the spectrum at mixer output.

So that's 2 Input signal @ RF + 1LO = means I need 3 Signal Generators... Right ?

 

[jiripolivka]

So that's 2 Input signal @ RF + 1LO = means I need 3 Signal Generators... Right ?

Yes, if the tested mixer does not come with its LO source. The LO signal generator should have the correct frequency and the specified output power to correctly operate the mixer.

 

[vfone]

Very important for accurate measurements, any IP3 setup should have pad attenuators (about 10dB) placed on the signal generators paths.

 

[luckyali]

may I know the purpose of these Attenuator Pads.

 

[backdoor_]

The pads are used to build up the isolation between the 2 sig gens.
The goal is to have a cleanest combined sig ( 2 tones) with no IP3 product. However, having too much pad will force the sig gen to work harder to compensate for the pads -> more harmonics.
I use isolators whenever I can instead of pads.