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IIP3 variation with input RF power in a mixer - RF Cafe Forums

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Post subject: IIP3 variation with input RF power in a mixer Posted: Wed Jun 04, 2008 3:26 am


Joined: Wed Jun 04, 2008 3:23 am

Posts: 11

I am working on a single resistive FET mixer. In the circuit, the input IP3 is decreasing by an amount of around 5-10dBm when i vary the input RF power from 0 to -20dBm, eventhough the input power for compression is greater than 15dBm. In the literature, what i have gone through shows that IIP3 is invarient of the RF power. So, can anyone please help me out in this regard. Does IIP3 changes with RF power? If yes, at what power level should we carried out the two tone analysis?



Post subject: Posted: Wed Jun 04, 2008 3:49 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello shireesha,

IP3 tests are performed in small signal. What can happen is that you reach to a signal level in which the Mixer does not work anymore in small signal region and experiencing some non-linearities.

Check also the following:

1. Do you provide the right LO level?

2. Are the ports correctly matched?



Post subject: Posted: Wed Jun 04, 2008 6:17 pm

Also check that the intermodulation is not being generated on your test bench. At a certain amount of power, IMD can be generated by the CW generators (if you are using a combiner to feed the two carriers, the isolation of the combiner is critical... you might need to add some circulators to increase the isolation and perform a correct measurement)

Also, at a certain amount of power, your spectrum analyzer might be intermodulating, try to lower the mixer level of your SA to check it..

You can clear all the test bench doubts by replacing the active device with a passive non intermodulating device (ex.. a resistive attenuator).

Take care



Post subject: Posted: Thu Jun 05, 2008 2:44 am


Joined: Wed Jun 04, 2008 3:23 am

Posts: 11

Hello Fsomma,

Thanks for your reply. Sorry, but i forgot to mention in the query that i am still at the simulation level. Suggestions provided by you will be very useful at the testing level. Can you suggest me what can i do in simulation.



Post subject: Posted: Thu Jun 05, 2008 2:57 am


Joined: Wed Jun 04, 2008 3:23 am

Posts: 11

Dear IR,

Thank you for the reply. What i feel is mixer will start experiencing non-linearities once the RF Power fed is nearer to the compression point. In the present design, Input P1dB is around +20dBm & the power what i am feeding is either -20dBm or 0dBm. So, do you think that it will experience non linearities?

LO Power fed to the device is in the range of 12-17dBm, with a small gate bias. All the ports are not perfectly matched, but the return losses are around 10dB.



Post subject: Posted: Thu Jun 05, 2008 7:37 am

Perhaps, because of some mismatches, your Input P1dB is not being +20dBm... check that by driving the device until having the IM3 tones at -30dBc from the fundamental tones, there, PEP power will give you your real Input P1dB.



Post subject: Posted: Tue Jun 10, 2008 6:45 am


Joined: Wed Jun 04, 2008 3:23 am

Posts: 11

Hi FSomma,

I have checked the P1dB in the way what u have suggested. At around 21dBm RF power, IM3 tones are -30dBc down from the fundamental tones.



Post subject: Posted: Wed Jun 11, 2008 9:00 am


Joined: Thu Sep 25, 2003 1:19 am

Posts: 50

Location: texarcana

In its linear region a mixer's IP3 "should" not be unaffected by RF power.

Of course, when RF power increases, the RF works against the LO bias the FETS or diodes. So at RF compression levels the affective LO power will decrease greatly.

In the mixers linear region, the LO power applied to the mixer may be much different than the power applied to the non-linear devices. So if your circuit is not matched properly compression may occur to soon.

But, you said,


21dBm RF power, IM3 tones are -30dBc down from the fundamental tones.

So your IIP3 is 36dBm? Sounds good to me!





Post subject: Posted: Wed Jun 11, 2008 10:30 am

Hello Shireesha,

1st: About the following: the 21dBm, are PEP, AVG, or it is the power of each tone?

shireesha wrote:

At around 21dBm RF power, IM3 tones are -30dBc down from the fundamental tones.

2nd: As I believe, while you are in the linear zone, the IM3 tones should increase on a 3 to 1 ratio respect to the change of input power, this, should keep your IP3 constant, I understand that that´s the "linearity" you are not achieving on the simulation and that I think you should... This is only to check, that your question is regarding to the fact that on your simulation you don´t have the 3 to 1 ratio....

Can you send an IM3(expresed on dBC) vs. Input Power curve?




Post subject: Posted: Sat Jun 14, 2008 3:22 am


Joined: Wed Jun 04, 2008 3:23 am

Posts: 11

Hi FSomma,

Regarding Input P1dB being 21dBm, i have checked the power of each tone, but not PEP or Avg. At 20dBm of RF Power, power of the fundamental signal was 13.175dBm and that in the IM3 signal is -17.83dBm. (diff= -17.83-13.175 = -31.005). Sorry, i tried but could not upload the curve.

As you have said, in the linear zone, IM3 tones should increase on a 3: 1 ratio respect to the change of input power. But my simulation don't have 3:1 ratio eventhough the P1dB shown in simulation is >20dBm.

Can you please tell me how many harmonics should be included for harmonic balance simulation of mixers, as i see that the number of harmonics of the tones are also effecting the IP3 performance.

Thanks & Bye!

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