IP3 testing - RF Cafe Forums

 Post subject: IP3 testing

Posted: Mon May 22, 2006 8:03 am 

I saw this on another forum a few days ago. Any comments about what it says? Seems to be a lto of different pinions out there. Sorry for the long length.

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"I think you will find that you do not directly measure IP3. IP3 is an extrapolated point which is probably not achievable in practice due to limiting and/or destruction of the device. The use of IP3 is to calculate distortion at low signal levels, not high signal levels.

It also matters, for 2 tone IP3 testing, what the combined power in both tones is. You get a different OIP3 prediction at different test powers.

OIP3 is a measure of the amount of unwanted spurious signals generated by a device that is operating near its compression point. Communications networks are sensitive to these unwanted spurious signals, as they can jam an adjacent communications channel, or distort a modulation scheme.
OIP3 is measured by having a device output two sine waves, and reading the unwanted spurious tones generated. The two sine waves superimpose. At some instances in time, the two superimpose as a maximum envelope--at which time they are being the most distorted (clipped envelope) by the devices limited P1db capacity.
So, the lower the P1db, the more the evelope of superimposed tones is distorted, and the worse the OIP3 will be.

The IP3 (referred to input or output) is just a figure of merit for the linearity of the amplifier. Amplifiers need to be linear so they can pass the undesired large adjacent signals (the jammers) while still amplifying the small desired signal (the channel of interest). The higher the IP3 then the less gain compression the amplifier will suffer... and less distortion from mixing of the jammer with the desired signal will fall next to the desired signal at baseband (and this 3rd order distortion can't be filtered). Receiver designers are more concerned with input IP3 because this gives them an idea how large a jammer (at the antenna) the amplifier can handle. If you're designing a Power Amplifier then you're more concerned with output IP3.
In the real world (in practice) when you design receivers the spec's are typically more specific than IIP3 or P1dB. If you have a good system's engineer then he will taylor the block specifications to more specific specifications like "the 3rd order distortion rejection with out of band jammer at X dBm = IMR3 spec or IP3 with specific tone power". The IP3 considering in band jammer is also spec'ed by determining the maximum acceptable signal swing hitting the baseband ADC (ie 250mV) and then back calculating the required input power hitting the input of the receiver to generate 250mV at the baseband output (considering mixer/filter/LNA gains)ie: Z dBm. Then you apply two test tones to the input of your receiver each with (Z-6) dBm and you should get 250mV max peak swing out of the baseband. Now, with the applied Z-6 dBm tones the 3rd order distortion products should not be higher than the noise floor of the receiver to avoid 3rd order distortion... thus giving you your in band IP3 specificaiton .The P1dB spec is not used to the extent which it is taught in school. I have only seen it used in one place. That is, making sure the amplifier gain does not compress by more than 1dB when the worst case (maximum power) jammer enters the receiver with the desired signal. The gain compression suffered by the desired low power signal is measured and when the gain of the desired signal is reduced 1dB then this is the gain compression breaking point.

The answer to your interview question (“what is the relationship between P1dB and IP3”) was 9.6dB as stated above. If any of you disagree then it's because you don't know the answer. It's standard textbook theory that you will learn in any graduate RFIC circuit design course. I don't know of a web page that shows this derivation but 9.6dB to 10dB difference between P1dB and IP3 is the widely accepted figure of merit. Any RF designer knows (or should know) this. I'm not sure about the data sheets you're looking at or how it was measured. The relation between IP3 and P1dB being 9.6dB is assuming you are measuring the IP3 with low power tones, meaning that if you increase the tone power by 1dB then the 3rd order products (IM3) will rise by 3dB. As you turn up your test tone power and they approach the 1dB compression point the device is no longer small signal non-linear and (IM3:desired) is no long 3:1 and this will cause the 9.6dB rule of thumb to not apply because your test tones are too hot and they're driving the device large signal non-linear.

To test the Input IP3 of your device you drive it with 2 tones w1 and w2. The output will have 3rd order distortion products 2w1+w2 and 2w2-w1 which is your IM3. Take either one of these tones and find the IMR3 which is your output tone power minus the adjacent IM3 power. Now, your IIP3 is the power of one of your input test tones (at the signal generator) plus 0.5*IMR3. If you're performing this test in the lab with the signal generator and spectrum analyzer you have to:
1. make sure there is no distortion generated by the signal generator. Typically you will have 2 signal generators (1 test tone per sig. gen.) and both tones are combined in a 2:1 power combiner that has only 3dB power isolation. Therefore, the test tones from opposite machines can enter the other machine and create 3rd order distortion in the signal generator itself. Make sure you use attenuators on the signal generator's SMA outputs (like 10dB pads) to ensure an extra 10dB isolation on each machine.
2. When you choose the test tone power level you don't want it too hot... because if it is then your distortion won't be 3:1 for IM3-vs-test tone slope. If it is, then you're not measuring small signal IP3. Make sure that if you increase the test tone 1dB then the 3rd order distortion increases by 3dB.
3. You can follow up your IP3 measurement by doing a P1dB measurement. Apply 1 tone at a XdBm and then at (X+10)dBm. See if the gain dropped by 1dB. If it didn't then increase X by 1dB and re-check the gain (at X+i+10)dBm. When your gain is down by 1dB then the X+i is your input P1dB point. Check to see if it is 10dB below your measured IP3!

One caevat is when you have high frequency distortion. The 9.6dB rule of thumb generally applies for frequency independent circuits (ie: no caps/inductors or frequency response). In a system with frequency response the a1 + a2^2 + a3^3 + .... analysis typically gets more complicated because you have to account for phase and frequency effects... requiring you to use volterra analysis. However, the 3:1 relationship between 3rd order distortion and test tone still applies unless your test tones are widely spaced to place your IM3 distortion far away to suffer different frequency response than the test tone.

AM to PM conversion effects on IP3 are typically only considered in power amplifier designs where you are basically more concerned with Adjacent Channel Power Ratio (ACPR), which can be related to IP3. If you measure the IP3 of your PA with small test tones then AM-to-PM conversion isn't as much an issue. However, small signal IP3 in a PA isn't very useful information because you care about large signal 3rd order and adjacent channel distortion... not small signal IP3. THerefore, the 9.6dB rule of thumb isn't really applicable because in a PA your spec and required linearity is derived from large signal distortion, not small signal distortion which is 3:1."

 Post subject:

Posted: Mon Oct 16, 2006 12:16 pm 

Hello,

After reading this long description, I have to say that most of it is true. However there are some points on which I disagree.

1. The ratio between P1dB and IP3 is not necessarily 9.6dB. This is a known mistake that many people do. The relationship is not constant and is dependant on the technology of the device.

2. According to the IEEE the IMR3 needs to be at least 20dB, meaning that the IM3 products need to be 20dB lower than the fundamental signals in order to get the right IP3 results.

More opinions are welcome!

_________________
Best regards,

- IR

 Post subject: OIP3 similar to P1dB?

Posted: Wed Mar 21, 2007 7:16 am 

I just did some measurements on a PA made of GaAs material. I got P1dB of ~25dBm and an OIP3 of ~25dB @ -20dBm input power as well. Is this possible? Can you explan to me pls. I made all the proper measurement setups for these measurements. Why is it that my OIP3 inproves at -10dBm input power? I got a 30dB OIP3.

 Post subject:

osted: Wed Mar 21, 2007 11:03 am 

Hello,

Please give me more details aout your amplifier:

Gain, input signal level of per tone.

Then I can let you know if your results make sense or not.

Consider the following:

1) There is an OIP3 (Output IP3) and IIP3 (Input IP3), the ratio between them is: IIP3=OIP3-Gain.

2) Valid test of IP3 is done is small signal: The difference between the fundamental signal and the 3rd products should be at least 20dBc according to the IEEE standard.

 Post subject:

Posted: Sat Mar 24, 2007 11:18 am 

Hi,
Thanks for your reply.
The two tone are at -20dBm.

2f1-f2 / f1 / f2 / 2f2-f1
-46.52dBm / -5.74dBm / -5.76 dBm / -46.29dBm

Output cable loss = 10.48dB. OIP3 ~= 25dBm, Gain = 25dB from network analyzer, P1dB from Network analyzer-power meter config = 24.6dBm

I also tried at -19dBm.
2f1-f2 / f1 / f2 / 2f2-f1
-44.69dBm / -4.76dBm / -4.76dBm / -44.39dBm

Output cable loss = 10.48dB. OIP3 ~25.54dBm.

The third harmonic does not increase 3dB per every dBm of input power. I asked around and they seem to think that the power amplifier is a cascaded device and the stages within the PA actually has phase cancellation which does not result in a 3dB increase of the third harmonic per dBm increase of input power. Is this really true? Through this explanation, I was made to believe that the IP3 cascaded formula is not accurate as it assumes distortion as being additive. I am really lost.

 Post subject:

Posted: Sat Mar 24, 2007 12:35 pm 

Hello,

Your results will make sense only if you take into consideration the loss of the cable in your output. Then you will get an output power of approx. +5dBm, the IM3 products will maintain the same -40dBc in relation to the fundamental and then your OIP3 will be +25dBm. By the way why do you use such a cable anyway. These output powewr are very low and can't damage your Spectrum Analyzer. In addition you can use an attenuator instead of a long cable.

It does not make sense that the P1dB is the same as OIP3. It must be lower. IP3 is a theoretical point which is reached by a linear extrapolation of the fundamental and 3rd order prodcuts slopes (Pout vs. Pin). I assume that your P1dB is in the range of +15dBm.




The IP3 formula relates only to the 2f2-f1, 2f1-f2 products; as these products fall into the operation BW. Mathematically it can be proved that the amplitude coefficients of these spectral components have a relation of 2:1 to the fundamental signals: If you increase the amplitude of f1 by 1dB then the 2f1-f2 will increase by 3dB.

The 3rd harmonics signals (3f1, 3f2) have a different amplitude coefficient therefore their behavior is different.

 Post subject: Re: IP3 testing

Posted: Thu Apr 22, 2010 12:51 pm 

Hi IR,

Would you post the number of the IEEE standard (or reference) you refer to for measurement of IP3?

Thanks a lot,

Dave

 Post subject: Re: IP3 testing

Posted: Sat Oct 09, 2010 11:45 pm 

The answer for most typical engineering grade measuremetn would be the less of the fundamental tone - highest IMD product tone. But to account for baseband(audio) response most commercial and mil grade specs all the higher fundamental against the highest IMD product. MIL-STD-188-144B and TIA-603. This gives a more relaxed spec to allow for the ripple inband. The spec is for equal level tones on the input at 100% rated power.

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