Understanding Third Order Intercept 

Thanks! Really appreciate the feedback!

Remarkable comment also!

Thanks for the feedback!

We're happy for feedback anytime :) Thanks!

Glad it's helpful - thanks!

Thank you!

My pleasure - thanks for watching!

Thank you!

您对了!谢谢您

You're right! IP3 is the BLUE plot and the FUNDAMENTAL is the ORANGE one.

Yes, you are right! based on the TOI formula, it make sense

Thanks!

Thanks for the suggestion!

GL ES 73! Paul, KO4LZ

Happy to help - thanks for the feedback!

Thanks. Some of the presentations that are currently planned include the ones you listed (noise figure, spur search, phase noise) as well as things like ACLR, EVM, noise power ratio, group delay, CCDF, etc. There are also series on analog and digital modulation and power measurements that should be ready soon.

@@pauldenisowski eagerly waiting for it sir ! continue the good work

@@pauldenisowski eagerly waiting for it Sir ! great job

By the way, the first of many phase noise presentations was just posted: see "Understanding Phase Noise Fundamentals" in the Test and Measurement playlist.

re: "Can you explain how the frequencies of the two tones in the two-tone test are chosen? " Short answer: Experience. Long answer: Determined by the DUT (device under test) such as freq operating range and so forth. Additional factors enter into the freq chosen, such as power supply bypassing and dynamic response, especially when freqs are close by such as in the audio range.

You want the attenuator to be before the first active device in the spec an. Many (most?) spec ans have an internal, user-controllable input attenuator so usually the procedure to add 10 dB of attenuation is simply done via the spec an user interface. The results would be the same if you were to screw an external attenuator to the spec an input port, although clearly this is slightly les convenient. [It should also be noted that instead of mechanical attenuators, instruments may also have internal electronic attenuators].

Thank you!

Sure. :) Essentially the question is whether the 3rd order intermodulation products are being generated within the device under test and/or in the spectrum analyzer. If the products are being generated external to the spec an, then an input attenuator will attenuate the fundamentals and the products equally. And since TOI/IP3 is calculated using the *difference* in levels between the fundamentals and intermod products, reducing them all by the same amount won't change the TOI/IP3 (assuming, of course, that you don't add so much attenuation that the products disappear under the noise floor ....). If the products are generated within the spec an, then an external attenuator will only reduce the level of the fundamentals. Recall that the amplitude of the third order products rises (or falls) much more quickly than the amplitude of the fundamentals (steeper slope). So attenuating the fundamentals at the input to the spec an will create a noticeable change in TOI if the products are being generated internally. Hope that helps!

Good question. Third order intercept is, in my experience, primarily an RF measurement. A common way of measuring linearity (or distortion) in audio applications is either SINAD or THD (total harmonic distortion). I've done videos on both of these, so please take a look!

The section on source isolation provides some guidance. Generally, either attenuators or circulators can be helpful in preventing undesired mixing.

I don't think I ever made a whitepaper version of this - do you think there would be enough interest?

To my best knowledge, the values "did" drop but the spectrum analyzer automatically offsets the value for you. By default the values you see on spectrum analyzer are with reference/calibrated to the input port of the analyzer. The analyzer has an internal attenuator that can be automatically or manually adjusted. By default the attenuation is set to auto.

Sure : ) If you look at the equation on slide 13, you see that TOI (or IP3) is a function of the power difference, P-delta, between the fundamental tones (Ptone) and the third order products (PIM3). In the case of external generation, both Ptone and PIM3 are all attenuated by the same amount, so P-delta stays the same and thus TOI stays the same. If the distortion is being internally generated, Ptone will be decreased (since the fundamentals passed through the attenuator) but PIM3 will be decreased even more: remember that the power in the products has a different (steeper) slope than the power in the fundamentals (see slide 9). A 10 dB decrease in the power of the fundamental tones results in a 30 dB decrease in the intermodulation products. So attenuating the fundamental means that P-delta will change and TOI will decrease *if* the distortion is being generated internally. We actually have an entire white paper: "Intermodulation Distortion Measurements on Modern Spectrum Analyzers" that covers IP3 measurements in much more detail. In particular, pages 15 and 16 mention how proper use of input attenuation can reduce the generation of intermodulation products in the first mixer in a spectrum analyzer. scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ef79/1EF79_1E.pdf If you watch our video "Understanding Basic Spectrum Analyzer Operation" it also mentions the importance of properly setting the reference level to avoid overdriving the first stages of the spectrum analyzer and creating IMD within the analyzer. Being aware of the possibility of IMD and adjusting your settings / levels appropriately is important in making good spectrum analyzer measurements. Hope that helps!

@@pauldenisowski Thank you, Paul. Your explanation is really helpful.

@@danganh870 Anytime :) Happy New Year!

The third order product only appears above a certain input power level (X-axis). The more linear a device is, the higher the input power level that's required before third order products appear. All other things being equal, a third order product line that starts further down the x-axis will produce a higher TOI / IP3 value (a good thing). If the fundamental and third order product both started at the origin, then IP3 = 0 dB and the amplitude of the third order product would be higher than the amplitude of the fundamental for *all* input power values (a very, very bad thing).