How should I scale intermodulation products? - Page 1
Forum section: electronics / rf-microwave

author: ByteMe
author_rank: Contributor
timestamp: August 28, 2017, 02:31:15 pm
content: I'm trying to automatically measure the amount of intermodulation based on a broadband spectrum capture(which is averaged over time). There is a given set of reasonably narrowband carrier waves, which are always present. If there's a non-linearity in the RF-path, such as a rusty bolt(which is basically a point contact diode) or an amplifier that's over/under driven, these will produce a set of intermodulation products.So far, I've managed to automatically calculate the composite second order(CSO) and composite trible beat(CTB) frequencies. The thing is: a lot of CSO/CTB frequencies are exactly the same. So there are e.g. 10 CSO products at 622.25 MHz and 2 CTB products at 623 MHz. But that means they scale differently.My idea to detect intermodulation is to numerically integrate the received power in a very narrow bandwidth around the precalculated CSO/CTB frequencies and try to find some thresholds for the result of that integral.My question is: how should I scale n CSO/CTB-products that come together in a single point? Should I just subtract 10*log_10(n) dB from that datapoint? Should I also include scaling for the binomial coefficients of the CSO/CTB products?Side question: are typical nonlinear distortions very frequency dependent? If yes, I would imagine that scaling for the number of products won't be very significant.PS: Yes, this is a literal copy of a question I put up on the Electrical Engineering Stack Exchange. I just figured this forum might be more active and there may be more knowledgeable engineers on this forum.

author: CopperCone
author_rank: Super Contributor
timestamp: August 28, 2017, 06:39:30 pm
content: I made a thread about nonlinear junction detectors that had some information posted that you may be interested in.I would like to foster discussion in both of these topics. The more discussion we have of the physics the better.https://www.eevblog.com/forum/rf-microwave/nonlinear-junction-detector-physicsspectral-purity-(harmonic-radarbug-detector/also, his stack exchange threadhttps://electronics.stackexchange.com/questions/326238/how-should-i-scale-intermodulation-products

author: rfbroadband
author_rank: Unknown
timestamp: August 28, 2017, 09:23:34 pm
content: you mentioned "automatically measure" and "automatically calculate"....Usually the test conditions for CTB/CSO measurements are well defined dependent on the standard you need to be in compliance with incl. Res. BW, Video BS settings of a SA and signal levels of the N tones including that potential tilt that needs to be applied during the test. If (for example) your test requires to apply 133 signals to your RF amplifier input (at a given signal level) you would measure the CTB with a spectrum analyzer at the output by turning off one the carrier and you would measure the residual signal level at the frequency of the carrier that was turned off. The ratio to the desired signal to the undesired IM signal is your CTB. This measurement allows you to evaluate and directly measure the CTB of your amplifier or system, it however does not! tell you which beats are responsible for the "overall" IM level that you just measured.If you are trying to predict the CTB/CSO or XMOD you will need to know (at a minimum):- IP2, IP3, IP5 vs frequency of your DUT, it can vary significantly.- you need to make sure you realize that the term IPx is a theoretical point where two lines 'intercept'. This means in order for an IP3 to be meaning full you have to operate the DUT at power levels where you can relay on the assumed 3:1 slope vs power of the IM3 products used for defining IP3.- If you operate your DUT at power levels that cause the IM3 products no longer to be on a 3:1 slope, you can't use IP3 specifications anymore for your estimates- you will need to know the phase of each carrier that is applied to the input of your DUT.In summary, the most practical way is to simply measure the CTB/ CSO etc. The real problem is to figure out what to do if your product doesn't meet the CTB spec. and the individual components that you use only provide you with narrow band IP3, IP2 specifications at power levels that deviate from the conditions that are of interest to you.google 'Matrix' and 'CTB/CSO' for some literature on CTB/CSO measurements. I hope this helps.