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How to get 89.5 dBM in this IP3 calculation
Hi,
I am learning some RF knowledge. The following is a post years ago on this group. I don't understand how the last poster got 89.5 dBM. Could you tell me that? Thanks in advance. http://groups.google.ca/group/rec.ra...3f 1163952dd9  am designing an RF stage that needs to have quite low intermodulation distorsion. When reading data sheets for various amplifiers they usually give a figure fo IP3. What I haven't found anywhere is if IP3 is related to the input or the output. Can ayone help me on this point? I need ntermodulation distorsion to be at least 100 dB down at the output of the amplifier at a level of 1.5 dBm, and I need a gain of approximately 3040 dB.  Tomas wrote: Is IP3 related to the input of the amplifer. In general, IP3 is related to the output for amplifiers and to the input for mixers. In the example using the MiniCircuits amplifiers, the output 3rd order intercept point will remain at essentially +47 dBM. With an output signal at the +1.5 dBM level, the distortion products will be at a level of 89.5 dBM. This is only 91 dB down from the output level and not the 100 dB desired. In order for the distortion products to be at a level of 98.5 dBM, which is 100 dB down, the output intercept point of the last amplifier must be at least 51.5 dBM. A MiniCircuits MAR8 will make things worse because of the lower OIP. Gain has nothing to do with these calculations. However, something to keep in mind is that increasing gain reduces dynamic range at the high signal level end. While an increase in noise figure reduces dynamic range at the low signal level end. 73, AL, w6wqc, 
#2




How to get 89.5 dBM in this IP3 calculation
On Sep 20, 8:18 am, fl wrote:
Hi, I am learning some RF knowledge. The following is a post years ago on this group. I don't understand how the last poster got 89.5 dBM. Could you tell me that? Thanks in advance. .... In the example using the MiniCircuits amplifiers, the output 3rd order intercept point will remain at essentially +47 dBM. With an output signal at the +1.5 dBM level, the distortion products will be at a level of 89.5 dBM. This is only 91 dB down from the output level and not the 100 dB desired. OK, how far did you get in the calc? If the 3rd order intercept is +47dBm, and the input is +1.5dBm, note that the TOI is 45.5dB higher than the input. For that to be the case, since 3rd order products increase in amplitude at three times the rate (in dB) as the input levels, and the input levels of the two tones used in the third order intermod test are assumed to be the same level, the distortion will be three times as far down as the two tone levels, measured from the intercept point. So for that +1.5dBm input, the third order intermod products are 3*45.5dB below the intercept point, or 2*45.5dB below the two tones. Note that this is from an ideal model where the output y(t) is related to the input x(t) as y(t) = a*x(t) + b*x(t)^2 + c*x(t)^3. Second order products, like the second harmonic and the sum and differences between frequencies, are caused by the x^2 term, and third order products like the third harmonic and 2*f1f2 (the usual third order intermod products) are caused by the x^3 term. But in actuality, the relationship also has higher order terms: x^4, x^5, and so forth. The x^5 term will also contribute to thirdorder intermod, and that causes the simple relationship of "increase the input by 1dB and the third order products go up by 3dB" to not strictly hold true. Amplifiers operated well below saturation generally conform very closely to the 3to1 rule, but ADCs and DACs most certainly do not, and neither do amplifiers that are driven into clipping. Cheers, Tom 
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