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Own IMD

IM MEASURING KIT

In this section we will focus on measuring OIP3 of designed measuring kit. Own OIP3 we need to know so that we can assess the suitability of the kit for measuring IMD of external passive circuits (BPF, LPF, HPF for example) with higher IP3.

During first measurement I used ferrite binocular FB-43-2402 with 8 turns bifilar in the combiner. OIP3 was measured at different levels of the output signal (from 0 to +22dBm). The following picture shows OIP3 dependence on the output signal level.


We see that in the first part the graph is almost linear, then the curve is more flatter and then changing rapidly downwards. What it means? Why so different dependence? Let´s try to find an answer.

I hope that most of you remember next math formula
                     y = kx + q     

It is an equation of the straight line.  Let´s add this function to the graph with k = 1 and q = 44.2


K = 1 means that every change the value on the x axis produces the same change on the y axis. Q = 44.2 indicates the initial value of IP3 for the output level 0 dBm. It is about the OIP3 value of pure VNA.

For the output signal level from 0 to abt. +12dBm measurement kit adds no intermodulation distortion. It is, because IMD3/2 in the known formula

                                                                   OIP3 = IMD3/2 + Po

practically doesn´t change its value in the range of output signal from 0 to +12dBm, as is visible from the graph.

For levels above abt. + 12dBm, the graph starts to turn, reaches a maximum and then declines sharply. The question is, what causes this change. Nelinearity of the amplifier? NO! The physical explanation for this phenomenon should be found in the ferrite material non-linearity only. The sharp drop in the curve means that the ferrite core is gradually saturates. As we can see from the next picture, changing of transformer type in the combiner moved the curve to the higher values of OIP3.


One more picture with both graphs for comparison...


Conclusion.
The designed kit is suitable for measuring IMD of passive circuits with higher IP3.  Its own OIP3 reaches excellent value close +60 dBm in the range of output signal level from +16 dBm to +18 dBm (measured in 14 MHz band - in other bands the curve can be different). For example, if I need to measure BPF with expected OIP3 around +50 dBm, in such a case is necessary to use output signal levels starting at least  from +8 dBm (to eliminate kit IMD effect). As an example have a look on the next picture....

 
 
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