Sunday, December 16, 2007

Why are 3rd Order Intercept Points of Interest: A Continuing Discussion

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1 comment:

wa5mlf said...

In a general sense, the upper end of dynamic range represents the largest tolerable signal. The popular representation of dynamic range, such as on these two figures, indicates the upper end to be where intermodulation products emerge. One reference calls this "spurious free dynamic range". If you define dynamic range in this manner, it appears to relate closely to the IP3.

The inflection point on the response curves, where the output level no longer increases with increased input level, is where compression takes place. It seems likely that some receivers may have a gradual inflection while others have a sudden or sharp inflection, depending on design. It is a point where the response becomes highly nonlinear. Some references define a 1 dB compression point where the actual output response is 1 dB below the extrapolated linear response.

It seems unlikely that any real receiver will have a perfectly linear response up to the inflection point. Electronic devices and circuits have characteristics that can be modeled as linear within a certain range. The wider the range of signal amplitudes, the more likely it is that the device or circuit must be modeled with a curve of some shape rather than a straight line.

A receiver's response, as depicted by these popular graphs, involves all stages of the circuit. It is not limited to the IF amplifier.