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sinx/x spectrum distortion using Johanson balun with CC2520

Other Parts Discussed in Thread: CC2520

We are using the Johanson balun specific to the CC2520. We have tried different matching inductors, from 1.5 to 3.9nH. 3.3nH gives us our best results for a good sinX/X spectrum, but at certain power levels the spectrum is distorted. Is this because the CC2520 conjugate match requirements are different at different power levels? Please see below for an example of the distortion, seen at 0dBm output setting. Thanks.

1882.3_3nH 5_18_11.doc

 

  • James,

    Your picture seem to indicate seems to be of a modulated output and its power spectrum.   Depending upon the code will generate different sinX/X outputs.  This is one way to look at the channel distortion.   Another way is to look at a single tone using a wide band spectrum analyzer (26GHz) and note the distortions. 

    The balun  functions is to  double the amplitude by phase aligning the two out of phase outputs of the Radio.   If the phase is not properly aligned then you would note this in the single tone.     Often designers ignore the series traces to fine tune or align the phase.  Since the incremental values of the dicrete inductors often are not of the resolution you must use the traces to achieve a better match.    The balun as you noted are purchased in increments of 50, 100, 200.  The inductor is tuning the impedance to match between the radio and this fixed value at one frequency.   If the Q is large for this circuit then acrossed multiple frequencies you will have some change.   If it isn't tune at the center channel frequency 2.45GHz then you could see lower or higher frequencies distorted in amplitude and phase. 

    Having different imedances at different output power levels is common in any device.    Hence small signal bandwidths and large signal bandwidths are always different, due to impedances change.    Carefully centering the channel at the highest output power will show the least amount of change as you decrease the power level. 

    Rgds,