Sunday, November 16, 2014

Prj133 Mixer at 1575MHz IF

The most significant challenge has been hand mounting the high frequency output balun.  The part is so small and only has 6 small pads that it has proven very difficult to seat properly.  Even the smallest amount of excess solder causes the part to not self align properly leaving one of the pads open.  Unlike some of the QFN parts I have used there are not enough pads to create enough surface tension to decently self align.  In the process of wicking off excess solder it is very easy to wick off too much and again wind up with an open.

In an attempt to address these difficulties, I started by mounting the balun first without any other components and measuring the resistance between the pads at the board level to ensure there were no opens.  The following is a picture of that.

This produced an output that was about 10dB lower than expected.  In retrospect, I may have been having problems using the external LO port and getting enough power into the mixer coupled.  In addition there uncertainties in drive level (+/- 5dB) and measurement level (+/- 2dB).  In short, I had myself convinced an open had appeared and proceeded to rework the part mounting.  This led to more problems with opens and alignment. Finally, I ended up tearing off a couple of pads by accident.  The part is so small with only 6 pads its easy to accidentally move it as the solder is not fully at temperature.  The figure below captures that saga.

The top part has missing pads and the bottom part is for reference.  Pin one indicator at left.  Pin 6 pad is missing while pin 1 is partially lifted. The trace to left of parts is 0.2mm, the C146 lands are for an 0805 part, the ground plane vias are 0.89mm diameter.  At this point, the discrete balun was looking pretty good ...

The overall mistake was to use orthogonal pads.  The datasheet for the balun recommends taking the corner pads out at a 45 degree angle.  This would have given enough vertical and horizontal pull to self align including the middle two pads.

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