Prj133 Mixer at 1575MHz IF (Part 2)

Given the issues with the high frequency balun, I populated the synthesizer and discrete balun for a 1575MHz IF output.  The schematic can be found in this post. The balun values used are shown below.

Component	Value	Comments
C105/C113	120nF	DC blocking capacitors
L102/L103	6.8nH	DC supply chokes
L107/L108	10nH	4:1 Lattice Balun inductor
C106/C107	1pF	4:1 Lattice Balun capacitor

The following is a picture of the final assembly:

Synthesizer (ADF4351 - left off center) and Mixer (LTC5510 - right center) using a 1575 MHz discrete balun (upper right).  Power and digital interface at left via 2x5 header.  

This was then used to sweep 200MHz about an IF of 1575MHz using various RF input frequencies.

The measurement device has an uncertainty of +/-2 dB.  The input RF level should remain fixed, however, its level can be pulled a dB or so with load reactance.  So the +/- dB wobble across the IF band is not a surprise.  The level falling off with increased frequency is also not a surprise given the board loss with the LO at 3.2GHz for the higher RF input cases. 

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.

Prj133 Mixer at 315MHz IF

Admittedly, using a 10.7MHz IF is not very challenging, however, it was a good first step to understand the PCB, the LTC5510, and the balun responses.  The next step was to use a higher IF.  315MHz was chosen due to the wide spread availability of low cost SAW filters.  The following captures that construction.

Initial measurements showed what appeared to be some shunt capacitance between the differential output lines.  Based on frequency response roll offs it looked like about 2pF.  This is higher than I had hoped.  I cut the traces to the pads for the high frequency SMT balun hoping this would alleviate some of that (it did not).  The DC feed chokes for the differential outputs were reduced from 1uF to 39nF in an effort to use them to cancel the shunt capacitance.  The table of final values used is shown below.

4:1 315 MHz Balun Part	Value	Comments
C105/C113	1nF	DC blocking capacitors
L102/L103	39nH	DC supply chokes
L107/L108	47nH	4:1 Lattice Balun inductor
C106/C107	5pF	4:1 Lattice Balun capacitor

 
Measurements were taken with a 1220MHz RF input at -15dBm while the LO was high side swept.  Those results are shown on the following graphs.

Based on the above figure the mixer gain is on the order of a dB or so at the frequency response peak.  This is within expected results all things considered (the uncertainty of the test equipment is +/-2dB).  The LO bleed through is roughly -40 dB while the RF bleed through is -25dB.  The data sheet at this supply voltage and with these frequency ranges indicates both should be <= -40dB.  The LO is fine while I suspect the RF to output port isolation is lower than the device due to PCB level issues (layout/proximity/groundplane on a routed 2 layer board).

I have noticed that a higher frequencies it is difficult to get enough power into the LO port.  The dog leg input is not helping me here but was only for early testing and sampling.  I suspect the "stray capacitance" I thought I was tuning out was a function of measurement uncertainties and LO power input roll off.  The final version will populate the on board synthesizer which has a nice impedance controlled direct differential input with the necessary power output. At this point the behavior is good enough and within expectations to move on to using the high frequency output balun.