Saturday, November 1, 2014

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.

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