Thursday, February 6, 2014

The Farhan Minima RF Mixer - Cont'd

With a little work, I was able to shrink the PCB for my mixer (see previous post). The smaller coils, smaller JFET foot prints allow for optimization and part placement. The PCB has be reduced to 1.0 x 1.4 inches.

Etched, Solder Wiped and Drilled,
Ready for Cut and Parts
The Completed Mixer
The Two J310 JFETs are located in the Center

JFET Bias Circuit is on the Left
The Two Coils are, of course, Obvious 
Testing will follow as tests are devised.

I spent some more time with LTSpice. I changed the LO from a small sign wave to a 70mV square wave, it works much better. Then I could correctly observed the results via an FFT plot. For the simulation I used a frequency of 27MHz for the LO and 7MHz for the input signal. As can be seen in the FFT plot, the output contains the desired 20MHz signal and lots of higher harmonics. The 20MHz peak is the difference (27 - 7 = 20) and the 34MHz peak is the sum (27 + 7 = 34), the 27MHz LO is suppressed, as it should be.

Lower Left is the Farhan Mixer Circuit
Upper Left is the Full FFT Plot
Upper Right is the Expanded FFT Plot Centered on 27MHz
Lower Right is the Output Plot at the 50 ohm Load
Note: I think a normal Diode Ring Mixer requires about 700mV P (~7dbm) of LO input, or something greater than the 0.6v to turn on the diodes. I think for this mixer the JFETs can use a much smaller LO signal.

I really like working with LTSpice.


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