The Farhan Minima RF Mixer - Cont'd 2

If you look at the schematic of the Mixer shown as used with LTSpice (see previous post), you will see the center of the lower transformer is not grounded. The original Farhan circuit grounds this point. When I first copied it, that is, created the circuit within LTSpice, I inadvertently forgot the ground connection.

But the LTSpice simulation worked as expected so I assumed the circuit was correct. Later while cross-checking the circuit for another issue, I noticed the forgotten ground. But, then when the transformer ground was connected, the LTSpice simulation did not show the expected 20MHz output via the FFT. Now I am really confused.

To allow the created circuit on the PCB to be tested with and without the transformer grounded, I inserted two Zero Ohm resistors that will be left out of the ground path for initial testing. I am sure they will be needed as I am sure Farhan knows his circuit much better than I.

Without the DC ground path I wonder how the LTSpice solution is discharging the electron build up from the effects of rectification at the junction of the JFET. Perhaps the other JFET junction provides a DC leak to ground. Regardless, I have something to do experiments with.

Also, I laid out the circuit board for smaller coil forms (BN-43-2402), but I could not find my 36 AWG wire to wind the cores.  When found, I plan to load another PCB with the small cores.

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UPDATE: Feb 14, 2014
I found a role of #26 AWG wire and decided to try to wind a tri-filer winding on the very small BN-43-2402 binocular core. I was only able to get about three turns on the core, I needs 8 turns for this project. I will continue looking for my much smaller wire.

UPDATE
I had previously mis-posted the part number of the Binocular Cores that I have used. the number and links are now correct.

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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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A new Audio Amp for DC Receiver

With my new found abilities of home PCB construction (see previous PCB posts), I have put together the circuit layout for a Audio Amp for my Direct Conversion QRSS Receiver. It has not tested yet, but I am hopeful, . . . (that it does not turn out to be an oscillator?).

 Audio Band Pass Amp using LM324N

Yes, there are four jumpers on the board, plus one under the IC, I did not want to make this a two sided board.

And if you look very close, you can see I forgot one two solder joints on the capacitors to the right of pin 8, and 14.

Forgot Solder Near Pin 8 - This is an Easy Fix!
Note: These are 1210 pads with 1206 parts, also note that there are traces or Ground Plane under many of the parts, it is easy to create solder paste shorts under the parts - as I have found out!

Now for some testing and circuit check out!


UPDATE 1:  Well, . . It is, . . . . an Oscillator! . . . . . I will have to do some more checking on the layout, and maybe some "LTSpice" simulations. Maybe it is just a short, or a miss mounted part - alot more checking is needed.

UPDATE 2:  I think I need to look for another circuit for my "Band Pass" audio amplifier, the circuit that I used was cobbled together without much thought. What I need is about 60-80db of gain with; 200 to 2000Hz Band Pass.

UPDATE 3:   A very old Navy friend of mine, Richard Davis, after seeing the photo, offered this bit of advice on FaceBook: "Don't use Taiwanese electrolytics! ;-)". He must be very observant, as I used the Caps that I had in the junk box. Maybe, I should look at the selection of Caps closer, or maybe the International friends will also have comments.

I found the following Active Band Pass Filter Calculators:

http://www.wa4dsy.net/robot/bandpass-filter-calc

http://www.captain.at/electronics/active-filter/

UPDATE 4:   I found one layout error, the output feedback cap of the third stage op-amp was connected to the bias line and not the the inverting input cap as it should. That would make the op-amp oscillate!

I may change the component values used to implement a 6 pole Chebyshes Filter as the Filter Calculator above suggests, the schematic and layout stays the same (after I fix the layout error found above).

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