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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The Farhan Minima RF Mixer

I have been wanting a simple Mixer Circuit for experiments in my electronics shop, and most recently for use with my Small Signal Amplifier (see previous post).

Alan - K6ZY at the last Puget Sound QRP (pQRP) meeting mentioned he was considering building a Homebrew Farhan VU2ESE Minima Transceiver project.  After looking at the schematic for the Mimina, it appears that the Mixer Circuit would work very nicely as an stand-alone mixer for my experimental use. Very good documentation of the mixer's operation is provided by Farhan. The mixer used in the Minima has been dubbed the “KISS Mixer” by Chris Trask in his paper.

Minima Mixer Circuit in LTSpice
(See file below)

To play with the idea of building an experimental mixer, I created a LTSpice simulation circuit to start my understanding of it operation.

So far I have not gleamed much information from the LTSpice simulation, but maybe I have something wrong with my circuit; bias, configuration, signal levels or expected output. Or, maybe LTSpice can not deal with mixed signals correctly, but  I will continue working with LTSpice to learn more.

In preparation for my experiment and use of the Mixer Circuit, I have started the initial design of a PCB using DipTrace. Keeping with my ever present Goal of building electronic projects as small as I can, this initial design is larger than I think the final design will be, as I have currently configured it to used larger components than necessary. I think I can find small SOT-23 J310 JFET packages and smaller transformer cores. But this is a start.

Mixer Layout in DipTrace

 I have configured the PCB as a single-sided circuit as it can be produced with simple Homebrew Toner Transfer Method. But perhaps, using double-sided will be best for reducing the over all size.

This mixer as a stand-alone device will be very useful in my electronics shop - Thanks Farhan.

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UPDATE: Aug 7, 2014 07:46

Here is my "SpiceMixer01.asc" file that I have been playing with.

Version 4
SHEET 1 880 680
WIRE -224 -352 -704 -352
WIRE -160 -352 -224 -352
WIRE 144 -352 -80 -352
WIRE 256 -352 224 -352
WIRE 256 -336 256 -352
WIRE -224 -320 -224 -352
WIRE -704 -288 -704 -352
WIRE 304 -288 224 -288
WIRE 144 -272 144 -288
WIRE 256 -272 144 -272
WIRE -224 -224 -224 -240
WIRE 144 -224 48 -224
WIRE 256 -224 256 -272
WIRE 256 -224 224 -224
WIRE 464 -224 256 -224
WIRE 544 -224 528 -224
WIRE -704 -192 -704 -208
WIRE -576 -192 -704 -192
WIRE 48 -192 48 -224
WIRE 304 -192 304 -288
WIRE -704 -160 -704 -192
WIRE -224 -144 -352 -144
WIRE -352 -128 -352 -144
WIRE 0 -128 -32 -128
WIRE 400 -128 352 -128
WIRE 544 -128 544 -224
WIRE 608 -128 544 -128
WIRE 544 -96 544 -128
WIRE -352 -80 -352 -128
WIRE -224 -32 -224 -64
WIRE -144 -32 -224 -32
WIRE 48 -32 48 -96
WIRE 48 -32 -64 -32
WIRE 176 -32 48 -32
WIRE 304 -32 304 -96
WIRE 304 -32 176 -32
WIRE -224 0 -224 -32
WIRE -352 32 -352 0
WIRE 544 32 544 -16
WIRE 400 128 400 -128
WIRE 400 128 224 128
WIRE 144 144 144 128
WIRE 256 144 144 144
WIRE -32 192 -32 -128
WIRE 144 192 -32 192
WIRE 256 192 256 144
WIRE 256 192 224 192
WIRE 320 224 320 192
WIRE -224 256 -720 256
WIRE -176 256 -224 256
WIRE 144 256 -96 256
WIRE -224 288 -224 256
WIRE 224 304 224 256
WIRE -720 320 -720 256
WIRE -224 400 -224 368
WIRE -720 432 -720 400
WIRE -560 432 -720 432
WIRE -720 448 -720 432
FLAG 176 32 0
FLAG 224 304 0
FLAG -352 32 0
FLAG 256 -336 0
FLAG -224 -224 0
FLAG -224 400 0
FLAG -352 -128 Bias
FLAG -224 -352 SigIn
FLAG -224 256 LO
FLAG 608 -128 SigOut
FLAG 544 32 0
FLAG -704 -80 0
FLAG -720 528 0
FLAG -576 -192 SampleSig
FLAG -560 432 SampleLo
FLAG -224 80 0
FLAG 320 224 0
SYMBOL ind2 128 144 R270
WINDOW 0 40 34 VTop 2
WINDOW 3 65 78 VBottom 2
SYMATTR InstName L1
SYMATTR Value {Lo}
SYMATTR Type ind
SYMBOL ind2 128 208 R270
WINDOW 0 34 32 VTop 2
WINDOW 3 60 86 VBottom 2
SYMATTR InstName L2
SYMATTR Value {Lo}
SYMATTR Type ind
SYMBOL ind2 128 272 R270
WINDOW 0 35 38 VTop 2
WINDOW 3 63 77 VBottom 2
SYMATTR InstName L3
SYMATTR Value {Lo}
SYMATTR Type ind
SYMBOL ind2 128 -336 R270
WINDOW 0 38 38 VTop 2
WINDOW 3 63 78 VBottom 2
SYMATTR InstName L4
SYMATTR Value {Ls}
SYMATTR Type ind
SYMBOL ind2 128 -272 R270
WINDOW 0 34 29 VTop 2
WINDOW 3 60 76 VBottom 2
SYMATTR InstName L5
SYMATTR Value {Ls}
SYMATTR Type ind
SYMBOL ind2 128 -208 R270
WINDOW 0 32 38 VTop 2
WINDOW 3 61 80 VBottom 2
SYMATTR InstName L6
SYMATTR Value {Ls}
SYMATTR Type ind
SYMBOL njf 0 -192 R0
SYMATTR InstName J1
SYMBOL njf 352 -192 M0
SYMATTR InstName J2
SYMBOL cap 160 -32 R0
SYMATTR InstName C1
SYMATTR Value .1uF
SYMBOL voltage -352 -96 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5V
SYMBOL voltage -224 -336 R0
WINDOW 3 24 44 Left 2
WINDOW 123 24 72 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value SINE(0 10uV 7Meg)
SYMATTR Value2 AC 1mV
SYMATTR InstName V2
SYMBOL voltage -224 272 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value PULSE(0 70mV 0 0 0 {PulseWidth} {Period})
SYMBOL res -176 -336 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 -5 56 VBottom 2
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL res -192 272 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 -5 56 VBottom 2
SYMATTR InstName R2
SYMATTR Value 50
SYMBOL res 528 -112 R0
SYMATTR InstName R3
SYMATTR Value 50
SYMBOL res -720 -304 R0
SYMATTR InstName R4
SYMATTR Value 100K
SYMBOL res -720 -176 R0
SYMATTR InstName R5
SYMATTR Value 1K
SYMBOL res -736 304 R0
SYMATTR InstName R6
SYMATTR Value 10Meg
SYMBOL res -736 432 R0
SYMATTR InstName R7
SYMATTR Value 1K
SYMBOL res -160 -16 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R8
SYMATTR Value 4.7K
SYMBOL res -240 -16 R0
SYMATTR InstName R9
SYMATTR Value 8K
SYMBOL res -240 -160 R0
SYMATTR InstName R10
SYMATTR Value 2K
SYMBOL cap 464 -208 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C2
SYMATTR Value 10n
TEXT -512 -280 Left 2 !.tran 0 10uS 9uS
TEXT 392 -416 Left 2 !.param Ls = 10000nH\n.param Lo = 10000nH\nKo L1 L2 L3 1\nKs L4 L5 L6 1
TEXT -800 -408 Left 6 ;WA0UWH
TEXT -272 448 Left 2 !.param Freq = 27Meg\n.param Period =  1 / Freq\n.param PulseWidth = Period / 2 * 1.05

-- Home Page: https://WA0UWH.blogspot.com

Small Signal RF Amp - Rev 03 - Cont'd

Update added at end of post.

I connected a reasonably calibrated 100uVolt RMS 50 ohm RF source to the input of my Small Signal RF Amp (see previous post). The output of the amp with a 50 ohm load was 20mVolts P, for a Gain of about 43db. Similar gain was seen at frequencies from 1 to 26 MegHz, with more on the lower frequencies, and less gain on higher frequencies. Currently, I do not have proper equipment to do a complete characterization of the amp.

Test Setup Show 20mVolts  RMS
With 50 ohm Load On the Left
100uVolt  RMS Source from Sig Gen on the Right

I am a happy camper !

I think this amp will be used in several of my future projects, stay tuned!

The amp was originally designed for 12 volt supply, but I think a local zener diode regulation at 9 volts (or battery) will be useful for use within an actual project. Regulation will help avoid supply side noise from effecting the performance. The above test was done with a 9 volt battery.

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UPDATE: Feb 04, 2014

I checked with the LTSpice Simulation, it suggests with 100uV RMS at 10MHz as input, I should only see 5mVolts P at the output. I am measuring about 4 times that as reported above, I wonder why?

More investigation needed.


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Small Signal RF Amp - Rev 03

Update added at end of post.

I corrected the layout of my Small Signal RF Amp as described in the previous post. This time I printed both the Front and Back side of the layout for use with the Toner Transfer Method.

This photo of the etched board was taken with back light, which shows good alignment with the front side hole images. Actually, only two holes will be drilled, they will be used to mount the power header.

View through the board via Back-Light

The Front of the Completed Board

The Back

Now for some performance tests.

But, for a quick test, I tried it as an input amplifier for a receiver, it passes signals, but more proper testing is needed. I am sure it will work best as an IF Amplifier.

Some Project Background

For a transfer, I normally use a modified laminator (modified to set higher temperature at 350F or 176C). But for this project I wanted to use the Clothes Iron method. My previous attempts to use the clothes iron were less than satisfactory, probable because I did not know what I was doing.  Now with more knowledge and understanding of what makes for a good transfer, the results speak for it self. It is all about; Board Preparation, the right Temperature, Time, Pressure and Transfer Media. I will go into the details in a future post.

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UPDATE: Feb 2, 2014

I just noticed an "0" was missing from the date/time on the back side image, but then it was missing on the upper back-light image also, which became the finished board. It must have just not transferred - go figure?


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Small Signal RF Amp

I have had an idea for an small signal RF Amplifier that I want to try. LTSpice suggest the gain will be good and the linearity appears to be good, or at least that is what the LTSpice FFT plot suggests. This Amp is similar to those that I have used in my Digital 15 Watt Power Amp, but biased for linearity.

My Goal for this circuit is Low Part Count and good Performance. And, as always, a major goal for all of my projects is to make them as Small as my Eyes, Nerves and Methods permits. Yes, I know there are published circuits that may work better, but this is my attempt.

This layout is a single-sided board, with ground plane on the backside. The SMA connectors, connects all ground planes together at the circuit board edge. A layout for a commercial manufactured board would include many via's to assist with ground plane connections.

The first Toner Transfer (TT) version of the circuit had a schematic error (my screw up, I was just to quick on the print button). The second version is shown here.

Etched and
Toner was Removed Underwater with Scotch Bright

Solder Wiped
and Ready for Cut and Part Installation

But alas, I made the power pads too small and they pulled up with only a slight tug on the cable, layout will need to be re-done.

Circuit Complete
with Jury Rigged Power Cable
The PCB is 0.6 x 1.1 inches

The circuit performance tests will have to wait until tomorrow. I will have to devise some interesting tests. Noise Figure, Gain and Large Signal behavior are things I am interested in checking.

On this Rev 02, I forgot to add a Power LED, so I am now working on Rev 03, which will correct all of the known layout problems.

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