New Cores for the RF MIxer

After building my Farhan Minima RF Mixer, and before taking it to Jack's LAB for performance evaluation, I replaced the two large cores with much smaller BN-43-2402 binocular cores. I had originally planned to use the smaller cores but I could not find my roll of very small wire with which to wind them, and therefor the first configuration sported the larger cores wound with 22 AWG wire-wrap wire. The RF Mixer worked, but . . .

Old Large BN-43-302 Cores

The new smaller cores fit the board much better, and they have also worked well on my BFO SSB Modulator/Detector board as well.

New Small BN-43-2402 Cores
as Original Planned

The small cores with the small wire are more difficult to wind, but with patience it can be done. The circuit requires eight turns of trifilar wire. With the small wire that I have (I do not know the gauge) only eight trifilar turns will fit (the circuit requirements are met).

One neat trick that I learned (in the distant past); before winding a DIY bifilar or trifilar wire together, stain one (or more) full length wires with colored Marks-a-Lot pen(s). The enameled magnetic wire takes stain reasonable well. A large felt tip pen and a slightly stretched (taught) wire makes the task easy. After twisting the wires and winding the cores, the stain will help make end identification much easier.

If I remember correctly, pre-stained filer wire is available for purchase, but I like DIY colored stained trifilar wire, because I can use any gauge of wire that I have available.

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UPDATE
Based on the online Toroid Calculator, I may have only needed 4 turns on the BN-43-2402 Binocular Cores.

UPDATE
I had previously mis-published the part number of the Binocular Cores that I am using, the number and links have been corrected.


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Farhan Minima - SSB Modulator/Detector

My order of Dual 1N4148 diodes (MMBD4814SE) were received in the mail. I created a Double Sided Toner Transfer PCB and loaded it with parts. See previous post.

This is my implementation of one of the Farhan's Minima Transceiver modules.

SSB Modulator/Detector
Double Sided Toner Transfer PCB
1x1.2 inches

The Dual Diode is the small SOT-23 device in the center right, the binocular core mixing transformer it center left, and the RFC is the binocular core in the lower right.

SSB Modulator/Detector

Soon I will have enough connectors and adapter to connect everything together in a nice neat test string of modules.

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Minima Transceiver Module Testing

Jack - WA7KMR and I spent some time in his Lab, using his Test Equipment to characterize the first few modules of my homebrew implementation of Farhan's Minima Transceiver. We tested my; RF Mixer, Crystal Filter and IF Amplifier (see previous post).

Jack at the Controls

We first connected each module as a standalone device to ensure each worked as expected.  We were both most interested in the Crystal Filter performance measurements. My previous measurement attempts with the AIM-4170B were inconclusive, other than the shape of the bandpass.

Crystal Filter

With Jack's Lab Equipment we measured the bandpass and insertion loss of the Crystal Filter. The bandpass is 6KHz, and insertion loss is about 3db, with about 2db of ripple. With some tweaking, I think the ripple could be reduced.

We later checked the operation of the standalone RF Mixer, and then the IF Amplifier. The RF Mixer worked similar to one of Jack's known mixer. The amp provided about 30db of gain at 20MHz.

We connected all three modules together and I very pleased with the results.

RF Mixer, Crystal Filter, and IF Amplifier

Output of the IF Amp
Horizontal = 20MHz Center,  5KHz / Div
Vertical  = -30dbm at top line, -110db at bottom, 10db / Div
LO was 200mV RMS
The RF Signal was Detectable down to -90dbm

The Results: I am a very Happy-Camper.

More of my "Minima" module implementations will follow.

Thanks Jack, for the measurement help.

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Mixer and Amp Performance Measurement

I took my Homebrew Mixer and Amplifier (see previous posts) to Jacks Homebrew Meeting tonight. I was hoping that Jack and his Lab could help provide meaningful measured performance data.

Jack has several very nice pieces of LAB Grade test equipment. Unfortunately, his tracking Oscillator-Spectrum Analyzer was missing a cable or not working, and could not be used.

But with other equipment, we were able to spot check performance with an HP Oscillator with Attenuator and a HP Frequency Sensitive Voltmeter with its Attenuator. The two attenuators agreed within 0.25db. We also had a standalone attenuator for gross signal level adjustment.

The results; the Amplifier measure about +30db gain from 1 to about 27MHz (which was the highest Freq the Oscillator was calibrated for). The Amplifier would "quiet" the Voltmeter with as little as 115dbm input signal. Actual Noise Figure performance was not measured. This overall measured results are similar to what LTSpice suggested, but less than I measured at a Load on my Oscilloscope. NOTE: I may be mixing in my mind; Power Gain with Voltage Gain performance data, I may need to rethink this measurement.

The Mixer performance data and method was less precise, and accurate measurement will have to wait until Jack has his other equipment working.

More performance testing is necessary.

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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

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