Build Log
I was looking for a first home-brew project when Farhan announced the Minima and ended up being one of the early builders. I bought the first parts and started building on January 24th 2014 and listened to my first QSOs on February 27th.
Building and Testing the Crystal Filter
I used an Arduino connected to a AD9850 DDS board to generate an input signal and characterize the crystal filter (source code of my signal generator). I did not have the equipment to characterize the crystals properly so the selection was just random.
Lessons learnt
How to properly measure RF power with an oscilloscope. The importance of 50 ohm impedance matching. How to measure the output impedance of a generator. Building an LC network for impedance matching. 50 ohm termination.
Winding My First Toroids
I got myself an AADE L/C Meter to measure inductance. For $100 you get a very nice kit to build in one quick evening including front panel, buttons and decals. Don't buy one of the cheap LCR meters on Amazon — most of them cannot measure anything smaller than a few nano-farads or micro-henris. We work with much smaller values in RF.
Winding the toroids is not very fun but it is pretty easy. I have found toroids.info to be a great resource.
| Value | Qty | Core | N-Turns (Mark) | N-Turns (Thomas) |
|---|---|---|---|---|
| .33 uH | 2 | T37-6 | 10 | 9 |
| .39 uH | 2 | T37-6 | 11 | 10 |
| .66 uH | 2 | T50-2 | 12 | 10 |
| .78 uH | 3 | T50-2 | 13 | 11 |
| 3 uH | 3 | T68-6 | 25 | 24 |
| RFC | 3 | FT37-43 | 25 | 24 |
Building the Low Pass Filters
Glued the toroids in place, soldered them and adjusted inductance before locking with nail polish. Used QRPme MePads.
Building the Main Board
Calibrating the Oscillator
When I first started my Minima, the output frequency was far from spot-on. I did some research and made a pull-request to solve this problem: Wiki page on Si570 Calibration · Pull-Request to add auto-calibration at startup.
You still need to tell the software what the factory calibrated frequency of your Si570 is.
Building the KISS Mixer
Make sure you wind those transformers really well. Use a drill to wind the three wires into a trifilar winding. I have tried lots of other methods but trust me, a small drill with a hook is the best solution.
Building the BFO Mixer
Followed Farhan's instructions to match the diodes. I used my Fluke high-resolution mode (0.1 mV) which was probably overkill because I realized after having measured 20 diodes that just blowing on them or touching them would change the reading by several millivolts. If you can get them to match at +/- 1 mV, you should consider yourself very happy!
Building the RF Pre-amp
Nothing special here. Just a bunch of transistors. I did manage to get the last output resistor wrong (22 ohms instead of 220 ohms). I was wondering why that last transistor was heating so much... I realized my mistake when a few days later the resistor started getting brown.
Building the BFO
Building the Audio Pre-amp
Building the Audio Amp
Used an LM386.
Debugging the Minima in RX
When I put everything together, my Minima showed very little sensitivity. I could hear a strong signal that I injected from a VFO in the antenna — or even another emitter a few meters away — but I could not get any real signal and the bands were awfully quiet. It did not sound like a "receiver".
I went through each stage, measured gains, checked bias on the transistors, compared with other builders, rewound my transformers 3 times, etc.
Long story short: I had misplaced one capacitor in the BFO. Instead of putting a 100pF between Q9 base and emitter, I had put it between the base and the collector. The result is that the BFO signal was much weaker than it should have been. Once I fixed this:
Crystal Filter on the Spectrum Analyzer
I secured access to a spectrum analyzer (Rigol DSA-815-TG). With almost 6 kHz bandwidth, it's probably going to be a bit too large and I will have to re-work it.
Building the TX Stages and Boxing the Minima
I added the TX pre-amp and the Mic pre-amp.
With everything on the board, it is starting to get crowded:
I don't really do CW at the moment but I decided to include the CW tone generator anyway. There is a lot of fun stuff we could do with Arduino + CW. Next piece is the RX/TX relay and the few transistors that go around it.
Finally I did some metal work and wired all the control signals between the Minima and the RF Board.
At the moment my Minima is stuck in transmit but this is a known problem (you need an external pull-up resistor on the PTT line). I had not done any real testing of the TX stages so I just plugged the Minima in the SA and looked at the output.
This is what a 28.200 MHz signal modulated with the CW tone looks like. The peak at 48 MHz is the local oscillator and it should be much more attenuated.
