Less is more, part 1: hearing is believing
by Antony Chazapis
The word “minima” in Greek stands for “message”. A nice name for a transceiver. However, I guess Ashhar Farhan, VU2ESE (of BITX fame), was thinking of “minimal” when he chose the name for his new design. Excluding the Arduino-based interface and the use of an Si570 for the local oscillator, all other components are common and easy to find. No integrated circuit for the mixer and no one-chip audio amplifier, but transitors, resistors, capacitors, diodes and toroids. A clean, modular architecture. The minimality of character in hardware extends to the user interface as well: a potentiometer to change the frequency and a single button to perform all other functions. Clever.
A simple transceiver, but a complex project nevertheless. I decided to start and go as far as I can. This will undoubtedly be an educating and rewarding process. I am thrilled just by the thought of making a contact with a transceiver built by myself!
I tried to make YO4HHP’s PCB, but it did not come out well. I think that it had to do with the gap between the copper traces being too thin for my PCB lab. I was disappointed by the result and did not try for a second time. Instead, I went on to design a new set of PCBs. Having the schematic already in Eagle format proved extremely helpful. I though that it would be better to build the transceiver in parts, completing and testing each one, before moving on to the next. So, I started with the audio modules.
The first board contains the CW tone generator, the microphone amplifier and the audio final amplifier, as discrete circuits. These, in turn, are connected through the second board, which provides transmit/receive switching and the audio pre-amplifier, used to boost the received signal after demodulation.
After etching and populating the PCBs, I connected a speaker, an audio potentiometer, a microphone, and power. The audio amplifier worked well with my iPod, although the maximum volume was not enough to fill the room with music. I even tried with another speaker. Putting the audio pre-amplifier in the path did not make any significant difference. I can not really be sure of its function without proper tools, like an oscilloscope. The same applies to the microphone preamplifier. The CW tone generator produced an audible output with 5V at the input.
The next instalment will be on the interface. I have already started altering the original code for a slightly different layout of the elements on the LCD. I also plan to use a rotary encoder for frequency selection and one of those cheap AD9850 modules for the local oscillator, in place of the Si570. Until then, check out WA0UWH’s blog, who has ventured on an unbelievable quest to build the Minima from tiny boards, using only surface-mount parts, while documenting each step in detail.