Direct-conversion indicates that the desired signal is mixed directly down to baseband using a local oscillator (LO), with no intermediate steps. Double-sideband means that signals above and below our local oscillator frequency are superimposed on each other and output at baseband. Because of this, a DSB receiver can be used as an AM receiver, though we were only using it to receive single-sideband communications.
The design we chose to build was based on the Porta 40, from the November 2012 edition of Amateur Radio Australia by Peter VK3YE. The schematic for the circuit is not online, but an explanation is on YouTube and a similar design is the DC40. The components for the design could be purchased from a local electronics store, which made it attractive for fast prototyping and experimentation. As we couldn’t obtain 7.2MHz resonators quickly, we used 7.158MHz crystals. This limited our tuning range, but this was acceptable for our purposes.
With the parts in hand I built it up on some blank PCB material, starting with the local oscillator. While I built the LO Mark wound the transformers that were required for the mixers. We tested the LO with David’s old HP oscilloscope and found it resonating at 7.143MHz, close enough to the 7.158MHz we were expecting.
The pre-amp was pretty straight forward, with the exception of a mistake in the published schematic where the Zener diode was drawn with reversed polarity.
Next up was the mixer. The hardest part of building this part was remembering which of the three windings had to go where. I recommend using different coloured wire for the windings, which reduces confusion and allows the design to be checked before powering on.
Finally the audio amp was built, and we tested the entire setup by connecting to the antenna on a pole, and the output to the soundcard on Mark’s laptop. Using some SDR software (Spectravue), we were able to tune to a SSB voice transmission right at the edge of our 24kHz of bandwidth provided by the soundcard. Not only had the design worked first time, but we were about to hear two operators from Victoria having a conversation using the SDR software!
Aside from using it as a single frequency analog voice receiver I plan to use this radio as a downconverter to demonstrate a SDR based implementation of FreeDV of Android.
Thanks to Peter VK3YE for publishing his design, and to David VK5DGR and Mark VK5QI for their help building and testing the radio.