Radio amateur Peter "DK7IH" Rachow has published a new build, this time taking a four-year-old pocket-size transceiver and applying the skills and knowledge he's built up since to re-engineer and improve the design.
"When the project of building a very small transceiver was accomplished 4 years ago," DK7IH writes, "I still lacked lots of skills in setting up electronic circuits using SMD technology. The radio's craftmanship had been a defective to a certain degree (there were still lots of things to learn when using SMDs on Veroboards), the inside looked more or less 'messy' and the performance was not sited in the premium league."
"Particularly the receiver was prone to IMD [intermodulation distortion] problems when signals on the band were strong. But because I liked the outer appearance of the radio a total revision of the inside had to be performed."
The core concept behind the "shirt-pocket transceiver" remains unchanged: The design features a 14MHz frequency layout, a Microchip ATmega328P microcontroller for control and to run an on-board OLED display panel, and uses the same compact housing.
"The major changes that were used to improve the radio are: Usage of a Si5351 clock oscillator as VFO and LO instead of an Xtal controlled LO and AD9835 as VFO," DK7IH explains. "Only one SSB filter (commercially made) instead of 2 ladder filters; Dual-Gate MOSFET as 1st mixer (instead of SA612); 2SC2078 as push-pull pair in the final TX stage."
"All SMD components are now mounted to the underside of the board; TBA820M instead of bipolar equipped push-pull audio amplifier; Cabinet size has been enlarged slightly (about plus 0.5 cm in length); Proper cabling instead of 'spaghetti' arrangement; Copper band has been used to improve radio frequency grounding."
The revisited project comes nearly half a year after DK7IH completed an impressive high-frequency six-band single-sideband (SSB) transceiver build, designed to be suitable for experimental projects and based around a Microchip ATmega128. The full build log, including schematics and a clever tip on printing front-panel labels, can be found on DK7IH's blog.