40M DSB Transceiver

du1vss's picture

I was inspired by the MDT40, a double side band suppressed carrier transceiver designed by VK2DOB because of its simplicity and how he simplified the use of a balanced modulator. His circuit uses a balanced modulator that serves two purpose that is to generate a double side band signal during transmit and also serves as a demodulator to detect incoming ssb signal during receive. Another advantage of the dsbsc is that it does not need a mechanical filter like the one found in a typical ssb transceiver. A dsbsc transceiver is able to communicate to any ssb transceiver without noticing any difference from the resulting signal and audio, the only thing is that a dsbsc is generating and receiving on both sidebands.

Although the design is straightforward, I am having difficulty in sourcing out the parts employed in the MDT40. The first is the 7.2MHz ceramic resonator in the oscillator stage. Having a crystal instead of a ceramic resonator offers just a few hertz of pulling the frequency so I find this insufficient for its intended use. Most of the parts in the power amplifier section uses transistors that aren't available here in Manila so my circuit version uses a locally available transistors. I found also that using individual boards for each stage will make the construction and testing more easier.

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