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SDT21 simple digital transceiver

Product no.: SDT21

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simple "digital" monoband transceiver SDT21

The goal of this project was the development of a simple to build CW monoband transceiver based on the direct superheterodyne principle with crystal control a la "PIXIE" or "Rockmite", but with much better technical data. To minimize the circuit effort, the concept should be as highly integrated as possible and realizable without hand-wound coils. Furthermore, for the SMD phobics, SMD placement was completely omitted this time. Despite these specifications, it was possible to fit the entire circuit on a double-sided PCB with dimensions of 69x52mm, which allowed it to be installed in the small TEKO A2 package.

The main focus of the circuit is based on the little known fact that digital inverters or gate devices of the 4000 CMOS series can be operated as analog amplifiers by a circuit trick, important here is the use of the unbuffered IC variant (index UB). Related information can be found in Appendix 5.

The usual buffered version (Index B) has an additional buffer stage after each input and before its output stage, which improves the switching behavior in digital operation, but leads to uncontrollable oscillation tendency in analog operation because of the much higher stage gain. For the same reason, devices of the 74HC series are not suitable for this special application.

With two CMOS-ICs of the type CD4011 a total of 8 gates are available with which the important function blocks of the receive and transmit part of the SDT 21 are realized in a simple way.

The buffered IC version (CD4011B) can be used in the transmit section, since all stages are fully digital.

Due to the relatively small size of the project we have omitted the usual and extensive introduction as well as a division into different assemblies.

Please order the desired "set of band specific parts" according to the desired frequency!

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