rf digital


Posted on Feb 6, 2014

It is important yet to mention that the 74HC240 can drive directly an IRF510 FET transistor being able to give until 10W. Another application is using it to drive a balanced diode mixer, which needs a considerable power drive, easily supplied by a 74HC240 gate. We did some tests with a Philips 74HC240, with a cost of almost R$1. 00 (U$0. 30 in sept/


rf digital
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2003), we use also the 74HCT240 with the same results, we also used a 74HC244 (SMD) as a buffer because it does not work as oscillator (it is not an inverter gate, but it has the same pin numeration as the 240), and the tests with the 74F240 showed this type does not work in this conditions! Another important fact is that each gate output impedance is about 16 Ohms, that is, connecting 4 in parallel we will have around 4 Ohms output, the matching is usually done with a low pass tuner with a low Q (around 1) or even with broad band transformers or trifilar coils. The digital oscillators generate square waves which are not proper for transmitting and most of the mixer, but filtering them with low pass filters we get rid of the harmonics (for the one who does not know: the square waves are formed by the combination of a wave with its odd harmonics = getting rid of the harmonics we get the desired senoidal wave). Another interesting fact is the use of EXCLUSIVE-OR - XOR as mixer for square waves, easing its use in super heterodyne receivers and the known universal VFOs. For the one who likes experimenting, this work with digital signals was always scarcely explored. We built this project in about 3 hours and by the end of the tests we got a QSO with Rio de Janeiro, with 0. 3W getting a 579. The construction was done by the dead bug method and the rig was stuck (using thermal conductor goo) on an aluminum board, serving as a...




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