Automatic Preselector

A PC-controlled receiver is very convenient, especially for DRM, since it makes it easy to locate your favourite stations. However, this advantage is lost if you have a manual preselector and must tune the antenna by hand: an automatic preselector is what you need!With a PLL-controlled receiver things are very simple. The PLL tuning voltage can be used simultaneously to control the first RF stage. Instead of one varicap diode two are used. If DDS tuning is used, then although we have better phase purity, which is a decisive advantage in DRM applications, there is no tuning voltage available for the first stage. What is needed is a circuit to generate a suitable tuning voltage automatically.