Traditionally, in a crystal detector radio tuned circuits, a mechanical type variable capacitor is used. For those of you who would like to eliminate this mechanical component, here is a modern version of the classic detector set. This radio, as shown on Figure 1, uses a varactor diode instead of the usual mechanical rotary device. The varactor is also known as a variable capacitance or a varicap diode. It provides an electrically controllable capacitance, which can be used in many different circuits. Varactors are small and inexpensive, which makes their use advantageous in many applications. Its disadvantages are a lower Q (quality), nonlinearity, lower voltage rating and a more limited capacitance range. A tuned circuit with a higher Q has a narrow pass-band that makes it better able to pick out a station of many equally strong. A lower Q tuned circuit has a wider pass band. It allows more neighbor stations through and makes listening to either radio stations frustrating. Frequency change with a varactor diode equipped tuned circuit is as simple as a voltage change.
Build Accurate LC Meter and start making your own coils and inductors. This LC Meter allows to measure incredibly small inductances making it perfect tool for making all types of RF coils. LC Meter can measure inductances starting from 10nH - 1000nH, 1uH - 1000uH, 1mH - 100mH and capacitances from 0.1pF up to 900nF. The circuit includes an auto ranging and reset function to make sure the readings are as accurate as possible ... [more]