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![]() Consumer AM-FM broadcast tuner with varicaps highlighted The three leaded devices depicted at the top of the page are generally two common cathode connected varicaps in a single package. Some designs of electronic security tag readers used in retail outlets require these high capacitance varicaps in their voltage-controlled oscillators. These newer devices allow electronically tuned AM broadcast receivers to be realized as well as a multitude of other functions requiring large capacitance changes at lower frequencies, generally below 10 MHz. In time, varicap diodes were developed which exhibited large capacitance ranges, 100–500 pF, with relatively small changes in reverse bias: 0–5 V or 0–12 V. These types were – and still are – extensively used in television tuners, whose high carrier frequencies require only small changes in capacitance. TERE BIN DIL NAIYO LAGDA MP3 SONG DOWNLOAD PAGALWORLD FULLEarly varicap diodes usually required a reverse voltage range of 0–33 v to obtain their full capacitance ranges, which were still quite small, approximately 1–10 pF. These circuit configurations are quite common in television tuners and electronically tuned broadcast AM and FM receivers, as well as other communications equipment and industrial equipment. ![]() Separation of grounds is often done to (i) prevent high-frequency radiation from the low-frequency ground node, and (ii) prevent DC currents in the AC ground node changing bias and operating points of active devices such as varicaps and transistors. ![]() The DC ground is shown as a traditional ground symbol, and the AC ground as an open triangle. A third circuit, at top right in diagram, uses two series-connected varicaps and separate DC and AC signal ground connections. When designing tuning circuits with varicaps it is usually good practice to maintain the AC component of voltage across the varicap at a minimal level, usually less than 100 mV peak to peak, to prevent changing the diode capacitance too much, which would distort the signal and add harmonics. This reduces the overall capacitance and the capacitance range by half, but has the advantage of reducing the AC component of voltage across each device and has symmetrical distortion should the AC component possess enough amplitude to bias the varicaps into forward conduction. (See lower left circuit in diagram.) The second varicap effectively replaces the blocking capacitor in the first circuit. Another common configuration uses two back-to-back (anode to anode) varicap diodes. TERE BIN DIL NAIYO LAGDA MP3 SONG DOWNLOAD PAGALWORLD SERIESSometimes, with very high-Q tuned circuits, an inductor is placed in series with the resistor to increase the source impedance of the control voltage so as not to load the tuned circuit and decrease its Q. Since no significant DC current flows in the varicap, the value of the resistor connecting its cathode back to the DC control voltage can be somewhere in the range of 22 kΩ to 150 kΩ and the blocking capacitor somewhere in the range of 5–100 nF. This can be accomplished by placing a DC blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the varicap diode in series with it and by applying DC from a high impedance source to the node between the varicap cathode and the blocking capacitor as shown in the upper left circuit in the accompanying diagram. The DC bias voltage must be blocked from entering the tuned circuit. ![]()
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