Converter Xtal

Fig. 3, Panadopter switching.

Sive the most frequency change with a given capacity change. A limit to this is reached when the Q* of the circuit becomes too low to allow oscillation. Some types of diodes will have a lower and may be the limiting (actor on the frequency range covered.

In the circuit shown, a regular power rectifier diode (silicon) is used as the varicap. One with ^ 100 to 750 volt peak inverse rating is adequate. Individual units will have somewhat different characteristics as to dc bias needed for a given capacity and capacity range available. Almost any will work but try a few ii available and see if one of them might be better than the rest.

A regular varicap or varactor may be used but be careful not to exceed the piv rating. The circuit shown will exceed these ratings and should be modified at the fiX" points in the high side of the diode bias control and the baud width control. A suitable resistor may be inserted at these points to limit the voltage the controls may place across the diode, These resistors may run to several megohms. Leave out the diode until you have installed the resistors by trial and then run the controls all the way up and measure the dc bias with a YTYM and the sweep with your scope and be sure it isn't too much for the varactor you choose. This is not a problem with the power diodes used in the circuit as shown. More frequency range may be obtained with the varactors but other problems appear. The RF voltage at the grid can override the small bias used. A 100 pf at 4 volt diode tapped down on the coil between cathode and ground thru a 250 pf condensor with dc bias of about 4 volts (using 10 meg resistors in the positions) will cover all of 2 meters

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