In the figure we show a Varicap-controlled VCO capable of generating signals in the range of a few megahertz up to more than 200 MHz. Although frequency control is achieved when using a varicap like the BNB809, even a common diode like the 1N4002 works as such, but with a smaller range of performance. The coil, C2 and C3 depend on the central frequency generated according to the following table:
Frequency Range (Mhz)
L1
C2
C3
1 a 4 MHz
40 turns
100 pF
220 pF
4 a 10 MHz
12 turns
47 pF
100 pF
10 a 50 MHz
8 turns
22 pF
27 pF
50 a 100 MHz
6 turns
10 pF
10 pF
100 a 150 MHz
3 turns
4,7 pF
4,7 pF
The coil is wound with 28 AWG wire in the form of 1 cm without core. We observed that the values in the table are approximate and changes may be necessary depending on the tolerances. The transistor also admits equivalents like 2N2218 and 2N2222 to obtain a slightly higher power for the signal. This signal can be taken either from the collector of the transistor or from a second coil wound over L1. The number of turns of this coil depends on the desired impedance, but in general it will be between half and a quarter of the turns of L1. The capacitors must all be ceramic.
