Here's a dandy circuit for a VCO and buffer that operates across the entire
FM broadcast band (88-108 MHz). I stole the main idea from the local oscillator in a radio shack scanner (pro2004). I like this design because
it doesn't require a tapped coil, it tunes very broadly, it's stable,
and it has a nice, hot output.

FM BROADCAST VCO AND BUFFER

VR1 <G>-+-/\/\/\--+--+-----------+--------+--R6--+----+-----+-----<+12VDC>
| ^ ^ | | | | | |
C2 | CR2 C8 R4 | c R8 | c C11
+---+ | | | |/ | |/ |
| <G>+--+ +--C5--+---+--| +--| <G>
audio R1 | | | |\ | |\
in | | | C6 | e | | e
O--C1---+--R2--+--C4--+ R3 | | | |
| ^ | | +----+--C9--+ +--C10--O RF out
C3 CR1 L1 | C7 R5 R7 R9
| | | | | | | | <G>-----+------+------+------+---+----+------+----+-----------<G>

Q1 Q2

+ connection
<G> ground connection
^ cathode of a diode

C1, C2, C8, C11 .1 uF
C3, C4 .001 uF
C5, C6, C7 39 pF NPO or silver mica
C9 10 pF NPO or silver mica
C10 22 pF
CR1 ECG616 varactor (tuning) diode
CR2 9v Zener
L1 5T #20 wire, 1/8 inch I.D., adjust spacing for tuning range
Q1 2N3663
Q2 2N3904
R1, R2 47 K
R3 22 K
R4 15 K
R5 1 K
R6 390 ohm
R7, R8 4.7 K
R9 100 ohm
VR1 100 K, linear taper

As with all VHF circuits, pay particular attention to construction technique.
I recommend cutting little square islands on one side of a two-sided copper-clad board. Use the remainder of that side as the ground plane,
and leave the bottom side to serve as a shield. If you keep all lead
lengths short, the circuit is quite stable.

With the parts listed here, effective frequency range extends well beyond
the FM broadcast band in both directions. If a 6V zener is substituted
for CR2, the circuit will run from a 9V battery, with a slightly smaller
tuning range. The output is hot enough that the signal can easily travel
a city block using just a clip lead for an antenna.




--- Mystic BBS v1.12 A43 (Linux/32)
* Origin: HAMRADIO telnet lu9dce.dynu.com (21:5/101)