61 lines
2.1 KiB
Plaintext
61 lines
2.1 KiB
Plaintext
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Here's a dandy circuit for a VCO and buffer that operates across the entire
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FM broadcast band (88-108 MHz). I stole the main idea from the local
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oscillator in a radio shack scanner (pro2004). I like this design because
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it doesn't require a tapped coil, it tunes very broadly, it's stable,
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and it has a nice, hot output.
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FM BROADCAST VCO AND BUFFER
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VR1
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<G>-+-/\/\/\--+--+-----------+--------+--R6--+----+-----+-----<+12VDC>
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| ^ ^ | | | | | |
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C2 | CR2 C8 R4 | c R8 | c C11
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+---+ | | | |/ | |/ |
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| <G>+--+ +--C5--+---+--| +--| <G>
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audio R1 | | | |\ | |\
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in | | | C6 | e | | e
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O--C1---+--R2--+--C4--+ R3 | | | |
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| ^ | | +----+--C9--+ +--C10--O RF out
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C3 CR1 L1 | C7 R5 R7 R9
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<G>-----+------+------+------+---+----+------+----+-----------<G>
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Q1 Q2
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+ connection
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<G> ground connection
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^ cathode of a diode
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C1, C2, C8, C11 .1 uF
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C3, C4 .001 uF
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C5, C6, C7 39 pF NPO or silver mica
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C9 10 pF NPO or silver mica
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C10 22 pF
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CR1 ECG616 varactor (tuning) diode
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CR2 9v Zener
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L1 5T #20 wire, 1/8 inch I.D., adjust spacing for tuning range
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Q1 2N3663
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Q2 2N3904
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R1, R2 47 K
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R3 22 K
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R4 15 K
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R5 1 K
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R6 390 ohm
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R7, R8 4.7 K
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R9 100 ohm
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VR1 100 K, linear taper
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As with all VHF circuits, pay particular attention to construction technique.
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I recommend cutting little square islands on one side of a two-sided
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copper-clad board. Use the remainder of that side as the ground plane,
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and leave the bottom side to serve as a shield. If you keep all lead
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lengths short, the circuit is quite stable.
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With the parts listed here, effective frequency range extends well beyond
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the FM broadcast band in both directions. If a 6V zener is substituted
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for CR2, the circuit will run from a 9V battery, with a slightly smaller
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tuning range. The output is hot enough that the signal can easily travel
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a city block using just a clip lead for an antenna.
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