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About
this Project
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If
you have ever wanted to build or replace your transmitter's
variable frequency oscillator (VFO) with a voltage
controlled oscillator (VCO), but had no idea where
to start then this small project should give you
a couple of hints. The above schematic shows how
simple it is to create a variable capacitor with
just two varicap diodes, 100K resistor and 10K potentiometer.
|
Varicaps
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| Varicap
Diodes
|
|
Model |
[pF] MIN |
[pF]
MAX |
[V]
MAX
|
[mA] |
Application |
|
2 |
42 |
32 |
0.2 |
FM |
|
2 |
42 |
30 |
0.2 |
FM |
|
2 |
16 |
30 |
0.2 |
FM
/ TV |
|
2 |
36 |
30 |
0.2 |
FM
/ TV |
|
5 |
32 |
30 |
0.2 |
VHF |
|
2.5 |
6.8 |
30 |
0.2 |
FM
/ TV |
|
4.5 |
25 |
30 |
0.2 |
VHF |
|
1.8 |
32 |
20 |
0.2 |
FM |
|
11.3 |
30 |
30 |
0.2 |
VHF |
|
2 |
42 |
30 |
0.2 |
FM |
|
2 |
18 |
35 |
0.2 |
UHF |
|
1.8 |
18 |
30 |
0.2 |
UHF |
|
4 |
20 |
30 |
0.2 |
VHF |
|
4.3 |
32 |
30 |
0.2 |
VHF |
|
17 |
560 |
12 |
0.05 |
AM |
|
2 |
42 |
30 |
0.2 |
FM |
|
2 |
16 |
30 |
0.2 |
FM
/ TV |
|
2 |
36 |
30 |
0.2 |
FM
/ TV |
|
2 |
42 |
30 |
0.2 |
FM |
|
1.8 |
18 |
30 |
0.2 |
UHF |
|
5 |
32 |
28 |
0.2 |
VHF |
|
6 |
30 |
30 |
0.02 |
VHF |
|
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Why
would I want to use Varicaps or VCO?
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With
voltage controlled oscillator you can precisely
control the frequency of the FM oscillator with
a potentiometer instead of using a variable capacitor.
If you use VCO, you can also digitally tune the
frequency of the oscillator by using what's called
Phase Locked Loop (PLL) which is a digital circuit
that prevents any frequency drifts.
|
BB105
Varicap Test
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| Volts |
Capacitance |
|
0 |
174 pF |
|
0.4 |
172 pF |
|
0.5 |
108 pF |
|
0.6 |
96 pF |
|
0.7 |
71 pF |
|
0.8 |
56 pF |
|
0.9 |
44 pF |
|
| Volts |
Capacitance |
|
1 |
34 pF |
|
1.1 |
29 pF |
|
1.2 |
28 pF |
|
1.3 |
27 pF |
|
1.4 |
26 pF |
|
1.5 |
25 pF |
|
1.6 - 1.8 |
24 pF |
1.9 |
23 pF |
2 - 2.4 |
22 pF |
2.5 - 2.6 |
21 pF |
|
| Volts |
Capacitance |
|
2.7 - 3 |
20 pF |
|
3.1 - 3.4 |
19 pF |
|
3.5 - 3.8 |
18 pF |
|
3.9 - 4.6 |
17 pF |
|
4.7 - 5.1 |
16 pF |
|
5.2 - 6.3 |
15 pF |
|
6.4 - 6.9 |
14 pF |
7 - 7.6 |
13 pF |
7.7 - 8.9 |
12 pF |
9 > |
11 pF |
|
|
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This
test has been made to show you how varicap's
capacitance can be varied by applying different
voltage ranges. Green tables (on the right)
shows that when applying voltage ranges from
1 to 9V, varicap gives out a smooth capacitance
increments, which is what you need in a precise
FM oscillator tuning. The red table on the other
hand (on the left) shows that voltage ranges
from 0 to 0.9V do not give smooth capacitance
increments and capacitance that can not be used
in FM oscillator.
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Sample
Voltage Controlled Oscillators:
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USB IO Board
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200m 4-Channel 433MHz Wireless RF Remote Control
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100m 4-Channel 433MHz Wireless RF Remote Control
Four button RF remote is used to turn ON / OFF four different devices independently. Any of the four outputs can be configured to work independently in either toggle or momentary mode. Outputs are buffered by BC549 NPN transistors and can drive low voltage devices directly or be connected to either 5V or 12V relays (or motors) to control appliances that use 110V / 220V mains voltage or any voltage of your choice. Multiple remote systems can be used independently to control more than four appliances in the same location by changing the address code on 433MHz receiver and remote. It is also possible to use several remotes to control the same appliance such as garage door. |
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