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Testing and using Varicap Diodes (Variable Capacitance Diodes) in VCO oscillators.




About this Project
 

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
 

  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



Why would I want to use Varicaps or VCO?
 

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
 
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

 

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.


Sample Voltage Controlled Oscillators:
 






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