how to use a varicap

[albert]

Hi,

I'm implementing a modulator using a VCO. In order to tune to
different frequencies, a varicap is used. The varicap should be
connected between two pins of the VCO IC, by varing the reverse
voltage between the two diodes of the varicap, different values of
capacitance can be obtained. However, when the pins of the varicap are
connected to a volatage source, at the same time, the corresponding
pins of the VCO IC are also connected through the voltage source.
Consequently, the varicap is bypassed! How to use a varicap on a
breadboard then? Maybe this is a stupid question, but I really have
little practical experience.


Thanks a lot!

 

[Thomas C. Sefranek]

You can decouple the DC supply from the diode with either an inductor or a
resistor.
(Feed the control voltage through resistors or coils.)

--
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| __O Thomas C. Sefranek WA1RHP@ARRL.net
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"albert" <wo918@hotmail.com> wrote in message
news:3aab1e1f.0310010120.566ba7fb@posting.google.com...

Hi,

I'm implementing a modulator using a VCO. In order to tune to
different frequencies, a varicap is used. The varicap should be
connected between two pins of the VCO IC, by varing the reverse
voltage between the two diodes of the varicap, different values of
capacitance can be obtained. However, when the pins of the varicap are
connected to a volatage source, at the same time, the corresponding
pins of the VCO IC are also connected through the voltage source.
Consequently, the varicap is bypassed! How to use a varicap on a
breadboard then? Maybe this is a stupid question, but I really have
little practical experience.


Thanks a lot!

 

[John Fortier]

"albert" <wo918@hotmail.com> wrote in message
news:3aab1e1f.0310010120.566ba7fb@posting.google.com...

Hi,

I'm implementing a modulator using a VCO. In order to tune to
different frequencies, a varicap is used. The varicap should be
connected between two pins of the VCO IC, by varing the reverse
voltage between the two diodes of the varicap, different values of
capacitance can be obtained. However, when the pins of the varicap are
connected to a volatage source, at the same time, the corresponding
pins of the VCO IC are also connected through the voltage source.
Consequently, the varicap is bypassed! How to use a varicap on a
breadboard then? Maybe this is a stupid question, but I really have
little practical experience.


Thanks a lot!

The varycap needs to be isolated from the chip. Use capacitors to achieve
this. The overall capacitance will be the two isolating capacitors and the
varicap in series, so you will need to amend your calculations accordingly.
The connection sequence should be - chip terminal, isolating capacitor,
varicap, isolating capacitor, other chip terminal.

You will also need to ensure that your bias voltage is completely free from
any interfering signal. Since the back biased varicap has a very high
impedance it's possible to use a high resistance coupled with a decoupling
capacitor to reduce any bias line interference to a level where it will no
longer cause problems. Connect the resistors between your biasing voltage
and the varicap and the decoupling capacitor between the bias voltage supply
and ground. You will need to use a larger electrolytic and a smaller
ceramic capacitor in parallel to cover all frequencies which may interfere.
I would suggest 1 megohm resistors, 10 nF ceramics and 10uF electrolytics.

If the circuit isn't balanced across the varicap, that is,if one leg of the
varicap is connected to ground, the above holds true, but only applies to
the non earth end of the varicap.

Hope this helps

John