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!

 

[Robert Lacoste]

Just add a DC blocking capacitor between the varicap and the IC...

Robert.

"albert" <wo918@hotmail.com> a écrit dans le message de
news:3aab1e1f.0310010120.6fe92385@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!

 

[Lizard Blizzard]

albert wrote:

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.

Isolate the DC of the chip from the varicap with capacitors that are
much larger, say 220 or 470 pF ceramic caps, preferably NPO. Isolate
the RF signal from the DC varicap controlling voltage with resistors,
say 47k or so.

chip RF circuit
View with courier font o o
| | isolation caps
--- --- 220 or 470 pF
--- ---
47k | varicap | 47k
DC control o------/\/\/\-----+-----|<-------+-----/\/\/\----o gnd.

Thanks a lot!

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