Help with Varactor Diode

[abuhafss]

Here is a portion of a circuit in which I want to replace the variable capacitor with a varactor diode like MVAM108. I know a little basics about varactors. Please help how to incorporate MVAM108 in this circuit so that it can be tuned with control voltage.

p.'s. http://www.tsf-radio.org/forum/im/133895mvam109.pdf

 

[Harald Kapp]

Varactor diodes are explained here. See section "varactor circuits" for applications.

Your circuit may not be suitable for the application of a varactor diode: A varactor diode's capacitance is controlled by the DC bias across the diode. Your circuit has an inductor in parallel to the diode. This inductor is a short-circuit for DC (apart from a typically low wire resistance of the coil). Any DC source that wants to apply bias to the diode will therefore be short-circuited by the inductor.
You may try to put a capacitor in series with the inductor to de-couple it from the DC bias of the varactor.

 

[KrisBlueNZ]

Yeah, a simple option would be to break the connection from the bottom of the inductor to the 0V rail, put a capacitor (e.g. 0.1 µF) across the break, and apply the control voltage across that capacitor.

 

[abuhafss]

You may try to put a capacitor in series with the inductor to de-couple it from the DC bias of the varactor.

Is this your suggested configuration?

 

[abuhafss]

Yeah, a simple option would be to break the connection from the bottom of the inductor to the 0V rail, put a capacitor (e.g. 0.1 µF) across the break, and apply the control voltage across that capacitor.

Is this your suggested connections?

 

[KrisBlueNZ]

Not quite. Move the cathode of the varicap to the other end of the inductor, then it's right.

 

[abuhafss]

Not quite. Move the cathode of the varicap to the other end of the inductor, then it's right.

Hope this is OK.

 

[KrisBlueNZ]

Now you have the resistor connected to the wrong place. Connect the resistor to the top of the 0.1 µF capacitor.

 

[abuhafss]

Now you have the resistor connected to the wrong place. Connect the resistor to the top of the 0.1 µF capacitor.

You mean the junction of the inductor and the 0.1μF cap?
In that case the varicap will be reverse biased thru the inductor?

 

[KrisBlueNZ]

Yes, and yes.

 

[Harald Kapp]

Now you have the resistor connected to the wrong place. Connect the resistor to the top of the 0.1 µF capacitor.

Kris: For DC this should make no difference.

 

[KrisBlueNZ]

Kris: For DC this should make no difference.

With a 100k series resistor, it won't make much difference, no. I didn't actually suggest using a series resistor. My suggestion was to apply the control voltage straight to the capacitor. This avoids any effect on the Q of the tuned circuit. With a 100k resistor, that effect would only be tiny, but there would be a small delay waiting for the capacitor to charge. My suggestion was intended to avoid both problems.

 

[Harald Kapp]

O.K., I think I see

 

[abuhafss]

And how the varicap would replace the capacitor in this circuit?
And where the control voltage will be applied?