varactor diodes series connection

[jgraygoza]

Good day.

I am working on an assembly that uses varactors connected in series on the transmission line.
They are there for the purpose of keeping a fixed bandwidth.
But it seems that they are not staying on because
the bandwidth changes as the voltage increases.
Is this even possible?
Either that or

Tne frequency varactors are alreay set into
The frequency range.

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Thank you for reading this message.

 

[thylacine1975]

I'm not sure I understand your question, and in particular the purpose(s) of the varactor diodes in "keeping a fixed bandwidth".

Nonetheless, the first thought that comes to mind is that naturally - something - (in your case the "bandwidth") - will change as [some] voltage changes, since that's what varactors *do*. They change their capacitance as the reverse bias across them changes.
If they are in series with a transmission line then yes, you will (definitely) observe all sorts of voltage-variable lumps/bumps/dips in the transmission characteristics. (If you don't want voltage-variable characteristics, use a fixed capacitor

So having said that, I don't understand:
* Why the varactors are there,
* Their intended application, and
* What the problem is! (...and thus how to help

Post a schematic or a picture and/or measurements and we'll see if we can be of a little more use!

 

[jgraygoza]

My apologies. When I wrote this post I was half asleep and my mind was somewhere else.
My intention is to build a filter that can keep a bandwidth as constant as possible.
The topology is using shunt LC resonators coupled with capacitors.
Thus, the series varactors.
As the frequency goes up, the coupling capacitance has to change in order ti couple it properly and
keep the same bandwidth.
I have a couple of varactors I've used and they somewhat keep the bandwidth constant, but
I am running through some issues.
It's pretty complicated to keep the bandwidth from opening too much.

That's why I ask if this is even possible or I am just daydreaming.
As far as moving the Fc, that's no problem. I have taken care of that.
The coupling is what makes my situation complicated.

Thank you very much for reading this message.

 

[biff44]

You should post your circuit, especially how you are applying DC bias. Reverse biase diodes have no conducting current. So if you have 3 series diodes and apply 10 volts to the entire circuit, you can NOT assume the diodes get an equal bias. One diode may have 9 volts across it, another 1 volts, and the third zero volts. Obviously, if you do not have a "stiff" biasing network on every diode, you can not expect it to have repeatable RF performance.