I have designed a dualband bandpass filter with HFSS, but when I tune the varactor, the center frequency doesn't change. could there be any anything Ineed to do right please?

 

A varactor is tuned with a variable voltage. Your circuit does not have a variable voltage and I do not see an inductor.
What frequency?

 

A varactor is tuned with a variable voltage. Your circuit does not have a variable voltage and I do not see an inductor.
What frequency?

my frequency is 1.05GHz in the first passband and 2.0GHz in the second passband

 

I get dizzy when I go up that high.

 

I have designed a dualband bandpass filter with HFSS

To see a schematic of your design would answer a lot of questions.

 

To see a schematic of your design would answer a lot of questions.

below is the HFSS design. I only used the HFSS software to design

 

Apparently you are driving the varactor through a resistor. Where the ground return for the (anode?)?

 

Apparently you are driving the varactor through a resistor. Where the ground return for the (anode?)?

yes, the varactor is between the resistor and the resonator

 

Please check out the Ansoft High Frequency Structure Simulation (HFSS) tutorials which provide valuable insight for use in your own HFSS simulation setups.

 

Is there a DC path through the resonator to ground?

 

no please

Is there a DC path through the resonator to ground?

no please, I only have a ground for the resonator, but not a DC path per se

 

You need the DC path to tune the diode. No path -- no tuning.
Check ARRL handbook for varactor circuits and application

http://www.arrl.org/arrl-handbook-2020

You can find older versions on Amazon or other sites. Really old ones from the 1930's might command a premium price.

 

Or to put it another way, how can you have DC voltage across the diode if the circuit is open?

 

You need the DC path to tune the diode. No path -- no tuning.
Check ARRL handbook for varactor circuits and application

http://www.arrl.org/arrl-handbook-2020

You can find older versions on Amazon or other sites. Really old ones from the 1930's might command a premium price.

ok, thanks

 

Or to put it another way, how can you have DC voltage across the diode if the circuit is open?

the path i only have a patch that is from the resistor to the ground. but the diode is fixed between the resistor and the resonator please. I really like your inputs, carry me on Sir, help me. I need to hand in my thesis

 

Here is an article with example circuits
https://www.electronics-notes.com/a...de/varactor-varicap-diode-circuits-design.php

The varactor must have a DC path to Ground

 

Here is an article with example circuits
https://www.electronics-notes.com/a...de/varactor-varicap-diode-circuits-design.php

The varactor mus have a DC path to Ground

ok thanks

 

I also lack an equivalent schematic of the circuit.

 

Kjeldgaard has the main point. Progress in helping you will be more and more difficult if not impossible without a schematic. Anything we can read will be find.

How are you applying bias to the varactor?

 

Kjeldgaard has the main point. Progress in helping you will be more and more difficult if not impossible without a schematic. Anything we can read will be find.

How are you applying bias to the varactor?

ok, let me be humble and tell my problem. I don't actually know how to apply the bias to the varactor, I would be glad if I am helped on how to design the bias circuit. currently, I only removed the modeled resistor from the HFSS design and I was able to tune the frequencies, but I doubt if that will be correct for fabrication. please teach me hoe to do it. I would be very thankful