schematics is a starting point. no schematics no discussion. all electrical design starts with electrical circuit. eventually this gets translated into a mechanical representation. show us how exactly are you "tuning" your filter right now. i see no inputs or or components that can be tuned mechanically.

components have values. also diodes are polarised devices. this link explains what varactor is and how to use it in a circuit:

https://en.wikipedia.org/wiki/Varicap


note, varactor bias is a DC. also varactors need to be reverse biased or your diode will turn on and not work as a capacitor. resistor need to be high value. this need to be AC coupled to your RF circuit to avoid influence on DC bias used for tuning.

help me. I need to hand in my thesis

very interesting motivation but should not impact approach to problem and your workflow. good luck

 

schematics is a starting point. no schematics no discussion. all electrical design starts with electrical circuit. eventually this gets translated into a mechanical representation. show us how exactly are you "tuning" your filter right now. i see no inputs or or components that can be tuned mechanically.

components have values. also diodes are polarised devices. this link explains what varactor is and how to use it in a circuit:

https://en.wikipedia.org/wiki/Varicap


note, varactor bias is a DC. also varactors need to be reverse biased or your diode will turn on and not work as a capacitor. resistor need to be high value. this need to be AC coupled to your RF circuit to avoid influence on DC bias used for tuning.




very interesting motivation and explains your workflow. good luck

thank you

 

There are many ways to bias a varactor -all of them reverse biased.

Can you show us a schematic of the oscillator without showing the varactor. We can then probably tell you what to change to insert the varactor into the circuit and properly bias it.

 

It seems that time is of the essence. I am trying to guide you in the proper direction take a moment and do more research I understand this is your thesis but you are on the verge of plagiarism you have to throw us a bone!!! schematic or bom as stated before. And I sincerely hope you fare well.

 

schematics is a starting point. no schematics no discussion. all electrical design starts with electrical circuit. eventually this gets translated into a mechanical representation. show us how exactly are you "tuning" your filter right now. i see no inputs or or components that can be tuned mechanically.

components have values. also diodes are polarised devices. this link explains what varactor is and how to use it in a circuit:

https://en.wikipedia.org/wiki/Varicap


note, varactor bias is a DC. also varactors need to be reverse biased or your diode will turn on and not work as a capacitor. resistor need to be high value. this need to be AC coupled to your RF circuit to avoid influence on DC bias used for tuning.




very interesting motivation but should not impact approach to problem and your workflow. good luck

thank you

 

There are many ways to bias a varactor -all of them reverse biased.

Can you show us a schematic of the oscillator without showing the varactor. We can then probably tell you what to change to insert the varactor into the circuit and properly bias it.

this is how I built the circuit, I don,t know how correct it is, please

 

To make it easier for people reading your schematic, you may want to use one of these symbols for the varactor:

(I use the one on top but they are all good.)

Your circuit is fine. It should work as drawn if the control voltage is positive and the anode of the varistor is grounded. Not quite sure how to interpret your representation of the varactor but if the cathode is grounded you will not have a voltage variable capacitor - it will be more like a current controlled resistor.

In a picture:

Your resistor should probably be in the range of 10k to 100k.

 

To make it easier for people reading your schematic, you may want to use one of these symbols for the varactor:
View attachment 198278
(I use the one on top but they are all good.)

Your circuit is fine. It should work as drawn if the control voltage is positive and the anode of the varistor is grounded. Not quite sure how to interpret your representation of the varactor but if the cathode is grounded you will not have a voltage variable capacitor - it will be more like a current controlled resistor.

In a picture:
View attachment 198279
Your resistor should probably be in the range of 10k to 100k.

ok, Dick thanks a lot. my circuit is just like what you posted now. I am grateful

 

An oscillator needs an active transistor to cause oscillation. The circuit has no active part so it is not an oscillator, instead it is a voltage-controlled tuned LC circuit.

 

An oscillator needs an active transistor to cause oscillation. The circuit has no active part so it is not an oscillator, instead it is a voltage-controlled tuned LC circuit.

yeah, this is a bandpass filter

 

1. are you sure about diode polarity?
2. what is the voltage range you use to tune it? (and what is polarity of that voltage)
3. what is the max amplitude of the signal you feed to this filter?

 

1. The polarity shown in post #27 is correct.

2. The voltage required is a function of the diode's capacitance vs bias curve and the desired tuning range. The diode is operated in reverse polarity because the depth of the depletion region between the anode and cathode is modulated by the amount of reverse bias, thereby modulating the capcitance of the junction.

3. @nana_asempa1 this is your question for sure.