I have done a modulation circuit by square wave and after this modulation circuit i put a band pass filter to make a sinus wave but i am wanted to do that when the carrier frequency changed, on modulation, the band pass filter should set its center frequency to new carrier frequency how can i do that can anyone explain that?

 

I some what do not get what you are saying.

Mind post the schematic with a bit more details ?

 

for example i have signal and i pass it through a band pass filter then i change my signal's frequency and send it again what i wanted is the filter sense this change and arrange its center frequency to new signal frequency. i am realy stuck with it is it possible to make?

 

If you know the new carrier frequency then switch to a different filter when you change the frequency. Why is the carrier frequency changing?

Detecting the frequency and changing the filter is a complex task that would likely require a microprocessor.

 

Sounds like he's doing FM modulation to me. By how much does the fundamental frequency change? If you want to filter harmonics ( to change the square wave to a sine wave ) then you shouldn't have to worry about changing the filter. As long as attenuation of the next harmonic is good, just use a fixed filter. As my friend crutschow said, it's a very complex task to change the filter fequency.

Please give us more details, and we can offer batter help.

 

it is actually my term project's one of the requirements. i am doing am modulation with square wave and passing it through a band pass filter. and they want me to do that when carrier frequency(it is about 1Mhz) change unexpectedly (such as increased to 2Mhz) my filter detect the new frequency and should arrange its center frequency to 2Mhz. but i don't know how much will change the frequency. also i am working with high frequencies i think i can't use opamp in filter, i should make rlc filter.

 

and they want me to do that when carrier frequency(it is about 1Mhz) change unexpectedly (such as increased to 2Mhz) my filter detect the new frequency and should arrange its center frequency to 2Mhz. but i don't know how much will change

Who are you talking about? Who is defining these requirements? I don't think this is a reasonable requirement. How are you going to change R's L's and C's? Variable components are available, but they require a manual tuning. Are you going to use a robot to tune the filter? If not, how are you going to do that? It sounds like a bad plan to me.

 

There are two ways I can think to do this. One, make an RC filter where the R is a combinaiton of a light dependent resistor and a light source that is energized in a manner that is proportional to the frequency. The other is an RC fliter where the R's and/or C's are switched in using analog switches and a binary scheme. Remember, you don't need the center frequency to be the same as the modulation frequency, you only need the filter set to attenuate 2nd and higher order harmonics.

You might get a measure of success with either of these methods, but it won't be easy.

 

i said it is my term project. generally term project are given by instructors in my country. if you are not able to think about it just say "i don't know" it is not a hard thing sometimes you need to use it.
your second post is what i thought yesterday but i can't narrow the bandwidth enough second harmonics of square wave also pass through the filter

 

if you are not able to think about it just say "i don't know" it is not a hard thing sometimes you need to use it.

Don't lecture those who are taking the time to try to help you. You get a bad reputation, and nobody will want to help.

 

This may not be relevant but I have heard that you can make the self oscillation frequency of a filter (like a chebyshev) frequency dependant by putting the capacitors in series with resistors. I must stress that this is an unfounded idea I've heard of in the past but never had reason to use or test myself, the changing resonant frequency suggests to me that the cutoff or center frequency is also shifting.

 

there is a difference, you just didn't try to help me in that post or i couldn't get it.
"Who are you talking about? Who is defining these requirements? I don't think this is a reasonable requirement. How are you going to change R's L's and C's? Variable components are available, but they require a manual tuning. Are you going to use a robot to tune the filter? If not, how are you going to do that? It sounds like a bad plan to me."

 

Part of helping is giving advise on the feasability of the approach. If the plan is bad, then alternatives should be examined. Asking questions and getting answers gives me a good idea of what lattitude we have here. Since you take my questioning so personally, I'll offer you no more advice.

 

but these capacitors react all the frequencies of square wave's harmonics is it still doable?

 

thank you for all your helps anyway

 

I did what you want to do with a data stream many years ago. We used a bitslice processor to recognize the data and to change the filter. You would call it a DSP now. I could look up the old paperwork and put you on the proper path, but your attitude, which is expressed in your typing as well as words, is poor, so I won't help you.

 

For the most part an analog filter made from passive components is not adjustable. Even an active analog filter made from operational amplifiers and passive components is not adjustable either.

There are variable resistors, capacitors, and inductors, but the adjustment is performed manually with a screwdriver or other mechanical tool. The varactor diode is a variable capacitor that can be adjusted with a control voltage. There were also variable resistors based on an EEPROM device called the EEPOT. I've never heard of a variable inductor that was controlled electrically.

Using DSP techniques I can see doing demodulation, decoding, FFT and such things along with changing the characteristics of a digital filter, or adjusting the properties of some analog components like a varactor diode or an EEPOT.

Maybe this helps and maybe it doesn't. I do second the comment that you might want to be careful about biting the hand that is trying to help you.

 

hey come on what are you guys saying i never bited a hand trying to help me i have never done such a thing in my life. the guy was just judging my post and angry with me about something about my project. i answered his first angry post and second helping post you can all go back and read it. now thank you for all your helps. i think somehow we couldn't agreed