Hey there, im relative new to electronics and i am having a probably stupid question right now to which i cant really seem to find an answer.
I am doing a small project where i want to only let through a specific area of frequencies (band pass filter) in a pretty low frequency range (between 1 and 2khz). A passive filter would be the simplest choise, but hence my project has nothing to do with audio i dont have a normal sine wave as an input. Instead i have a "sine wave" with no negative voltages (the entire negative part of the wave is 0V instead). So basically looks like a sine wave going through a single diode.
Would this actually work at all since the entire negative part is missing?

Thank you for you time.

 

Welcome to AAC!

It would be clearer if you posted pictures of what you're trying to describe.

It would also be helpful if you used paragraphs to organize your thoughts and help us pick out circuit description and question(s).

 

Preferably the filter should be first, then the diode to rectify the signal. The filter will not really work with pulsating DC.

 

"Would this actually work at all since the entire negative part is missing?"

It completely depends on what you are trying to do. As a general rule.......you only need one half.....either the positive or negative.

But there are applications where both are needed.

 

Welcome to AAC!

It would be clearer if you posted pictures of what you're trying to describe.

It would also be helpful if you used paragraphs to organize your thoughts and help us pick out circuit description and question(s).

Thank you for you reply.

My problem is that I want to use a passive band pass filter (frequency filter) to only let a specific frequency through (1KHz-2KHz).
The actual Filter looks like this:

R1: 5k
R2: 5k
C1: 32nF
C2: 16nF


Since this is meant for a sine wave as an input and I have a rectified sine wave as the input I am not sure that
the filter will work.
The Input sine wave to the filter would look like this:

I can not test this circuit right now because of lacking parts, but I just wanted to know if this frequency filter could in concept filter a rectified
sine wave or if it only works with a full sine wave.

 

Preferably the filter should be first, then the diode to rectify the signal. The filter will not really work with pulsating DC.

The problem is that the original signal is already rectified without any way of changing that (it's an IR receiver which only can output a positive voltage).

 

Ok. So what is your application? Communications? Remote control? There may be other ways of reaching your goal. Bandpass filtering may not be the best way to do it.

 

Ok. So what is your application? Communications? Remote control? There may be other ways of reaching your goal. Bandpass filtering may not be the best way to do it.

It's actually more a experiment than a real project that has any use.

I want to send a specific frequency over an IR emitter and then receive it with the receiver. The frequency of the emitter is variable and i want
a response on the receiver side when the frequency is in a specific area (1KHz-2KHz). The Output of the receiver should go into an OP Amp.

But I only need an output from the Op Amp when the frequency is in the right spot, so the best thing I found were passive frequency filters.

Is there actually a circuit besides those passive filters that can filter frequencies below and above a defined area with the input signal only being
a rectified sine wave, when the passive filters won't work on something like this?

 

If you pass the rectified sine wave through a bandpass filter, you will recover basically the original sine wave at a smaller amplitude. If you expand the fourier series of your rectified signal, you will find that the major component is f1, or the original 1000Hz signal. Your filter will greatly attenuate the other components of the series.

 

Microcontroller comes immediately to mind.
A counter or timer that is compared to a high and low value, and only performs the function when the conditional value is correct.

Doing it with analog methods, I'm not so sure. Much more work that way I think.

Anyone else have a suggestion?

 

Thank you for you reply.

My problem is that I want to use a passive band pass filter (frequency filter) to only let a specific frequency through (1KHz-2KHz).
The actual Filter looks like this:

R1: 5k
R2: 5k
C1: 32nF
C2: 16nF


Since this is meant for a sine wave as an input and I have a rectified sine wave as the input I am not sure that
the filter will work.
The Input sine wave to the filter would look like this:

I can not test this circuit right now because of lacking parts, but I just wanted to know if this frequency filter could in concept filter a rectified
sine wave or if it only works with a full sine wave.

You wouldn't need physical parts if you used a simulator.

 

Ok. Maybe you can utilize LM2907. Frequency to voltage converter and use it in the tachometer input mode.
Follow with voltage comparator and some logic gates to enable your signal when the proper voltage levels are present.
The tach input takes pulses that are referenced to ground so rectified sinewave is perfect.

 

I'm not sure I understand what you are working with. IF you have a IR receiver.......that outputs pulses......that vary from 1 to 2 KHz...........and say you want a high only when those pulses are say 1200 Hz?

Is that correct? IF that is correct..........it's depends on how much you can vary the 1200 Hz figure.

If it has to be exactly 1200 hz.........then a processor is necessary. However.........if the 1200 Hz can be plus or minus 100 hz............then I believe an ACTIVE filter would suffice.

At those frequencies.......forget a passive filter.

 

...........i want
a response on the receiver side when the frequency is in a specific area (1KHz-2KHz). The Output of the receiver should go into an OP Amp.

We need more specifics about what you exactly want to see as the frequency of your signal varies.
What output voltage do you want from the op amp at what frequencies?
Without values we are just spinning our wheels trying to guess what you need.

 

Ok. Maybe you can utilize LM2907. Frequency to voltage converter and use it in the tachometer input mode.
Follow with voltage comparator and some logic gates to enable your signal when the proper voltage levels are present.
The tach input takes pulses that are referenced to ground so rectified sinewave is perfect.

So basically I could wire the input signal to the LM2907 and receive a voltage depending on the frequency. Then follow it up with a Dual Comperator and define two voltages on both Input 1 and Input 2 of the Dual Comperator.
Let's say the voltage at Input 1 would represent the minimum frequency and Input 2 the maximum frequency.

Now if the minimum frequency is reached Output 1 will go high and if the maxiumum frequency is reached both Output 1 and Output 2 will be high.
This is then followed up with an EXOR Logic Gate. If Output 1 and Output 2 are low the logic gates output will be low too. If Output 1
is high and Output 2 low the logic gate will be high. And if both Outputs are high the logic gate will be low again.

Since I have practically no experience with logic gates I am not sure if this is right.

Do a have to amplify the output of the logic gate or would it be enough to lets say drive a small led?
And what input voltage does the LM2907 need at the tach+ input? The signal is only a few mV, so should i preamplify it before wiring it to
the LM2907?

 

So basically I could wire the input signal to the LM2907 and receive a voltage depending on the frequency. Then follow it up with a Dual Comperator.........

You could use a window comparator at the LM2907's output.
Its output will be high when the input is above the low level and below the high level, and low otherwise.

Do a have to amplify the output of the logic gate or would it be enough to lets say drive a small led?

A typical comparator output, such as an LM337, can drive a small, high brightness, LED.

And what input voltage does the LM2907 need at the tach+ input? The signal is only a few mV, so should i preamplify it before wiring it to
the LM2907?

The data sheet tells all.
It states:

so you will need to amplify the signal to at least 40mV.
They tested it at 250mV so that would be a good value to shoot for.