square wave -->sine using passive bandpass filter

[windell747]

Hi,

I'm designing a circuit that will take a square wave generated by a microprocessor and feeds it through a bandpass filter to get a sine wave. The application will be to provide a tone for morse code to a handheld ham radio on board a small satellite. Right now im considering remaining simple by using a 1st order passive bandpass filter that uses a capacitor and inductor in series. Assuming this I have a few questions.

1) How should I choose the impedance of the bandpass filter? I do not know the impedance of the GPIO of the microprocessor. Does this impedance matter much?

2) What is the difference between the parallel and series configuration for the passive 1st order bandpass filter?

3) Should I filter more to get a truer sine wave? Any experienced hammers out there? I will be transmitting the morse code tone from a ham radio from a 400km orbit (LEO). I will then feed the tone through a morse decoder to convert the morse to text using a freeware program.

4) Finally, I wanted to use a passive filter to save power, however, I'm open to an active filter that uses an opamp. Are there any advantages to this?

Any guidance would be greatly appreciated!

Thanks,
Windell

4)

 

[windell747]

Thanks VE1BLL. I was once told about the passband filter within the radio. I wasn't sure if I should just assume that this already exists in the radio. The radio is the VX3R. Is there any way I can look this up for sure?

Thanks,
windell

 

[IRstuff]

Is this for school?

TTFN

FAQ731-376

 

[windell747]

is there any freeware that you may know of that I can use to simulate the performance of my filtering circuit? I don't have the radio with me as I'm on vacation from the lab so I don't have a chance to do some testing of the audio quality of sending a square wave into the mic input of the radio. To be safe ill include the lowpass filter just in case. I'll include a bypass in the circuit that can skip over it if it is sufficient to just send the square wave into the mic input.

Right now my circuit looks like the following.

two passive low pass filters cascaded (thanks Skosgurra)...
then a voltage divider at the end to attenuate the signal down to 5mV
then a voltage follower
then a capacitor in series with the output of voltage follower.

I have a couple questions...
1) would it be fine to put a capacitor with a really small capacitance at the output of the opamp to filter out the DC components? Does the value of the capacitor matter much?
2)What would be the best way to determine the values for the components in the lowpass filters? I know that the equation is f0=1/(2piRC) but that leaves a large number of combonations for R and C.
3) The impedance requirement for the mic specifided by the radio's documentation is 2000ohms. How do I maintain this requirement with the capacitor at the end of the voltage follower? I do know the impedance of the output of the opamp and it is about 30ohms.

I appreciate the help!
Thanks,
Windell

 

[Skogsgurra]

OK. At last, we got something that looks like a specification.

You do not need the attenuator to get the right output level (the 5 mV). Just make your low pass filter (the two RC links) output that voltage. You do that by adjusting the f0 to get the right output level. That gives you optimum filtering action as well.

You do not need the opamp or voltage follower. It is possible to select components in the filter so that your output impedance is around 2 kohms. An exact fit is not necessary. Usually, anything "kilohmish" will work.

We have been doing this for quite a while now and I appreciate the way you try to grasp the material. So, I have made a short run with data that should satisfy your requirements. Input is 3.3 V 50 % 1020 Hz square from a micro. Output is around 6 mV at around 2 kohms (a little lower actually, because of source impedance being quite low).



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[windell747]

thanks,

windell

 

[Noway2]

Here is an idea that might be of benefit that approaches the problem from a different perspective.

Instead of using a square wave out of your microprocessor at the desired fundamental frequency, try PWM'ing a higher frequency pulse output with a modulation of the desired sinewave. This will help reduce the amount of effort requried by the filter. As I understand it, the technique is commonly used in PWM inverters to generate a low frequency sinewave.

 

[Skogsgurra]

I second that, Noway!

A much better approach

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

This is a little late but I think this link goes through a Sallen-Key filter design fairly well.

http://www.t-linespeakers.org/tech/filters/Sallen-Key.html

 

[Madcow]

How about a D2A converter, a tone generator from a phone or an oscilator going to an op-amp with an enable?