Frequency detection

[codmw2]

Hi,

I am building a frequency detection to detect the following frequencies: 20kHz, 40kHz, 60kHz, and 80kHz.

I am using phototransistors to detect the light sources and have a transimpedance followed by a coupling circuit and a gain stage which then feeds into my bandpass filter which covers all four frequencies.

My circuit works but when i add a bit more gain to get a further distance (of about 6-8 feet) i start getting a lot of noise problems. I am just curious as how to get rid of my noise problems. The noise is causing the output waveform to change the frequency. And i have tried sheilding my phototransistors which electrical tape.

If anyone has any info. on how to get rid of the noise that would be greatful. By the way, I am using an LM837 op amp for my transimpedance stage. I have access to the LM837, LM741, and TL081 op amps.

 

[crutschow]

Depends upon the source of the noise. What does it look like?

All electronic circuits have intrinsic noise (even a resistor) and there's only so much you can do to minimize it. If it's from the op amp, the LM837 is fairly low noise at 4.5nV(root)Hz, but you could try a very low noise device such as an AD797 which has <1nV(root)Hz noise (less than the noise of a 100Ω resistor).

Post a schematic of your circuit.

 

[Hero999]

You could use a tone decoder based on the NE567 or CD4046.

 

[audioguru]

Light dimmers and fluorescent light tubes and compact fluorescent light bulbs produce noise at the frequencies you are trying to detect.

 

[codmw2]

Well I have actually tried to make use of the LM567 tone decoders and then seem to work fine but only at detecting the frequency within 1 foot and then moving the LED back to about 6-8 feet. But when i try to detect it first from 6-8 feet it does not read it. I have to bring the LED in then move it out slowly so it can be able to read it. I will try to get a schematic of my circuit up soon

 

[Hero999]

The NE567 should be good to 500kHz so maybe you need an amplfier?

Do you need to distinguish between different frequencies?

You could set the NE567 to 80kHz and use a band pass filter and a non-linear circuit to generate harmonics so it triggers on any multiple of 20KHz up to 80kHz, that way you only need one tone decoder IC.

 

[codmw2]

the only problem about using only one tone decoder is the bandwidth is only a maximum of 18% of the center frequency so i think i would still have to use multiple tone decoders. And i have already tried adding gain. But i will try later on today as I will be going to the lab to work on it.

 

[Hero999]

No you don't, a single tone detector will do.

If you distort a 20kHz sine wave, you also get harmonics: 40kHz, 60kHz, 80kHz 100kHz etc.

You use a highpass filter with a sharp roll-off to stop it from detecting 10kHz and below which will also produce 20kHz, 30kHz, 40kHz ect.

 

[codmw2]

alright hero. Thanks. I'll see what i can do.

 

[codmw2]

I am using a 4th order chebychev I high pass filter but how does it distort the 20kHz to use it with only one tone decoder to get the harmonics?

 

[Hero999]

You need so distort the signal after the filter.

This can be done using a non linear amplifier such as a common emitter amplifier driven to the point of clipping. You'll probably need to experiment a bit with the non-linear element to give you a rich enough source of both odd and even harmonics to trigger the tone generator.