how to overcome noise caused by dot matrix scanning effect

hai all,
now i am build a dot matrix which its injected by audio. for the audio I use Fm modulation (carrier frequency = 50khz)and for the wave i choose square instead of sine. i connect the square audio to clock pin of CD4017(decade counter) and the output to every row of my dot matrix. so, the idea is every change of the square wave will change the row too (row scanning). But the effect of scanning influence the audio at receiver. There is new sound with high frequency. When the output audio connect to one LED (without scanning), the audio at receiver it's OK. So how to overcome the scanning Effect. I used High pass filter (fc=33kHz in the front of receiver) and Lowpass Filter (fc= 15khz) at the end receiver.

thanks
Elits

Re: how to overcome noise caused by dot matrix scanning effe

The most likely place for the noise to be coupled is through the power supply. As the LED's are switched on and off, you are creating power supply spikes that could easily be coupled through the common power supply connection to the audio stage. A preamplifier is a very likely place for such noise to be picked up because of the high audio gain normally associated with such a stage (but it is not the only possible place).

Try filtering the power supply. You should take a look at the power supply with an oscilloscope to see the ripple while scanning. Once you have an idea of the frequency range of the ripple, you can design your filter for optimum results.

If you don't have a scope available, to make a quick check you could tack a 10uF (or some other large value) capacitor in parallel with a .1uF (or something between .01 and 1.0) capacitor and place it across the power supply at the audio stage. If the intensity and/or frequency of the noise changes, you have confirmed the source.