how to solve this problem ?

It's an audio transmission system, you can't send plain data via it - you need a modem either end.

Where did you find a ready made receiver for square waves? What did you expect to hear without modulation?You probably have an FM receiver and are detecting the harmonics from the square wave you say you are transmitting.

A good FM radio can receive a square-wave that is from about 200Hz to about 2000Hz from a good FM transmitter.

Square-wave frequencies below 200hz will have tilt and curvature because the response drops below 50Hz. Frequencies above 2000hz will not have steep sides on the square-wave because the response drops above 15kHz.

I guess originally I read the OP differently than you did . To quote, "my problem is that I build a RF transmitter at frequency of 96MHz ... but when I transmit a square signal I dont receive it propeply...I get a distorted signal..."

I read that to mean the OP was transmitting a square wave at 96 MHz and was getting only noise. That didn't make sense to be doing, so I asked what he expected without modulation. Obviously, you read the post to mean that the modulation was a square wave, but the carrier at 96 MHz was a typical carrier.

In retrospect, I agree that your interpretation makes more sense than mine did. Still, it would help if the OP would clarify what he was sending as a signal. John

A "ready-made" receiver for 88Mhz to 108MHz has an audio output, not a square-waves RF output.

It would help if the OP said what is the frequency and attached a 'scope picture of the distortion. Maybe the transmitter is over-modulated.

I read the OP as keying the transmitter on and off with a square wave, and of course the receiver will not demodulate such a signal.

Lefty

Well, I take it that he is feeding a baseband square wave into the transmitter's FM modulation input. As AG said, the distortion you get is dependent on several things. The transmitter audio frequency response is the first thing. For square waves of a certain frequency, say like 1000 Hz, you need good low end response down to, oh, maybe about 20Hz or so to keep the tops of the squares flat and not drooping. Also, to keep the edges reasonably straight up and down, you need very good high frequency response. For a 1000 Hz square wave, you would want flat audio response up to more than 20 KHz.

Now, the next problem is that of the bandwidth of your receiver. FM receivers like the one you are using have IF filters that limit your frequency response of the receiver. The wider you modulate your transmitter, the more likely your receiver's IF filters will distort the audio that gets through. So you have to actually adjust your transmitter modulation to as low a level as is practical to minimize distortion of your square wave. Even so, the receiver's IF isn't wide enough to give you much bandwidth, so you are very limited to low frequency square waves, like up to maybe 2 or 3 KHz.

Maybe the "ready-made" FM receiver is the junk sold at The Dollar Store. Everything comes out distorted.

hey all,thank you very much for replying my Question,but some of the replies sounds kind of derision why people ???
My question is : Iam sending a square wave of frequency 1KHz to modulate a carrier of 96MHz,then I receive the modulated signal by a ready made receiver of a band 88 - 108 MHz and I tune it on 96MHz so I have to get the sent square wave as I sent it,,but I receive it a distorted square wave,,so what did I wrong ??

☼☺☼coool☼☺☼ said:

I receive it a distorted square wave,,so what did I wrong ??

Click to expand...

We told you already. Maybe the level of your square-wave signal into the transmitter is too high then the transmitter is over-modulated. Then the radio will also be over-modulated and will have severe distortion.

We asked for you to post an oscilloscope pic of the distortion. You have not even described the distortion.

Also, please post the schematic of your FM transmitter for us to see if it has pre-emphasis that matches the de-emphasis in the FM radio.

thanks for reply,,,Iam gonna send you what you asked me to in the comming days I hope you to wait for me....thanks