Re: Need help about 38khz Oscillator
38KHz is only used INSIDE the encoder, it should not appear at the output so the simulation is correct. If you were to transmit 38KHz using an FM transmitter it would use almost twice the width of an FM channel so you wouldn't be able to pick it up on a normal radio. Instead, the frequency is divided by 2 and 19KHz is transmitted instead. In the receiver, the 19KHz is doubled in frequency to recreate the 38KHz needed to operate the decoder switch.
The frequencies were chosen for that very reason, 19KHz is deemed to be just above the average human hearing range so it wouldn't be objectionable to listeners. Most receivers have audio low pass filters that limit the top frequency to around 16KHz anyway. In a radio system, higher frequencies in the modulation require wider bandwidths so going higher than 19KHz would simply waste band space. 19KHz was chosen as a good compromise. The trouble with actually switching the channels at 19KHz is it would make strange noises as it created beat notes with higher audio frequencies, for example a sound at 15KHz would produce a (19-15) 4KHz sound as well. By switching channels at 38KHz it avoids the beat notes and by dividing it by 2 allows it to be transmitted at 19KHz while still allowing the original 38KHz to be re-created in the receiver by doubling it.
Brian.