Electronic – Does this look like amplitude modulation
Modulation
I have designed a transmitter and a receiver for amplitude modulation. Does the signal in the image looks like amplitude modulation?
Best Answer
Yes, there is some amplitude modulation. There is also a significant shift in the DC level of the carrier, seeming to correspond with the carried signal. In other words, this looks like amplitude modulation plus the baseband signal.
For most uses of amplitude modulation the additional baseband signal added to the carrier at the transmitter is irrelevant since it's at much lower frequency. It will get automatically filtered out by the resonant output filter of the transmitter, and the antenna system. Even if not, it should get filtered out by any competent receiver.
In your case, the modulating signal is only about 1/5 of the carrier frequency. If this signal is transmitted over the air, then that could result in significant out of band transmission, which may cause interference with other devices and be illegal.
If this is the signal inside your receiver, then it has problems. By the time you are trying to demodulate a AM carrier, it's DC level should be nice and constant. This is a basic assumption of most AM demodulation techniques, like the common diode detector.
As a side note, only 5:1 ratio of carrier to baseband signal is very slim. Depending on what the baseband signal is, it might result in unacceptably low signal to noise ratio. The spectrum of the whole AM signal will also be very broad relative to the carrier frequency. Usually you want significantly more frequency ratio for AM. That for commercial AM radio is about 100:1, for example.
Your perfectly single-sideband suppressed-carrier modulated sinusoid certainly has a phase which can be measured. However, what you cannot tell is what the contributions of that measured phase from the audio input and the RF oscillator were.
There is another form of single-sideband modulation, in which not only one sideband but also the carrier component is transmitted. This provides a reference which can be used to synchronize the receive LO to the transmit one - normally done to insure exact tuning, but it would also give you the ability to recover the original audio phase.
It is also quite possible, especially with modern DSP gear, to transmit two separate audio channels, one on each side band. This is commonly called independent sideband modulation (ISB).
Many spread spectrum implementations are DSP based and capable of receiving multiple channels at once - GPS being a good example.
M is the amplitude of the modulating cosine (or sine to answer your
question)
A is the amplitude of the carrier sine (or cosine to reinforce the
answer!!)
φ is the phase displacement of the modulating sinewave but is
irrelevant all but mathematically
ωm and ωc are the frequencies of modulation and carrier.
Maybe I just don't recognize your formula but the answer is, like Jim Dearden implies swap them up or use the same because carrier and modulator are not going to be the same frequency when dealing with AM.
Best Answer
Yes, there is some amplitude modulation. There is also a significant shift in the DC level of the carrier, seeming to correspond with the carried signal. In other words, this looks like amplitude modulation plus the baseband signal.
For most uses of amplitude modulation the additional baseband signal added to the carrier at the transmitter is irrelevant since it's at much lower frequency. It will get automatically filtered out by the resonant output filter of the transmitter, and the antenna system. Even if not, it should get filtered out by any competent receiver.
In your case, the modulating signal is only about 1/5 of the carrier frequency. If this signal is transmitted over the air, then that could result in significant out of band transmission, which may cause interference with other devices and be illegal.
If this is the signal inside your receiver, then it has problems. By the time you are trying to demodulate a AM carrier, it's DC level should be nice and constant. This is a basic assumption of most AM demodulation techniques, like the common diode detector.
As a side note, only 5:1 ratio of carrier to baseband signal is very slim. Depending on what the baseband signal is, it might result in unacceptably low signal to noise ratio. The spectrum of the whole AM signal will also be very broad relative to the carrier frequency. Usually you want significantly more frequency ratio for AM. That for commercial AM radio is about 100:1, for example.