about modulation

I am unable to distinguish the difference between the constant-envelope modulation and the nonconstant-envelope modulation.
Can anyone tell me the definition?
And what kind of modulation is belong to nonconstant-envelope modulation? FSK, PSK...
thanks

cdonstant envelop modulation is the modulation type that the amplitude of the carrier is not modulate is it is imune for noise in amplitude

PSK is a constant envelop QPSK is also
QAM is not

khouly

Do you mean that phase modulation and frequency modulation are belong to constant-envelope modulation?
So, FSK PSK QPSK are all belong to constant-envelope modulation.
And I have a question.
I/Q-modulation-based transmitters can operate with both constant and nonconstant modulation, but VCO-based transmitters only operate with constant modulation.
Why is this?
Thanks so much.

I think VCO's function is to vary the frequency with respect to the input voltage. Thats what VCO stands for... (voltage controlled osccillator) sorry, im not familiar with I/Q modulators...but there are many books and tuorials out there that can help you with that problem...

I guess I/Q modulation means to down-convert passband signal to base band signal.
if it is true, I/Q modulated signal envelop depends on input signal envelop. if input signal is constant envelop modulated signal is also constant envelop and vice versa.
you can use communication books like Carlson or Haykin textbooks.