[sdiy] What is a quadrature oscillator good for?

[Cornutt, David K]

From: Andre Majorel [mailto:amajorel at teaser.fr]
> 
> Would that be FM or AM ? Is the switch instantaneous, and how
> often does it occur ?

In the applications that I'm familiar with, the
modulation rate is quite a bit lower than the
carrier frequency; for example, an 11 GHz carrier
modulated at 48 MHz.  I'm assuming that the switch
is discrete and instantaneous at each bit time,
but I'm not totally sure about that.  Basically,
the system generates an in-phase carrier (they
refer to this as the I channel) and a quadrature
phase carrier (referred to as the Q channel), and
the modulation causes switches back and forth
between the inverted and non-inverted carrier on
the I and Q channels to do the modulation.  Also,
the amplitude of the channels isn't always the
same; in some signals the I channel will have more
power than the Q channel or vice versa.  

I think it's more akin to FM than AM, but not 
quite the same.  This is stretching my knowledge
of the subject about as far as it goes.  I've
been trying to find some info about what it does
to the power spectrum, to compare it to what I
know about FM and AM, but I haven't come up with
anything yet.