AM removal from FM signal generated by IQ modulator

DETAILED DESCRIPTION The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
As described herein, a circuit is provided for use in a dual mode mobile phone for selectively one of phase modulating or frequency modulating an RF signal.
The circuit includes an IQ generator for generating one of first quadrature waveshapes or second quadrature waveshapes in response to a modulating signal; a quadrature mixer for mixing the first quadrature waveshapes or the second quadrature waveshapes with an RF signal to generate a phase modulated RF signal or a frequency modulated signal, respectively; and a limiter, such as at least one limiting amplifier, that has an input coupled to an output of the quadrature mixer for reducing an amplitude modulation component of only the frequency modulated signal.
The at least one limiting amplifier has an input coupled to a limit control signal that has a magnitude selected for setting a dynamic range of the at least one limiting amplifier.
In a presently preferred, but not limiting, embodiment of this invention the phase modulated signal is a .
pi.
/4-DQPSK signal.
Also described is an analog mobile phone that employs the IQ modulator and limited of this invention to generate a FM transmission signal, and that furthermore employs but a single variable local oscillator that is used to quadrature mix and up-convert the transmitted FM signal, and that is also used to down-convert a received FM signal to an IF signal.
The IF signal has a frequency equal to the channel spacing between the transmitted and received FM signals, or 45 MHz in an analog mobile phone compatible with IS-91.