Quadrature generator with image reject mixer

DETAILED DESCRIPTION While the conversion circuit shown in FIG.
5 may be a smaller and less expensive alternative to one that includes an image reject filter, it suffers from a susceptibility to changes in the frequency of local oscillator 5.
This susceptibility is a consequence of the nonideal behavior of phase shifter 20 with respect to changes in the frequency of the signal that drives it.
Specifically, as the frequency of local oscillator signal S20 drifts away from .
omega.
.
sub.
LO (e.
g.
because of local heating, changes in ambient temperature, electromagnetic interference, component aging, etc.
), the shift performed by phase shifter 20 may deviate from 90 degrees.
Variances during circuit fabrication may also cause a frequency error in the output of the local oscillator, producing a similar deviation of this phase relation from the expected value.
As explained elsewhere (e.
g.
, by Behzad Razavi in RF Microelectronics, Prentice Hall PTR, Upper Saddle River, N.
J.
, 1998, ISBN 0-13-887571-5 at Chapter 5, section 5.
2), an error in the output of phase shifter 20 may prevent image reject mixer 100 from canceling the unwanted image components, thereby allowing them to corrupt IF signal S30.
Even in the absence of an image component in RF signal S10, the error may cause distortion (e.
g.
phase distortion) in IF signal S30.
It is desirable to obtain a heterodyne conversion operation that is more tolerant of local oscillator frequency drift and deviation.
FIG.
6 shows a block diagram of a converter 300 according to an embodiment of the invention that is configured and arranged to receive a RF signal S10 and output an IF signal S130.
Converter 300 includes an image reject mixer 200 and a quadrature generator 400 that supplies in-phase and quadrature generator signals S140 and S150 (both signals having a frequency .
omega.
.
sub.
g) to image reject mixer 200.
FIG.
7 shows a block diagram of an implementation 200a of image reject mixer 200.
In this circuit, phase shifter 110 receives RF signal S110 and outputs I and Q signals as indicated