Single side-band mixer

DETAILED DESCRIPTION Applicant's invention resides, in part, in the recognition that a single sideband mixer including two supposedly 90.
degree.
phase shifters may be used to produce upper and/or lower sidebands with a high degree of sideband rejection even when the phase shifters have a phase angle error (.
theta.
), so long as the phase angle error of the two phase shifters are substantially equal and the amplitude of the two orthogonal outputs of each phase shifter have substantially equal amplitudes.
Thus, in contrast to the prior art, a single sideband mixer system embodying the invention includes the application of first and second input frequencies (.
omega.
1, .
omega.
2) to first and second 90.
degree.
all-pass phase shifters to produce first and second supposedly orthogonal outputs per phase shifter.
The outputs of the two phase shifters are then applied to inputs of first and second mixer circuits whose outputs are then combined to produce either the upper (the sum of .
omega.
1 and .
omega.
2) or the lower (the difference between .
omega.
1 and .
omega.
2) sidebands of the two input frequencies.
The all-pass type phase shifters are used to ensure that there is little, if any, amplitude imbalance in the 90.
degree.
degree phase shifter outputs.
The first and second 90.
degree.
phase shifters embodying the invention are preferably formed such that any phase error (i.
e.
, .
theta.
1) in the first phase shifter is essentially equal to any phase error (i.
e.
, .
theta.
2) in the second phase shifter and that the phase errors .
theta.
1 and .
theta.
2 track each other.
Accordingly, in systems embodying the invention, the first and second phase shifters are preferably formed within an integrated circuit and in close proximity to each other so that they track each other over time and varying ambient and voltage conditions.
The outputs of the first and second phase shifters are supplied to mixer circuits whose outputs are combined.
If the phase error of the two phase shifters track, the phase errors are significantly reduced when the mixer outputs are combined to produce an output signal V