The present invention relates to quadrature frequency converters, and more particularly to an automatic quadrature network with phase and amplitude detection to eliminate both phase and amplitude errors in a frequency converter output.
Quadrature (I/Q) frequency converters rely on having a pair of mixers which are driven by a quadrature local oscillator signal, i.e., two signals of the same frequency, but ninety degrees out of phase. One of the simpler methods of achieving the desired result is to input a single frequency to a quadrature network having two phase shifting paths, one path with a series resistor followed by a shunt capacitance (RC), and the other path with a series capacitance followed by a shunt resistance (CR). If the resistances are equal and the capacitances are equal, then there is a particular frequency for the single frequency input where the outputs of the two paths are of equal amplitude and in quadrature phase. Outside of the particular frequency, while the phase relationship is held, the amplitudes may vary.
One method of generating quadrature signals from a local oscillator (LO) uses amplitude detectors and a feedback control loop which adjusts either resistance or capacitance values in the RC and CR paths until the quadrature signals are matched in amplitude. This method is illustrated by U.S. Pat. No. 5,644,260 (DaSilva) and G.B. Patent No. 1,345,274 (Ratzel). However, this method cannot generate perfect phase quadrature and equal amplitudes when the non-varied elements of the network don't match, when the variable elements don't track exactly, or when parasitic elements exist. For example, because of some non-ideal components—an R is too big or too small compared to the other or a C has the same problem, etc.—the frequency of the RC circuit in one quadrature signal path is too low and the frequency of the CR circuit in the other quadrature signal path is too high. FIG. 1 is a graphic representation of the DaSilva implementation, which shows that at the specified frequency—100 MHz—the quadrature signals are equal in amplitude, but the phase differential between the two quadrature signals varies across the frequency spectrum—in this example at the LO frequency of 100 MHz the phase difference is 100° instead of 90°. In order to achieve the proper phase relationship, a static Vcal signal is used in conjunction with the detected amplitudes, the Vcal signal being selected from a table for the frequency of the particular local oscillator signal input. However, the Vcal signal cannot account for dynamic changes to the network due to time and temperature, so the network needs either frequent calibration or always has some phase error present.
An alternative method, if amplitude matching is not important, uses a phase detector and feedback loop to adjust one quadrature (RC) path or the other quadrature (CR) path until perfect phase quadrature is achieved, as illustrated by U.S. Pat. No. 4,908,532 (Chadwick). Ignoring amplitude matching, however, may cause amplitude errors, even though the signal is passed through limiting stages. Because the phase difference may be 90° over a wide range of amplitude differences, one side or the other might be so starved of signal that the limiting amplifiers cannot limit. This causes poor match between the two mixers, since the local oscillator level is not the same at the inputs to the mixers.
What is desired is an automatic quadrature network for a quadrature frequency converter that provides equal amplitude and ideal quadrature phase for quadrature signals derived from a local oscillator.