Broadband phase shifting circuit having two phase shifter branches connected in parallel

A broadband phase-shifting circuit, in particular for an IQ modulator, has two phase-shifting branches connected in parallel, to the input of which is supplied the input signal of which the phase is to be shifted, and which supply at their outputs output signals of which the phase is shifted over a predetermined angle, in particular of 90.degree.. The phase shifter of one phase-shifting branch is controlled depending on the frequency of the input signal in such a way that the phase angle between the two output signals approximately corresponds to the desired value (coarse control), whereas the phase shifter of the other phase-shifting branch is set to the desired phase angle (fine regulation by a phase detector connected between the outputs of the two phase-shifting branches.

BACKGROUND OF THE INVENTION
 The invention is directed to a broadband phase shifter circuit.
 More specifically, the invention is directed to a broadband phase shifter
 circuit, particularly for an IQ modulator, comprising two phase shifter
 branches connected parallel to which the input signal to be shifted in
 phase is supplied at the input side and that supply phase-shifted output
 signals at their outputs shifted by a predetermined angle, such as
 90.degree..
 Broadband phase shifter circuits of this species are known; for example,
 they are employed as 90.degree. phase shifters for IQ modulators (for
 example, according to U. S. Pat. Nos. 4,908,532, 4, 951,000 or 5,644,260).
 However, these respectively have various disadvantages. The fashioning of
 the phase shifters as RC elements with variable capacitance diodes and
 their regulation via a phase detector connected between the outputs (U.S.
 Pat. No. 4,908,532) results in indefinite amplitudes of the output
 voltages at the two phase shifter branches. The different amplitudes
 deteriorate the modulation precision of the following IQ modulator. The
 opposed phase correction of two identical phase shifters in the two phase
 shifter branches via a phase detector (U.S. Pat. No. 4,951,000) is limited
 to specific phase shifters and cannot be employed, for example, given
 simple RC elements. The regulation of the two phase shifter branches
 dependent on the difference of the amplitude of the two output voltages
 (U.S. Pat No. 5,644,260) is not exact enough with respect to adhering to
 the desired, exact phase relationship.
 SUMMARY OF THE INVENTION
 It is therefore an object of the invention to create a broadband phase
 shifter circuit that avoids these disadvantages and assures exact
 adherence to the desired phase angle, given amplitudes of the output
 voltages of respectively the same size in a broad frequency range
 regardless of the kind of phase shifters employed.
 To that end, in an embodiment the invention provides a phase shifter
 circuit in which the phase shifter of the one phase shifter branch is
 controlled dependent on the frequency (f) of the input signal such that
 the phase angle between the two output signals approximately corresponds
 to the desired value (rough control); and the phase shifter of the other
 phase shifter branch is regulated to the desired phase angle via a phase
 detector connected between the outputs of the two phase shifter branches
 (fine control).
 In an embodiment, the invention provides a broadband phase shifter circuit,
 comprising: two phase shifter branches connected in parallel to which an
 input signal to be shifted in phase is supplied at a common input side and
 which supply at respective outputs phase-shifted output signals shifted by
 a predetermined angle, wherein, each phase shifter branch comprises a
 phase shifter, the phase shifter of one phase shifter branch is controlled
 dependent on the frequency of the input signal such that the phase angle
 between the phase-shifted two output signals approximately corresponds to
 a desired value for relatively rough control purposes, the phase shifter
 of the other phase shifter branch is controlled to the desired phase angle
 via a phase detector connected between the outputs of the two phase
 shifter branches for relatively fine control purposes.
 In an embodiment of the invention, the phase shifter of the one phase
 shifter branch comprises an RC low-pass element and the phase-shifter of
 the other phase-shifter branch comprises an RC high-pass element.
 In an embodiment of the invention, the phase shifters in the phase shifter
 branches comprise all-pass filters.
 In an embodiment of the invention, there are included respective variable
 resistors for effecting the rough control and fine control.
 In an embodiment of the invention, the variable resistors comprise
 drain-source paths of field effect transistors.
 In an embodiment of the invention, there is included respective variable
 capacitances for effecting the rough control and fine control.
 In an embodiment of the invention, there is included the phase shifters of
 the two phase shifter branches comprise RL elements with variable
 resistors.
 In an embodiment of the invention, the phase shifters of the two phase
 shifter branches comprise RL elements with variable resistors and variable
 inductances.
 In an embodiment of the invention, the actuating variables belonging to the
 respective frequency for the rough control are stored in a memory and are
 read therefrom dependent upon the frequency of the input signal.
 In the inventive phase shifter circuit, one phase shifter branch, dependent
 on the frequency of the input signal, is roughly set to a value that
 approximately corresponds to the desired, predetermined phase angle. Only
 the second phase shifter branch is re-adjusted so finely via a phase
 detector that the desired, predetermined phase angle, for example
 90.degree., is ultimately achieved. This principle of rough pre-setting
 and fine readjustment is suitable for all possible phase shifters, both
 for simple RC elements as well as for all-pass elements, delay elements or
 phase shifter circuits upon employment of inductances as well. The only
 pre-condition is that the phase shifters employed can be respectively
 dimensioned such in the two phase shifter branches that the output signals
 at their two outputs respectively exhibit the desired phase angle. Since,
 according to the invention, fine regulation is undertaken via the phase
 detector directly dependent on the phase and not via the expedient of an
 amplitude comparison, the circuit is also insensitive to temperature
 effects and aging of the components. As a result of the combined rough
 control / fine adjustment, preferably via variable resistors in the phase
 shifters of the branches, an inventive phase shifter circuit is extremely
 broadband over up to two frequency decades.
 The invention is explained in greater detail below on the basis of
 schematic drawings with reference to an exemplary embodiment.
 These and other features and aspects of the invention will become clear in
 the following detailed description of a few typical exemplary embodiments
 with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
 FIG. 1 shows the schematic diagram of an inventive broadband phase shifter
 circuit, namely applied in an IQ modulator. The one phase shifter branch 1
 is formed by a low-pass with a variable resistor 2 and a fixed capacitor 3
 plus following limiter 4; the other phase shifter branch 5 is formed by a
 high-pass with a fixed capacitor, a variable resistor 7 and a following
 limiter 8. The low-pass generates a phase shift of -45.degree., the
 high-pass a phase shift of +45.degree.. A high-frequency input signal
 supplied to the common input 11 is divided via the two phase shifter
 branches 1 and 5 into two output signal components at the outputs 9 and 10
 that exhibit an exact 90.degree. phase shift relative to one another. The
 output 9 is connected to the mixer 12 for the I-input; the output 10 is
 connected to the mixer 13 for the Q-input of the IQ modulator. The outputs
 of the mixers are combined in an adder 14. The 90.degree. phase shifter
 generates the two signal components of the carrier signal supplied to the
 input 11 that are phase-shifted by 90.degree. relative to one another.
 A phase detector 15, for example a multiplier, whose output is connected
 via a controlled-gain amplifier 16 to the variable resistor 7 of the
 high-pass is inserted between the outputs 9 and 10 of the limiters. The
 variable resistor of the low-pass is controlled dependent on the frequency
 of the HF input signal. The two variable resistors 2 and 7, for example,
 are formed by the drain-source path of a field effect transistors that is
 variable via the gate terminal.
 The control of the resistor 2 dependent on the frequency ensues via a
 memory 17 in which the appertaining actuating variables U for the resistor
 2 are stored a digital values for the frequency values f of the HF input
 signal. The actuating variable U selected dependent on the input frequency
 f is converted via a digital-to-analog converter 18 into a corresponding
 analog actuating variable for the resistor 2, for example the gate control
 voltage for a field effect transistor. In the simplest case given a
 constant input frequency, the input of the respective frequency value f
 can ensue manually by the user; a corresponding frequency meter can be
 used given changing frequency. In most cases, the frequency of the HF
 input signal in such applications is present as digital value, and the
 appertaining actuating variable U can then be read directly from the
 memory 17 therewith.
 Via the resistor 2, the low-pass is roughly pre-set to a value that
 approximately yields a phase shift of -45.degree. with a precision of
 +5.degree.. Via the control circuit of the phase detector 15, the resistor
 7 of the high-pass is then re-adjusted such that the output signals at the
 outputs 9 and 10 exhibit the desired, exact 90.degree. phase shift, the
 output of the phase detector thus supplied the actuating variable 0.
 According to the inventive principle, not only can a mutual 90.degree.
 phase shift be exactly set, but any arbitrary phase angle between 0 and
 180.degree.. For example, this can occur in a simple way in that a
 corresponding comparison voltage is applied to the +-input of the
 controlled-gain amplifier 16, and the phase between the output signals is
 then regulated to this predetermined voltage offset.
 The size of the frequency steps under which the respective actuating
 variables U are stored in the memory 17 is based on the desired precision.
 The relationship between frequency f and actuating variable U is
 preferably determined via a one-time calibration event in which the
 respectively appertaining actuating variables U are determined in steps of
 approximately 10% frequency change for various frequency reference values
 within the band width in which the phase shifter is to be operated.
 Instead of the CR elements 2, 3 or, RC elements 6, 7, all-passes can also
 be utilized in the two phase shifters 1 and 5. The schematic diagram of
 such an all-pass is shown in FIG. 2. All-passes have the advantage that
 the output level remains constant regardless of the frequency or phase
 shift. Due to the constant frequency response of an all-pass, a specific
 operating point no longer has to be adhered to, as is the case given RC
 elements. As a result thereof, a larger frequency range can be covered
 with the same variation of the resistors.
 Given an all-pass according to FIG. 2, the amount of the gain is equal to 1
 when the differential amplifier 20 has a gain of 2. The gain is then +1 at
 low frequencies and -1 at high frequencies. By varying the resistor R, a
 phase shift can be set between 0.degree. and -180.degree. without
 influencing the amplitude. The phase shift amounts to .phi.=-2
 arctan(2.pi.fRC).
 The two limit frequencies of the all-passes utilized in the two phase
 shifters 1 and 5 are preferably shifted so far from one another that at
 least the desired phase difference can be set with adequate reserve. Due
 to the independence from the amplitude, the all-pass in the first phase
 shifter branch 1 can be set in far rougher steps; the difference phase
 must merely achieved the desired value of, for example, 90.degree.. For
 example, it is possible to set the first all-pass in the phase shifter
 branch 1 to -5.degree. and to adjust the all-pass in the phase shifter
 branch 5 to -95.degree. via the control loop. Given a frequency change,
 the phase in the phase shifter branch 1 changes to, for example,
 -50.degree. with a new pre-setting. The regulation then sets the second
 allpass in the phase shifter branch 5 to -140.degree.. As long as the
 range of control of the all-pass in the phase shifter branch 5 is still
 adequate, the pre-setting need not be modified.
 Such a phase shifter constructed with all-passes can thus, for example, be
 operated broadband in a frequency range from 100 MHz to 5 GHz by varying
 the resistors over a decade.
 The all-passes in the two branches 1 and 5 have the respectively same
 structure and, for example, are constructed according to FIG. 2; their
 limit frequency is merely selected correspondingly different. Instead of
 the rough control and fine regulation via the resistor R, the respective
 control and regulation given the all-passes can also ensue via a variation
 of the capacitance C that, for example, are [sic] fashioned as capacitance
 diodes. This is also possible given the RC elements 2, 3 or, respectively,
 6, 7 according to FIG. 1; here, too, variable capacitance diodes can be
 utilized instead of the variable resistors. A mixing of these control and
 regulating possibilities is also conceivable. The phase shifters could
 also be constructed as RL elements with variable resistors or variable
 inductances.
 Although modifications and changes may be suggested by those skilled in the
 art, it is the intention of the inventors to embody within the patent
 warranted hereon all changes and modifications as reasonably and properly
 come within the scope of their contribution to the art.