RC/CR automatic quadrature network

• US 5,644,260 A
• Filed: 10/13/1995
• Issued: 07/01/1997
• Est. Priority Date: 09/22/1993
• Status: Expired due to Term

• Current Assignee: Agilent Technologies, Inc.
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Teetzel, Andrew M., DaSilva, Marcus K.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US08/542,745
• Time in Patent Office: 627 Days
• Field of Search: 333/2, 333/18, 333/124, 333/139, 307/511, 307/512, 307/262, 328/55, 328/155, 332/103, 327/238, 327/240, 327/243
• US Class Current: 327/238
• CPC Class Codes: H03C 1/02 Details
• CPC Class Codes: H03C 1/52 Modulators in which carrier or one sideband is wholly or partially suppressed H03C1/28 - H03C1/34, H03C1/46, H03C1/48 take precedence
• CPC Class Codes: H03C 1/60 with one sideband wholly or partially suppressed
• CPC Class Codes: H03C 2200/0016 Pre-emphasis or de-emphasis circuits
• CPC Class Codes: H03C 2200/0058 Quadrature arrangements
• CPC Class Codes: H03C 3/00 Angle modulation H03C5/00, H03C7/00 take precedence
• CPC Class Codes: H03C 3/40 using two signal paths the outputs of which have a predetermined phase difference and at least one output being amplitude-modulated
• CPC Class Codes: H03C 5/00 Amplitude modulation and angle modulation produced simultaneously or at will by the same modulating signal H03C7/00 takes precedence
• CPC Class Codes: H03D 1/2245 using two quadrature channels H03D1/2209 takes precedence
• CPC Class Codes: H03D 3/007 by converting the oscillations into two quadrature related signals H03D3/245 takes precedence
• CPC Class Codes: H03H 11/16 Networks for phase shifting
• CPC Class Codes: H03H 11/18 Two-port phase shifters providing a predetermined phase shift, e.g. "all-pass" filters
• CPC Class Codes: H03H 11/22 providing two or more phase shifted output signals, e.g. n-phase output

Abstract

An IQ modulator incorporates a quadrature network that is responsive to a frequency dependent control signal and that has in-phase and quadrature signals that are of equal amplitude and in exact quadrature over a wide range of applied frequencies. The quadrature network includes RC and CR phase shifters whose C'"'"'s are fixed capacitors of equal value and whose R'"'"'s are made equal to each other and to the capacitive reactance of the C'"'"'s by the action of the frequency dependent control signal. The R'"'"'s may be FET'"'"'s. The frequency dependent control signal may be generated without express knowledge of the applied frequency by a servo loop that nulls out the amplitude difference between the in-phase and quadrature outputs from the quadrature network; it may also be generated from a look-up table as an express function of frequency. The frequency dependent control signal is split into separate instances that are applied to each R, and an offset may be introduced therebetween to provide extreme precision. The offset may be fixed or programmatically controlled as a function of frequency. Varactors may be used to provide variable C'"'"'s in cooperation with fixed R'"'"'s. The entire circuit may be fabricated as a GaAs IC for use in the frequency range of 200 MHz to 3 GHz.

Claims

1. A phase shift circuit comprising:

a first phase shifter having a first input that receives an input signal to be phase shifted, a first output at which appears a phase shifted first output signal having an amplitude that is related to the phase difference between the input signal and the first output signal, and a first control input that receives a first control signal to which the first phase shifter is responsive by varying the phase shift between the input signal and the first output signal;

a second phase shifter having a second input that receives the input signal, a second output at which appears a phase shifted second output signal having an amplitude that is related to the phase difference between the input signal and the second output signal, and a second control input that receives a second control signal to which the second phase shifter is responsive by varying the phase shift between the input signal and the second output signal;

a first amplitude detector, coupled to the first output signal, that produces a first level signal corresponding to the amplitude of the first output signal;

a second amplitude detector, coupled to the second output signal, that produces a second level signal corresponding to the amplitude of the second output signal;

a differential amplifier having inputs coupled to the first and second level signals and having an output; and

a circuit coupled to the output of the differential amplifier and having outputs coupled to the first and second control inputs, the circuit introducing a differential offset between the first and second control signals.

2. A phase shift circuit as in claim 1 wherein:

the first phase shifter further comprises a first variable resistance in series with a first capacitance to a signal ground, the first control signal determines the resistance of the first variable resistance, and the first output signal is the voltage across the first capacitance; and

the second phase shifter further comprises a second capacitance in series with a second variable resistance to the signal ground, the second control signal determines the resistance of the second variable resistance, and the second output signal is the voltage across the second variable resistance.

3. A phase shift circuit as in claim 2 wherein the first and second capacitances are of equal value.

4. A phase shift circuit as in claim 3 wherein the first and second variable resistances are FET'"'"'s exhibiting equal resistances whenever the first and second control signals are equal, and wherein the first and second output signals are in quadrature.

5. A phase shift circuit as in claim 1 wherein:

the first phase shifter further comprises a first variable capacitance in series with a first resistance to a signal ground, the first control signal determines the capacitance of the first variable capacitance, and the first output signal is the voltage across the first resistance; and

the second phase shifter further comprises a second resistance in series with a second variable capacitance to the signal ground, the second control signal determines the capacitance of the second variable capacitance, and the second output signal is the voltage across the second variable capacitance.

6. A phase shift circuit as in claim 5 wherein the first and second resistances are of equal value.

7. A phase shift circuit as in claim 6 wherein the first and second variable capacitances are varactor diodes exhibiting equal capacitances whenever the first and second control signals are equal, and wherein the first and second output signals are in quadrature.