Phase shifter with an RC polyphase filter

Details Phase shifter with an RC polyphase filter Phase shifter with an RC polyphase filter

2002-04-19

2003-10-21

Callahan, Timothy P. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Converting input current or voltage to output frequency

C327S103000

Reexamination Certificate

active

06636085

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase shifter for performing orthogonal modulation/demodulation in digital communications and, more particularly, to a phase shifter using an RC filter composed of a resistive element and a capacitive element.
2. Description of the Prior Art
In digital communications, an orthogonal modulation/demodulation technology plays an important role. In orthogonal modulation/demodulation, two orthogonal local signals or high-frequency (RF) signals are used. The phase shifter is a circuit for generating an orthogonal signal, i.e., a signal having a phase difference of 90°. The phase shifter must have the following characteristics: (1) small phase and amplitude errors, (2) broad in band, and (3) low in power consumption.
Phase shifters are usually classified as (A) a type using an RC filter composed of a resistive element and a capacitive element, and (B) a type using a frequency divider compose of a flip-flop. The phase shifter of the type (B) has an advantage of small phase and amplitude errors in a broad band. In the phase shifter of the type (B), however, there are fundamental constraints imposed. That is, because of a necessity of a reference signal having a frequency higher by frequency division ratio times, an upper limit of an operation frequency is restricted by an operating limit of the flip-flop.
On the other hand, in the case of the phase shifter of the type (A), because of a fixed RC time constant in an integrated circuit, an amplitude error is increased only with a simple combination of a low pass filter and a high pass filter, and thus a correction circuit such as PLL must be added. A phase shifter using an all pass filter is also available. However, this phase shifter was impractical, because a ratio of elements necessary for achieving an RC constant made manufacturing of an integrated circuit difficult in most cases. In addition, when a frequency becomes higher, an RC time constant must be reduced. However, there is a limit to a capacity value for accurate formation in the integrated circuit. Thus, a value of resistance must be reduced, making it necessary to lower input impedance of the RC filter. For driving the RC filer, a current proportional to an inverse number of impedance of the RC filter is necessary. Consequently, as a frequency is higher, consumption of current is increased.
As an RC filter capable of obtaining an orthogonal signal broad in a band, and small in both amplitude and phase errors, a polyphase filter having resistive elements and capacitive elements cyclically connected is known. However, the polyphase filter is a 4-phase RC filter, where input impedance is smaller than that of a normal single phase RC filter. Thus, the polyphase filter must be used in small amplitude, or by increasing a driving current.
FIG. 1
is a block diagram of a conventional phase shifter described in Japanese Patent Application Laid-Open No. 11 (1999)-298293. This conventional phase shifter includes a differential amplifier
101
for driving, cascade-connected RC polyphase filters (simply referred to as polyphase filters, hereinafter) of N stages
201
, . . . ,
20
N, and differential amplifiers
301
, . . . ,
30
N used as buffer circuits. In this conventional phase shifter, the polyphase filter
201
is driven by the differential amplifier
101
. However, since a large driving current cannot be supplied for the above-described reason, driving must be carried out by small amplitude. In addition, in order to achieve a broader band, a plurality of polyphase filters are cascade-connected. However, signal voltages are decayed at the polyphase filters
201
, . . . ,
20
N. Thus, the differential amplifiers
301
, . . . ,
30
N are provided between the polyphase filters to amplify the signal voltages. Because of such configuration, a circuit size was increased, creating a problem of an increase in power consumption. Especially, in a device driven by a battery such as a portable equipment, an increase in power consumption is not preferable, because it directly shortens use time.
SUMMARY OF THE INVENTION
The conventional phase shifter using the polyphase filters is advantageous in that a broad band can be achieved, and errors in both amplitude and phase can be suppressed. However, the necessity of providing the buffer differential amplifiers for driving with small amplitude has increased the circuit size, consequently increasing the power consumption.
The present invention is devised in view of such a circumstance, and an object of the invention is to provide a phase shifter improved for a reduction in power consumption, inheriting the advantages of the phase shifter using the polyphase filters, i.e., the capabilities of achieving a broad band, and suppressing errors in both amplitude and phase.
In order to achieve the above-described object, in accordance with an aspect of the present invention, there is provided a phase shifter, comprising: a voltage-to-current conversion circuit for receiving an input signal, converting a voltage value of the input signal into a current value, and outputting an input current signal; a polyphase filter composed of a plurality of cascade-connected stages of RC filters for receiving the input current signal, and outputting a polyphase phase-shifted current signal, each RC filter having resistive and capacitive elements connected alternately cyclically, one end of each resistive element being set an input terminal, and the other end of each resistive element being set as an output terminal; and a load circuit including a current-to-voltage conversion circuit for converting each current value of the polyphase phase-shifted current signal into a voltage value, and outputting a output signal. Since the voltage value of the input signal is converted into the current value, and supplied to the polyphase filter, the number of buffer circuits conventionally necessary for amplifying signals can be reduced, and consumption of power can be reduced.
According to the phase shifter of the invention, the voltage-to-current conversion circuit includes a differential amplifier composed of a first transistor having an emitter connected to a constant current source, one input signal as a balanced signal entered to a base, and a collector connected to one input terminal of the polyphase filter, and a second transistor having an emitter connected to the constant current source, the other input signal connected to a base, and a collector connected to the other input terminal of the polyphase filter, and a current signal corresponding to a voltage value of the input signal is supplied to the polyphase filter.
According to the phase shifter of the invention, the polyphase filter is constructed by cascade-connecting n stages (n≧2) of RC filters, each RC filter including a first resistor having one end connected to a first input terminal, and the other end connected to a first output terminal, a second resistor having one end connected to a second input terminal, and the other end connected to a second output terminal, a third resistor having one end connected to a third input terminal, and the other end connected to a third output terminal, a fourth resistor having one end connected to a fourth input terminal, and the other end connected to a fourth output terminal, a first capacitor having one end connected to the other end of the first resistor, and the other end connected to one end of the second resistor, a second capacitor having one end connected to the other end of the second resistor, and the other end connected to one end of the third resistor, a third capacitor having one end connected to the other end of the third resistor, and the other end connected to one end of the fourth resistor, and a fourth capacitor having one end connected to the other end of the fourth resistor, and the other end connected to one end of the first resistor, the first and second input terminals of the RC filer of the first stage in an input side are connected in common to enter one input current

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