Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1998-09-11
2003-03-18
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S302000, C455S102000, C455S553100, C341S118000, C332S103000, C332S119000
Reexamination Certificate
active
06535561
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to modulation systems and methods, and more particularly to dual-mode modulation systems and methods.
BACKGROUND OF THE INVENTION
Modulation systems and methods are widely used in transmitters to modulate an information input including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as “mixers” or “multipliers”. For example, in a mobile radiotelephone, a modulator is used for the radiotelephone transmitter.
As is well known to those having skill in the art, modulation systems and methods for digital input signals generally include a Digital-to-Analog Converter (DAC) that converts the digital input signal into an analog signal. A low pass filter, also referred to as an “anti-aliasing filter”, filters the analog signal to produce a filtered analog signal. A modulator modulates the filtered analog signal onto a carrier. The modulator includes a multiplier that is coupled to a local oscillator, such as a Voltage Controlled Oscillator (VCO), and to the filtered analog signal. The carrier including the filtered analog signal may then be transmitted by an antenna.
In modern communications systems, it is often desired to provide dual-mode modulation systems and methods that can modulate two types of communications signals. For example, in mobile radiotelephones, it is often important to provide a modulator that operates both in narrowband FM mode and in wideband Code Division Multiple Access (CDMA) mode. More particularly, in order to provide a mobile radiotelephone that can be used with both an IS-19 AMPS analog system and an IS-95 Direct Sequence Spread Spectrum (DSSS) wideband CDMA system, it is desirable to provide dual-mode modulation systems and methods.
Unfortunately, it may be difficult to provide a dual-mode modulation systems and methods that can handle the disparate bandwidths of the AMPS and CDMA signals. In particular, the narrowband AMPS FM signal has a bandwidth of about 12.5 KHz, while the wideband CDMA signal has a bandwidth of about 615 KHz, or about an order of magnitude wider.
In modem radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to share circuitry in dual-mode radiotelephones. Shared circuitry can decrease the number of components that are used in the modulator, thereby allowing a decrease in the size thereof. Shared components can also decrease the power consumption of the dual-mode modulation system, which can allow an increase in battery time. Finally, sharing of components can allow a decrease in component cost, thereby allowing a decrease in the overall cost of the radiotelephone.
FIG. 1
illustrates a first conventional dual-mode modulator. As shown in
FIG. 1
, an IQ modulator
10
, also referred to as a “quadraphase modulator” or a “quadrature modulator” includes a quadrature splitter
20
, also known as a 90° phase shifter, and a pair of multipliers
16
a
,
16
b
coupled to the quadrature splitter. A local oscillator
15
, such as a Voltage Controlled Oscillator (VCO), is coupled to the quadrature splitter
20
to produce 90° phased shifted local oscillator signals. I data
11
a
and Q data
11
b
are coupled to a respective multiplier or mixer
16
a
,
16
b
respectively. Digital input data is converted to analog data by I Digital-to-Analog Converter (DAC)
14
a
and Q DAC
14
b
, respectively. The outputs of the DACs
14
a
and
14
b
respectively are applied to low pass filters
12
a
and
12
b
respectively to provide the I and Q data inputs
11
a
and
11
b
respectively. The modulator modulates the input data on a carrier
13
, by summing the outputs of the multipliers
16
a
,
16
b
at summing node
218
, and transmits the modulated carrier
13
via an antenna.
The DACs
14
a
and
14
b
, low pass filters
12
a
and
12
b
and IQ modulator
10
may be used to modulate a high bandwidth CDMA signal such as a Direct Sequence Spread Spectrum (DSSS) signal onto a carrier. Since the signal is generated digitally, it is low pass filtered by filters
12
a
and
12
b
to let the information through while removing digitally generated spurs and noise.
In order to use the IQ modulator
10
of
FIG. 1
in a dual-mode, such as for narrow bandwidth FM signal, a separate FM DAC
19
and a separate FM low pass filter
17
may be provided. Baseband circuitry generates an FM voltage signal that is applied to the tune line of the VCO, to modulate the FM information onto the carrier for transmission according to the AMPS standard. Since the FM voltage signal is generated digitally, it is low pass filtered by FM low pass filter
17
to let the information through while removing digitally generated spurs and noise.
The low pass filter
17
generally has a different bandpass characteristic than the low pass filters
12
a
and
12
b
that are part of the CDMA modulator, due to the widely differing bandwidths of the FM and CDMA signals. Accordingly, in this dual-mode embodiment, a separate FM DAC
19
and a separate FM low pass filter
17
is provided. Modulation systems according to
FIG. 1
have been designed into many integrated circuit chip sets developed for CDMA standards that also include AMPS functionality. Unfortunately, this technique uses separate DACs and low pass filters, which may increase the size, cost and/or power consumption of the modulator.
A second dual-mode modulation system is illustrated in FIG.
2
. In this figure, an IQ modulator
210
including a quadrature splitter
220
, a pair of multipliers
216
a
and
216
b
, a summing node
218
and a VCO
215
are provided to produce a modulated carrier
213
. However, in contrast with
FIG. 1
, the DACs and low pass filters are shared for the dual-mode operation. In particular, the I DAC and Q DAC
214
a
and
214
b
respectively are used for both wideband CDMA and narrowband FM operation. Low pass filters
212
a
and
212
b
are also used for wideband CDMA and narrowband FM operation.
Unfortunately, due to the widely disparate bandwidths of the CDMA signal and the FM signal, the low pass filters
212
a
and
212
b
should have different band pass characteristics when in the different modes. In order to share the low pass filter, the band pass frequency is switched depending upon mode. Accordingly, while these switched filters
212
a
,
212
b
are used in both modes, they may be expensive to implement and may consume excessive power and/or area in a radiotelephone.
In high performance communications systems, it also may be desirable to provide high carrier suppression. In order to provide high carrier suppression, a low DC offset should be produced in the modulation system. For example, the required carrier suppression for FM modulation in an IS-19 AMPS analog system may be approximately −35 dBc. In order to provide an acceptable design margin, it may be preferred for the nominal carrier suppression to be −40 dBc, which can translate into a 14 mV differential DC offset signal when a 2V peak-to-peak differential information signal is generated in a balanced system.
Low DC offset in the digital input signal may be provided using conventional techniques. Unfortunately, however, the modulation system may generate its own DC offset. More specifically, the digital-to-analog converter and/or the low pass filter may generate DC offsets.
The DC offset that is generated in the digital-to-analog converter can be reduced using high performance digital-to-analog converters. Unfortunately, these digital-to-analog converters may be costly and complex. DC offset can be reduced in the low pass filter by providing a passive, off-chip filter with tight tolerance components. Unfortunately, such a passive off-chip filter may be costly and complex, and may consume excessive space in a portable radiotelephone.
SUMMARY OF THE INVENTION
It is therefore an object of the present in
Arpaia Domenico
Boesch Ronald D.
Ericsson Inc.
Le Amanda T.
Myers Bigel & Sibley & Sajovec
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