US20020075092A1 - Method and apparatus for generating digitally modulated signals - Google Patents
Method and apparatus for generating digitally modulated signals Download PDFInfo
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- US20020075092A1 US20020075092A1 US09/738,124 US73812400A US2002075092A1 US 20020075092 A1 US20020075092 A1 US 20020075092A1 US 73812400 A US73812400 A US 73812400A US 2002075092 A1 US2002075092 A1 US 2002075092A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
- H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
- H04L27/36—Modulator circuits; Transmitter circuits
- H04L27/365—Modulation using digital generation of the modulated carrier (not including modulation of a digitally generated carrier)
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Abstract
Description
- This invention relates to digital modulators, and, more particularly to digital modulators in which the modulation of a digital signal onto a carrier occurs entirely in the digital domain.
- Digital modulators are employed when it is desired to transform a digital data stream into an analog signal modulated onto a carrier or intermediate frequency signal. Digital modulators are used, for example, in cable modems, television set top boxes, Microwave Multipoint Distribution Systems, Local Multipoint Distribution Systems, Orthogonal Frequency Division Multiplexing, and Vector Orthogonal Frequency Division Multiplexing.
- In a classical implementation of a digital modulator, a pair of digital data streams, perhaps derived from a single data stream, are converted into analog signals and mixed as analog signals with the outputs of a quadrature oscillator to produce in-phase and quadrature-phase signals imposed on a carrier at the frequency determined by the oscillator.
- Other more recent digital modulators, known as direct digital synthesis (DDS) modulators, have provided similar digital data streams (which may be designated as the real and imaginary components of an original data stream) which are modulated onto a carrier signal entirely within the digital domain. This allows processing of the signal substantially entirely within one integrated circuit. Once the digitally modulated carrier is produced, it is converted to an analog modulated carrier by a digital to analog converter.
- This latter technique has several advantages, including the introduction of less distortion due to filter dissimilarities in the in-phase and quadrature-phase analog paths, better gain balance, and better phase balance.
- The oscillator of a DDS, however may introduce spurious signals and noise into the system, thereby reducing the effectiveness of the circuit. It would be desirable to retain the advantages of direct digital synthesis while at the same time reducing the spurious content of the oscillator.
- The above and other advantages of the present invention are attained by providing a method and apparatus for generating digitally modulated signals in which a serial data stream of digital signals to be modulated is provided, the serial data stream being converted into real and imaginary components which are then converted into a complex polar signal representing the serial data stream. A carrier of appropriate frequency is generated by an infinite impulse response filter and the polar signal is mixed with the output of the infinite impulse response filter to provide a representation of the complex polar signal modulated at the frequency generated by the infinite impulse response filter. Subsequently the imaginary component of the resulting representation is stripped from the signal and the real component of the resulting representation is applied to a digital to analog converter to produce an analog version of the serial data stream.
- FIG. 1 is a representation of a classical implementation of a digital modulator.
- FIG. 2 is a representation of a direct digital synthesis modulator.
- FIG. 3 is a block diagram of a digital modulator according to the present invention.
- FIG. 4 is a chart showing the input signal to the interpolator of the invention.
- FIG. 5 is a chart showing the output of the interpolator of the invention.
- The classical implementation of a digital modulator is shown in FIG. 1A discrete-time data signal is processed using digital signal processing techniques. A single digital data stream may be demultiplexed into two
data streams data streams pulse shaping filters 14 and 16. The filtered signals are applied tointerpolation filters analog converters low pass filters - An
oscillator 30 produces a carrier at a desired frequency that is applied to a quadrature phase shifter to produce the sin and cos components of the carrier. The sin and cos signals are mixed with the analog data streams inmixers summer 38 to produce a carrier which is modulated by analog signals which are representations of the original data stream. - In the representation of FIG. 1, elements10-20 are digital elements and elements 22-38 are analog elements.
- This modulation scheme has the disadvantage of being primarily analog and thus subject to distortion as a result of the difficulty of precisely matching analog filters and failing to take advantage of the simpler semiconductor processing techniques available in largely digital systems.
- FIG. 2 is a representation of a more recent digital modulator design, known as a direct digital synthesis (DDS) modulator. The modulator has provided two similar digital data streams40 and 42 (which may be designated as the real and imaginary components of a single original data stream derived as in the modulator of FIG. 1), but which are modulated onto a carrier signal entirely within the digital domain. This allows processing of the signal substantially entirely within one integrated circuit.
- The
data streams 40 and 42 are applied topulse shaping filters interpolations filters - The outputs of the
interpolation filters Polar conversion element 52 in order to create the amplitude and phase components of the baseband modulated signal. The phase modulation component is applied to the Numerically Controlled Oscillator (NCO) 54, thereby creating the phase modulation of the NCO created IF. The amplitude modulation component is applied directly to the Digital to Analog Converter 56 in order to create the amplitude modulation of the IF. The output of theDAC 56 is smoothed by theLow Pass Filter 58. - In this representation all the elements of the modulator are digital except for the Digital to Analog Converter56 and the
Low Pass Filter 58. - This latter technique has several advantages, including the introduction of less distortion due to filter dissimilarities in the in-phase and quadrature-phase analog paths, better gain balance, and better phase balance.
- The oscillator of a DDS, however may introduce spurious signals and noise into the system, thereby reducing the effectiveness of the circuit. It would be desirable to retain the advantages of direct digital synthesis while at the same time reducing the spurious content of the oscillator.
- FIG. 3 is a block diagram of a digital modulator according to the present invention. A digital data stream is generated by a source70 which may be the output of a vocoder, for example, or simply a digital data stream from the output of a computer. The digital data stream optionally may be subjected to error correction such as forward
error correction apparatus 72. - The encoded data stream is then demultiplexed by
demultiplexer 74 which creates two digital data streams designated as the real and imaginary portions of the digital data stream. Thedemultiplexer 74 may operate simply by taking every second digital signal to be designated as the real component and the others as the imaginary portion of the data stream to create a series of complex digital numbers. - The real and imaginary portions of the data stream may be optionally applied to
pulse shapers - The real and imaginary outputs of the
pulse shapers polar converter 80. If the pulse stream of the real portion of the rectangular waveform is a(t) and the imaginary portion is b(t), then the complex number at the input ofconverter 80 is a(t)+jb(t). Theconverter 80 merely solves the equations: A(t)=sqrt{a2(t)+b2(t)} and θ(t)=tan−1{b(t)/a(t). - In a normal implementation, the clock rate necessary for the generation of the IF and modulated signal must be considerably higher than the clock rate necessary to process the data. Because of this the
pulse shaping filters interpolation filter 82, and the rectangular topolar converter 80 will operate at a slower sample clock than the clock used to process the data to be modulated. The respective clocks are Fclkd<Fcdk, where Fclkd is the clock used in processing the data and Fdkm is the clock used to actually modulate the polar data onto the digital IF. - The outputs of the rectangular to
polar converter 80 are thus applied to an interpolator where the additional numerical points are added to the points of the digital data streams to provide a smoother representation of the data stream and to match to higher clock rate of the digital IF. For example, FIG. 4 shows a series of points on a graph representing the numbers generated at the output of the rectangular to polar 80. FIG. 5 is a representation of the same series of numbers after having been interpolated by a factor of four to one (four to one is an exemplary ratio only; the actual interpolation rate will be determined by the ratio of Fclkd to Fclkm). - An infinite impulse
response filter IIR multiplier 84. The output ofmultiplier 84 is applied tocomplex multiplier 88 where it is mixed with the output of theinterpolation filter 82. The output signal ofmultiplier 84 is also applied to adelay register 86, the output of which is fed back, delayed, to themultiplier 84 where the delayed value fromdelay register 86 is mixed with the input carrier phasor, exp{−j2πf0TM}. The infinite impulse response filter operates in accordance with the following equation: - Y(nT M)=δ(nT M)+exp{−j2πT M }y[(n−1)T M]
- The impulse δ initializes the IIR oscillator and the oscillation is sustained by feedback. Thus a one is stored in the
delay register 86 for the first cycle. So δ=1 and y[(n−1)TM]=0 resulting in a first output of 1. On the next cycle, δ becomes 0 (δ(n)=0). For subsequent groups, exp{−j2πf0TM} controls the frequency, fIF=f0·TM. - The modulation in the
complex multiplier 88 takes place in the digital domain. The output is a complex modulated signal: - Z(nT M)=M(nT M)e jφ(nT M)e −j2πf IF T M
- Rewriting this equation in terms of real and imaginary components:
- Z(nT M)=M(nT M) cos [n2πf IF T M+φ(nT M)]−j sin [n2πf IF T M+φ(nT M)]
- It can be seen that the digital version of the desired signal after modulation is the real part of Z(nTm), or
- Re{z(nT M)}=M(nT M)cos [n2πf IF T M+φ(nT M)]
- This signal is produced by the
apparatus 90, which simply takes the real part of the complex signal and drives the digital toanalog converter DAC 92 whose output becomes the analog version of the desired modulated signal. The output ofDAC 92 is further processed by a low pass filter to smooth the signal and eliminate some of the clock noise. - Thus has been provided a digital modulator with substantially wholly digital processing of a digital data stream which uses an infinite impulse response filter to generate the carrier frequency onto which the digital data stream is to be modulated.
- Although the preferred embodiment of the invention has been illustrated, and that form described in detail, it will be readily apparent to those skilled in the art that various modifications may be made without departing form the spirit of the invention or from the scope of the appended claims.
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Cited By (3)
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US20050238117A1 (en) * | 2002-04-23 | 2005-10-27 | Steven Washakowski | Method and device for pulse shaping qpsk signals |
US20070183304A1 (en) * | 2003-09-26 | 2007-08-09 | Jeong Eui R | Apparatus and method for digitally implementing a wideband multicarrier |
CN113359369A (en) * | 2021-05-11 | 2021-09-07 | 上海交通大学 | High-frequency anti-aliasing band-pass adjustable optical analog-to-digital conversion device |
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US6061551A (en) | 1998-10-21 | 2000-05-09 | Parkervision, Inc. | Method and system for down-converting electromagnetic signals |
US7515896B1 (en) | 1998-10-21 | 2009-04-07 | Parkervision, Inc. | Method and system for down-converting an electromagnetic signal, and transforms for same, and aperture relationships |
US6813485B2 (en) * | 1998-10-21 | 2004-11-02 | Parkervision, Inc. | Method and system for down-converting and up-converting an electromagnetic signal, and transforms for same |
US6370371B1 (en) | 1998-10-21 | 2002-04-09 | Parkervision, Inc. | Applications of universal frequency translation |
US7236754B2 (en) | 1999-08-23 | 2007-06-26 | Parkervision, Inc. | Method and system for frequency up-conversion |
US7039372B1 (en) | 1998-10-21 | 2006-05-02 | Parkervision, Inc. | Method and system for frequency up-conversion with modulation embodiments |
US6879817B1 (en) | 1999-04-16 | 2005-04-12 | Parkervision, Inc. | DC offset, re-radiation, and I/Q solutions using universal frequency translation technology |
US6853690B1 (en) | 1999-04-16 | 2005-02-08 | Parkervision, Inc. | Method, system and apparatus for balanced frequency up-conversion of a baseband signal and 4-phase receiver and transceiver embodiments |
US7110435B1 (en) * | 1999-03-15 | 2006-09-19 | Parkervision, Inc. | Spread spectrum applications of universal frequency translation |
US7110444B1 (en) | 1999-08-04 | 2006-09-19 | Parkervision, Inc. | Wireless local area network (WLAN) using universal frequency translation technology including multi-phase embodiments and circuit implementations |
US7693230B2 (en) | 1999-04-16 | 2010-04-06 | Parkervision, Inc. | Apparatus and method of differential IQ frequency up-conversion |
US7065162B1 (en) | 1999-04-16 | 2006-06-20 | Parkervision, Inc. | Method and system for down-converting an electromagnetic signal, and transforms for same |
US8295406B1 (en) | 1999-08-04 | 2012-10-23 | Parkervision, Inc. | Universal platform module for a plurality of communication protocols |
US7010286B2 (en) | 2000-04-14 | 2006-03-07 | Parkervision, Inc. | Apparatus, system, and method for down-converting and up-converting electromagnetic signals |
US7454453B2 (en) | 2000-11-14 | 2008-11-18 | Parkervision, Inc. | Methods, systems, and computer program products for parallel correlation and applications thereof |
US7020070B2 (en) * | 2001-04-10 | 2006-03-28 | Telefonaktiebolaget L M Ericsson (Publ) | Selectively controlled modulation distortion of an IQ-baseband signal |
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US7460584B2 (en) | 2002-07-18 | 2008-12-02 | Parkervision, Inc. | Networking methods and systems |
US6701134B1 (en) * | 2002-11-05 | 2004-03-02 | Rf Micro Devices, Inc. | Increased dynamic range for power amplifiers used with polar modulation |
US7109791B1 (en) | 2004-07-09 | 2006-09-19 | Rf Micro Devices, Inc. | Tailored collector voltage to minimize variation in AM to PM distortion in a power amplifier |
US7336127B2 (en) * | 2005-06-10 | 2008-02-26 | Rf Micro Devices, Inc. | Doherty amplifier configuration for a collector controlled power amplifier |
US7330071B1 (en) | 2005-10-19 | 2008-02-12 | Rf Micro Devices, Inc. | High efficiency radio frequency power amplifier having an extended dynamic range |
-
2000
- 2000-12-15 US US09/738,124 patent/US6441694B1/en not_active Expired - Lifetime
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20050238117A1 (en) * | 2002-04-23 | 2005-10-27 | Steven Washakowski | Method and device for pulse shaping qpsk signals |
US7346125B2 (en) * | 2002-04-23 | 2008-03-18 | Raytheon Company | Method and device for pulse shaping QPSK signals |
US20070183304A1 (en) * | 2003-09-26 | 2007-08-09 | Jeong Eui R | Apparatus and method for digitally implementing a wideband multicarrier |
CN113359369A (en) * | 2021-05-11 | 2021-09-07 | 上海交通大学 | High-frequency anti-aliasing band-pass adjustable optical analog-to-digital conversion device |
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