Pulse or digital communications – Apparatus convertible to analog
Reexamination Certificate
1998-10-23
2001-10-30
Pham, Chi (Department: 2731)
Pulse or digital communications
Apparatus convertible to analog
Reexamination Certificate
active
06310908
ABSTRACT:
TECHNICAL FIELD
The present invention relates to digital telecommunications system transmitters and in particular to a digital to analogue converter implementation for such a transmitter which directly produces an analogue intermediate or radio transmission frequency output.
PRIOR ART
A typical prior art transmission stage for a digital telecommunications system is shown schematically in FIG.
1
. In-phase and quadrature outputs from a digital signal processing (DSP) unit are converted to analogue signals which are low pass filtered to eliminate any high frequency components. The filtered analogue signals are up-converted to an intermediate frequency, and then combined to produce the full intermediate frequency (IF) signal. This IF signal is filtered and up-converted again to the final radio transmission frequency (RF) which is then amplified and transmitted. Such prior art arrangements require a large analogue section with a high number of components contributing to both the size and cost of the overall transmitter.
OBJECT OF THE INVENTION
It is an object of the present invention to provide an improved transmission stage for a digital telecommunications system.
SUMMARY OF THE INVENTION
The invention provides a digital to analogue converter implementation for coupling to a telecommunications digital signal processing means having a predetermined data output period, the converter implementation comprising:
a digital to analogue converter (DAC);
means for nullifying the output of said DAC over a predetermined part of each said data output period to form a pulse corresponding in amplitude to the data value associated with said period; and
a band pass filter coupled to said output;
wherein said output is bandpass filtered at a non fundamental frequency band to produce an analogue output signal in said frequency band.
For the purposes of the specification, nullifying the output of the DAC means forcing the output voltage to zero or a DC offset voltage representative of zero input.
Preferably said nullifying means is a zero-padding means coupled to the DAC input. For the purposes of the specification, the term zero-padding means adding zero value words to the DAC input for 2's compliment data or mid-scale value words representative of zero input for other data numbering schemes.
Alternatively said nullifying means is an analogue switch coupled to the output of said DAC and arranged to switch said output to zero or a DC offset representative of zero input for said predetermined part of each sample period of the DAC.
Preferably said predetermined part is at least 75% of said data output period. Preferably said predetermined part is at least 87.5% of said data output period.
Preferably the converter implementation further comprises means for forming a second pulse of opposite sense to said first pulse over a second predetermined part of each said data output period.
Preferably said second predetermined part is at most 25% of said data output period and said first predetermined part is at least 50% of said data output period. Preferably said second predetermined part is at most 12.5% of said data output period and said first predetermined period is at least 75% of said data output period.
Preferably said implementation further comprises an in-phase and a quadrature digital signal up-converter, and an in-phase and quadrature digital combiner connected to combine said up-converted in-phase and quadrature digital signals, the output of said combiner being coupled to the input of said DAC. Preferably said up-converters and said combiner form part of the digital signal processing means, although separate components could be used.
In a second aspect, the present invention provides a telecommunications transmitter comprising:
a digital signal processing means having a predetermined data output period and comprising: an in-phase and a quadrature signal up-converter; and an in-phase and quadrature combiner connected to combine said up-converted in-phase and quadrature signals;
a digital to analogue converter (DAC), having an input coupled to said combiner;
means for nullifying the output of said DAC over a predetermined part of each said data output period to form a pulse corresponding in amplitude to the data value associated with said period; and
a bandpass filter coupled to said output;
wherein said output is bandpass filtered at an intermediate or radio transmission frequency to produce an analogue output signal at said frequency.
Preferably said transmitter further comprises:
an RF up-converter coupled to the output of said filter;
an RF amplifier means coupled to the output of said RF up-converter; and
an antenna arrangement coupled to the output of said amplifier means.
In a further aspect, the present invention provides a method for up-converting and analogue converting digital telecommunications transmission signals having a predetermined data output period for a telecommunications transmitter comprising a digital to analogue converter (DAC) having an output coupled to a bandpass filter, the method comprising the steps of:
converting said digital signals to analogue signals using said DAC whilst nullifying the output of said DAC over a predetermined part of each said data output period to form a pulse corresponding in amplitude to the data value associated with said period; and
bandpass filtering said output at an intermediate frequency to produce an analogue output signal at said frequency.
REFERENCES:
patent: 5534828 (1996-07-01), Okada et al.
patent: 5783974 (1998-07-01), Koslov et al.
patent: 6052701 (2000-04-01), Koslov et al.
Gale Simon
Reed Christopher John
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Pham Chi
Tran Khai
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