Method and system for direct digital up-conversion in a cable modem
View Patent ↗Provided are a method and system for a module for a cable modem termination system. The module includes a digital modulator configured to block up-convert a plurality of digital channels and a digital to analog converter coupled, at least indirectly, to an output of the digital modulator to convert the digital channels to an analog format.
1. A module for a cable modem termination system (CMTS), comprising:
a digital modulator configured to up-convert, in a digital domain, a plurality of digital channels received in parallel, to one or more higher transmit frequency ranges in accordance with DOCSIS standards for CMTS downstream transmissions, the up-conversion being based on a first sampling frequency;
a digital to analog converter (DAC) coupled, at least indirectly, to the digital modulator to convert the up-converted digital channels to an analog format based on a second sampling frequency; and
a clock, coupled to the digital modulator and the DAC, configured to provide the first sampling frequency and the second sampling frequency, wherein the second sampling frequency is a multiple of the first sampling frequency.
2. The module of claim 1 , further comprising a serializer coupled between the digital modulator and the DAC.
3. The module of claim 2 , wherein the digital modulator includes an interpolation filter and up-converter logic.
4. The module of claim 1 , wherein the digital modulator is a digital up-converter.
5. The module of claim 1 , wherein the plurality of digital channels are adjacent to one another.
6. The module of claim 5 , wherein each of the channels includes a quadrature amplitude modulated (QAM) carrier signal.
7. The module of claim 1 , wherein the digital modulator includes an interpolation filter and up-conversion logic.
8. The module of claim 1 , wherein the digital modulator and the DAC are formed on a single integrated circuit (IC).
9. The module of claim 8 , wherein the digital modulator and the DAC are based upon a clock speed greater than or equal to two giga-samples per second.
10. The module of claim 1 , wherein the DAC is a 12 bit DAC.
11. The module of claim 1 , wherein the module includes built-in testability.
12. The module of claim 11 , the built-in testability comprising a direct digital frequency synthesizer configurable for being multiplexed with the up-converted digital channels and being provided as an input, at least indirectly, to the DAC.
13. An up-conversion system for a cable modem termination system (CMTS) configured to simultaneously up-convert a plurality of digital channels, comprising:
a combiner to combine the plurality of digital channels into a single digital bit-stream having an inverting and a non-inverting data component and representative of the plurality of digital channels;
a digital modulator to modulate, in the digital domain, the digital bit-stream to one or more higher transmit frequency ranges in accordance with DOCSIS standards for CMTS downstream transmissions based on a first sampling frequency;
a serializer to receive the modulated digital bit-stream from the digital modulator in parallel and to serialize the modulated digital bit-stream into a data stream based on a second sampling frequency;
a digital to analog converter (DAC) for converting the data stream into one or more up-converted analog radio frequency output signals; and
a clock to provide the first sampling frequency and the second sampling frequency, wherein the second sampling frequency is an integer multiple of the first sampling frequency.
14. The up-conversion system of claim 13 , wherein the combiner is of a heterodyne variety.
15. The up-conversion system of claim 13 , wherein the digital modulator and the DAC are formed on a first single integrated circuit (IC); and wherein the combiner is formed on a second IC.
16. The up-conversion system of claim 13 , wherein the digital modulator, the DAC, and the combiner are formed on a single IC.
17. A method for testing a block up-converting module within a cable modem termination system (CMTS), the module including a digital modulator coupled to a digital to analog converter (DAC) and configured to up-convert, in a digital domain, a plurality of digital channels to one or more higher transmit frequency ranges in accordance with DOCSIS standards for CMTS downstream transmissions, the up-conversion being based on a first sampling frequency, the method comprising:
providing first and second sampling signals to the block up-converting module;
multiplexing one or more fixed input words with an output of the digital modulator to produce a multiplexed signal based on the first sampling signal;
providing the multiplexed signal as an input to the DAC based on a second sampling frequency; and
comparing an output of the DAC with an analog signal based on the one or more fixed input words,
wherein the second sample frequency is an integer multiple of the first sampling frequency.
18. The method of claim 17 , wherein the fixed input words are representative of two or more independent frequencies.
19. The method of claim 18 , wherein the two or more independent frequencies are produced in accordance with direct digital frequency synthesis principles.
20. The method of claim 17 , wherein the multiplexing and the providing occur while the DAC is operating at speed.
21. A up-conversion module for up-converting a plurality of digital channels in a cable modem termination system (CMTS), comprising:
a combiner module, configured to combine the plurality of digital channels into a single bit stream;
an interpolation filter and up-converter module, logically coupled to the combiner module, configured to interpolate and filter a portion of the single bit stream to produce a modulated output stream at a first sampling frequency, wherein the modulated output stream comprises a plurality of bits for each bit of the portion of the single bit stream, and the modulated output stream for the portion is produced in parallel;
a serializer module, coupled to the interpolation filter and up-converter module, configured to serialize the modulated output stream;
a digital to analog converter (DAC) module, coupled to the serializer module, configured to convert the serialized modulated output stream to an analog format at a second sampling frequency; and
a clock, coupled to the combiner module, the interpolation filter and up-converter module, the serializer module, and the DAC module, configured to provide first and second sampling signals for synchronizing the up-conversion, wherein the second sampling frequency is a multiple of the first sampling frequency.
22. An up-conversion system for a cable modem termination system (CMTS) configured to simultaneously up-convert a plurality of digital channels, comprising:
a combiner to combine the plurality of digital channels into a single digital bit-stream;
a digital modulator to modulate, in the digital domain, the digital bit-stream based on a first sampling frequency, wherein the modulation is performed simultaneously on each of the digital channels of the plurality of digital channels, and the modulated digital bit-stream comprises a plurality of parallel data paths;
a serializer to serialize the modulated digital bit-stream into a data stream based on a second sampling frequency;
a digital to analog converter (DAC) for converting the data stream into an up-converted analog radio frequency output signals based on the second sampling frequency; and
a clock to provide the first sampling frequency and the second sampling frequency, wherein the second sample frequency is higher than the first sampling frequency.