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What’s up with down converters

Wireless infrastructure development is happening at a rapid pace in order to meet the needs of the burgeoning high-speed voice and data electronic products including PDAs, cell phones, and of course lap tops. Up/down converters fit nicely into the plans of many designers of these products as they offer filtering and tuning for multistandard signals, and even help reduce parts count by reducing the number of intermediate frequency stages.

“Digital up/down converters are finding a home in the communications industry and it's specifically the communications infrastructure that is driving most of the product development for these functions,” according to David Duff, product line director for Analog Devices. “The wireless cellular infrastructure segment is the largest, but there are also applications in general communication receivers, cable infrastructure as well as instrumentation and medical imaging applications,” he added.

Texas Instruments has noticed an up tick in designs using up/down converters. The company says it is seeing up/down converters used in defense/aerospace radio systems, wireless repeaters, cable modem radios and head-end systems and wireless infrastructure and instrumentation.

“Any digital communications system can use digital up / down converters,” agreed Steven Smith, Product Marketing Manager for Intersil. “Systems that require products to perform channel filtering, channel tuning, interpolation and gain control for transmit and receive paths will find higher levels of performance and programmability offered with digital up/down converters than with traditional analog conversion techniques,” he said.

Intersil says its products are well suited for the demanding cellular basestation infrastructure and broadband wireless access markets. The company's products can also be used in smart antennas, channelized receiver systems, spectrum analysis, software defined radios, linearized basestation amplifiers, point to point, point to multi-point systems, and satellite modems.

What's needed to make up/down converters work?
What makes the digital up/down converter special and what does it take to make it work? As you might imagine, it's a sophisticated technology to bring this capability to market. For example, TI uses its 95 nm process technology to implement the functions. “The really small features of the 95 nm process enable the higher levels of integration and the increased capabilities, all in a smaller size, said Maria Puig, Marketing Programs Manager of the High-Speed Communications products at TI.


Diagram of TI's GC5316 Up/Down Converter

“The digital up/down functions are a class of digital signal processing that can be implemented in a general purpose DSP for low bandwidth applications (AM/FM radio for instance) or in fixed function digital logic for higher bandwidth,” said ADI's Duff. “They can be implemented in customer specific ASICs, Application specific standard products (ASSPs), and programmable logic gate arrays (FPGAs). The choice of technology is generally driven by cost, but power dissipation and performance are also important issues. At one extreme; ASICs – have the lowest direct cost but have the highest non-recoverable expense (NRE) investment. FPGAs have the lowest NRE investment but the highest direct cost. Therefore application volume tends to dictate the best choice of technology, he said.

For high volume applications (handsets or other customer premises equipment), ASICs make the most sense, according to Duff. “This is especially true when power dissipation is a factor and/or mixed signal capabilities can be included on-chip (data converters, RF, and others). Low volume applications generally favor the flexibility and low investment needed for FPGAs,” observed Mr Duff. Fortunately, several standard functions used in digital radios have evolved into ASSPs available from several sources including Analog Devices. These ASSPs offer the direct cost advantage of an ASIC without the NRE investment.

The IP used in these systems may also dictate the best choice of technology. For example, if the OEM retains significant value in the IP, the company may want to develop ASICs internally using FPGAs as proof of concept. However, if the IP is readily available in the open market, or the semiconductor maker adds value, then the ASSP would be a better route.

Trends
Like most electronic designs there is a trend for more integration. TI says its customers need to take on more of the digital tasks such as interface, digital pre-distortion and crest factor reduction.

Intersil's Smith says the tendencies for additional system digital processing and the incorporation of analog to digital converters and digital to analog converters (D/A converter), to reduce overall system design complexities, will continue. For example, products such as the Intersil HSP50415, which incorporate a dual 12-bit D/A converter, separate I/Q filtering, and numerically controlled oscillators (NCOs), will continue to see increased applicability in future digital communication systems.

“For the down-converter function, the most notable trend, says ADI's Duff, is the increasing channel bandwidth to support the varieties of 3G standards and broadband wireless standards (802.16x). “Single carrier radio design, such as micro and pico-cells, will certainly drive the development of ASSPs. While multi-carrier radio designs will be supported with a mix of FPGAs, ASICs, and ASSPs, he said.

On the transmit side, there are a number of exciting development areas driving the definition of digital up converters. “For example, the dominant and emerging air interface standards are using modulation schemes that have both high linearity and high peak-to-average ratios. Both requirements have a significant cost associated with power amplifier (PA) design that can be mitigated thru digital signal processing,” Duff said.

“Additionally, the linearity concerns of the PA are being addressed with analog and digital pre-distortion schemes,” continued Mr Duff. “The intellectual property for these systems tends to reside with the individual OEMs and PA manufacturers. The architectures and technology used for pre-distortion tends to be specific to a particular PA, radio, and frequently even the end-customer. Accordingly, FPGAs and ASICS seem to be the dominant technology used,” observed Duff. “Finally, the peak-to-average ratio problem is being addressed by a number of technologies and is well suited to ASSPs (as well as ASICs), since the IP can be developed independently from the specifics of an individual PA or customer requirement,” he said.

What are the products?
Intersil's most recent release is the ISL5416, a wideband four-channel programmable digital down converter targeted towards high dynamic range requirements, such as cellular basestations, where the processing of multiple channels is required in a small physical space. The ISL5416 combines four channels in a single package, each including: a numerically controlled oscillator, a digital mixer, digital filters, an automatic gain control and a resampling filter.


Block diagram of the Intersil ISL5416

All channels are independently programmable and may be updated in real time. Each of the four channels can select any of the four digital input buses. Each of the tuners can process a W-CDMA channel. Channels may be cascaded or polyphased for increased bandwidth. Selectable outputs include I samples, Q samples, and AGC. Outputs from the part are available over the parallel, serial or microprocessor interfaces. Sampling rates are Up to 95MS/sec on the input, the internal path is 20 bits, there is also a programmable RF attenuator/VGA control, and a 32-bit programmable carrier NCO with >110dB SFDR. Filtering functions include a Multi-Stage Cascaded-Integrator-Comb (CIC) Filter, two programmable FIR filters (first up to 32-taps, second up to 64-taps), a half-band interpolation filter, and a re-sampling FIR filter. Overall decimation is from 1 to >4096. Evaluation boards and configuration software is available.

ADI's VersaCOMM product family recently added availability of the AD6654 ADC with integrated digital down converter. The 14-bit A/D converter incorporates digital down converter technology found in the AD6636 and is targeted at highly integrated multi-carrier receivers for 3G and 802.16 devices. The AD6633 is the latest digital up converter and incorporates ADI's proprietary crest factor reduction scheme (VersaCREST) to address the peak-to-average ratio problem in transmitters.


Block Diagram of ADI's AD6633

TI's GC5316 is a highly integrated digital up and down converter that is ideal for 3G applications including W-CDMA and cdma2000. The high channel density allows designers to reduce the size and complexity of their designs thus addressing the high density requirements of base station solutions.

The GC5016 is the most recent addition to TI's family of digital up and down converters. At its introduction, the flexible four-channel GC5016 was the first chip to provide both digital downconversion and upconversion functions in a single package. Targeted to a variety of platforms including cdma2000, W-CDMA, TDSC-DMA, PHS and GSM base station systems, the converter offers superior digital radio performance for applications such as wireless repeaters, cable modem radios, wireless instrumentation and defense-based digital radio systems.

Digital up/down converters have found a home in the communications industry. They will branch out to meet the needs of manufacturers that want to reduce parts counts while covering the requirements of several communications standards. It could even be in your next design plan.


Contact Information

Analog Devices, Inc.
Tel: 1/800-ANALOGD (262-5643)
www.analog.com

Intersil Corp.
Tel: (321) 724-7000
www.intersil.com

Texas Instruments Inc.
Tel: 800-477-8924, ext. 4500
www.ti.com

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