Direct down conversion, and its companion, very low IF (VLIF) or near-zero IF, are approaches that have essentially changed the face of the receive chain in a mobile handset. For the designer, it is no longer a question of whether or not to use one of these technologies, but which one is best for an application. Each offers reduced component counts, a simpler design, and lower overall costs, but each also takes a slightly different approach.
To create the most efficient handset design in an arena that is becoming more competitive everyday, designers must carefully consider the associated trade-offs and the level of integration before deciding on a solution. To help the wireless handset architect navigate the murky waters of direct conversion receiver design, this guide outlines the differences, disadvantages, and benefits to the variety of receive architectures currently on the market.
Replacing the superhet
To be sure, a major advantage of direct-conversion is the simplicity. Directly down converting from RF to baseband leads to multiple benefits, including lower cost, smaller circuit-board area, reduced component count, and a roadmap to ongoing cost reductions over time.
In a traditional superheterodyne receiver approach, channel selectivity is performed by one or more stages of passive filtering at IF between the actual RF carrier frequency and the baseband stages. Each filter adds cost and board space to the mobile handset design. And, each downconversion involves the addition of a local oscillator (LO) source (usually a phase-locked loop [PLL] and associated discrete components) and impedance matching for the inputs and outputs of the filters. In direct conversion, a single local oscillator is used and the filtering is done in silicon at baseband, either in analog or digital technology (or both). In very low IF or near zero IF, the RF signal is downconverted to 100 kHz or lower, which also eliminates the need for off-chip filtering.
"Direct conversion is a newly-hot old technology," says Stan Bruderle, chief analyst for Dataquest, adding that it has been used in pager designs for some years. He points out that the excitement is not just at the component level, where eleven companies had announced direct conversion or near-zero IF technology by August of this year, but also at the system level, with Alcatel, Ericsson, and Nokia announcing handsets using direct conversion or very low IF radio technology.
"The main advantages of these architectures are significantly reduced component counts and reduced cost," says Bruderle, "Handset manufacturers consider this a top priority as the baseband processing becomes more complex, having to support multiple digital modes of operation and process multimedia content as well." Direct conversion radios can also be designed for broadband operation, which allows support of multiband applications.
The advantages of a direct conversion/VLIF approach are clear, paving the way for multimode receivers as handsets move into third generation (3G) designs. "Using a superhet implementation for a multimode receiver would require multiple filters (one for each signal bandwidth) and a means to select among them, while direct-conversion receivers can do the filtering on chip, using silicon technology with its associated predictable cost and size reduction over time," says Doug Grant, business development director at Analog Devices.
"We strongly believe that for a GSM chip set to be competitive, it needs to be direct conversion receiver or VLIF," asserts Dr. Dan Schwartz, Radio Products Division chief architect at Motorola's Semiconductor Products Sector.
Cellular radio requirements are more stringent than other wireless technologies, such as WLAN. And, a direct conversion or VLIF radio can eliminate the IF SAW filter and its associated components. The next step in this evolution is integrating the VCO, and several vendors have announced plans or products that do just that.
Selecting a direct conversion solution
When selecting a receiver solution, in addition to price and footprint (including total part count), it is important to carefully examine performance, compatibility with other system chips, and integration. "In general, an engineer should consider whether the direct-conversion radio in question is actually capable and proven to meet the system specifications. This is most clearly and convincingly demonstrated by documentation showing that the chip in question has been used in a mobile terminal that has successfully passed full type approval testing in an accredited test lab," observes Grant.
Because of its complete elimination of an IF stage, there are some important performance characteristics unique to direct conversion receivers, including sensitivity and linearity.
"A direct conversion receiver has an intrinsic difficulty; it generates a DC offset. If it is not suppressed well, it causes a decrease in receiver sensitivity," reports Donald K. McClymont, Wireless Communications product line manager at Conexant Systems, Inc.Focusing engineering resources on this problem, direct conversion vendors have taken different approaches to their DC offset cancellation scheme. When examining the DC offset cancellation approach, it is important to understand how fast the DC offset correction can converge, how much time it takes to set up, and if this has any impact on current drain.
Since direct-conversion translates the radio signal to baseband, the majority of the gain and filtering are performed in a frequency band from DC to the signal bandwidth. In the process, intrinsic DC offsets in the signal path are amplified, and can degrade the dynamic range available in the circuit. In addition, DC offsets can be created if some of the on-channel local oscillator (LO) signal "leaks" to the RF front-end and is downconverted.
Doug Grant explains that intrinsic DC offsets can be handled several ways. In some applications, for signals with no useful information content at DC, the baseband signal can be AC-coupled. However, systems like GSM use modulation and system synchronization techniques that require the DC information. Additionally, the time-slotted operation in other systems makes AC coupling impractical. "In these systems, careful attention to the circuit design and layout is required to reduce the internally-generated DC offsets to a level that will allow signal-related DC content to pass," says Grant, adding, "Using techniques from the field of precision analog circuit design can minimize circuit offsets, and a means can be provided for adjusting most of the remaining offset."
In terms of DC offsets created due to LO signal leaks, care in the generation of the LO injection signal can minimize the problem. Most of the leakage occurs when the LO signal is routed across a circuit board. The track in the board acts as an antenna, and allows the LO signal to be self-detected, producing a DC error. Grant explains that one solution is to operate the VCO that ultimately generates the LO signal at a different frequency, and perform the necessary operation on-chip to produce the desired LO frequency. Another technique is to operate the VCO at twice the desired LO frequency, dividing the signal's frequency on-chip. The leakage from the VCO to the receiver's RF input then causes no self-detection because the VCO is not on the RF carrier frequency.
Of course, once the application moves to multi-band operation, the problem becomes even more complex and requires more sophisticated DC offset cancellation techniques. The point here is not necessarily to tell you how the vendors are doing it, but to point out that you need to be sure the DC offset problem is solved to your satisfaction.
Another specification that should be carefully examined is linearity or the second order intercept point (IP2) performance of the radio, which determines immunity to in-band blockers.
"IP2 is a key issue involving reverse isolation between the LO and antenna, while Rx yield is a function of meeting the GSM reference sensitivity measurement," explains McClymont, "Under typical conditions, the device should exhibit at least 3 dB of margin to allow a good yield."
Tyson Tuttle, product manager at Silicon Laboratories, Inc., notes that the IP2 requirements for GSM are very stringent. "In a direct conversion approach, you need a high degree of isolation to avoid having the interfering signals affect the LO," says Tuttle, "If an interfering signal turns off and on, it can degrade the signal-to-noise ratio (SNR). Unfortunately, the interfering signal usually occurs at baseband, adding to the baseband signal." As a result, the front-end receiver must be very linear.
Also, "If you create an out-of-band blocker or two-tone signal, what kind of contributions are there?" asks Schwartz, adding, " IP2 is not an issue for superhet radios, except under obscure circumstances, but it's a big deal for direct conversion."
Selecting a low IF receiver
Known as very low or near-zero IF, this class of receivers enables filtering to be performed on chip at frequencies in the hundreds of kHz range. This approach is not subject to the DC offset and IP2 concerns discussed with direct conversion receivers. "However, the disadvantage is that the radio front-end requires higher performance and must be capable of coping with high nearby interferers and image signals, which can affect power consumption," notes Grant.
In the low IF approach taken by Silicon Laboratories, for instance, the signal is mixed down to 100 kHz. "We get a benefit of a 20-dB improvement on IP2 offset problems, and it can be implemented in CMOS," reports Tuttle.
What about disadvantages to the low IF approach? "Superheterodyne offers the best performing radio for the lowest power," concedes Tuttle, "But, you are trading off some level of performance on certain parameters for the integration."
In addition to the concerns mentioned above, other selection criteria for a receiver include spurious emissions, current drain, standby performance, total part count including power management and baseband components, compatibility with the baseband controller and mixed-signal devices in the system, and level of integration.
"Above all, it is important to assess how well it matches your application," says Schwarz, "Just because it is a direct conversion receiver or VLIF does not make it the winning solution."
Many manufacturers in this arena have announced various levels of VCO integration. Eliminating the VCO module further streamlines the solution and minimizes RF engineering time. When an off-chip VCO module is used, it is necessary to take an analog signal off-chip, then come back on-chip with a high-frequency signal. This involves a PLL for the LO and for the transmit loop - not a simple design.
When the SAW filter and external VCO module are removed, the board's RF complexity is substantially reduced. For instance, in Silicon Laboratories Aero product, all of the VCOs are integrated on chip. For a dual-band radio, this would require a total of 18 components: eight bypass capacitors and 10 RF components (six of which are for the input network to the band select filters).
Streamlining the receiver solution through integration can improve board yield and simplify design.
The menu for today
There are several choices for direct conversion and low-IF receivers already available (see Vendor Listing) Analog Devices, for instance, announced its Othello chip set in 1999, which consists of two chips - the AD6523 transceiver and the AD6524 multi-band GSM PLL synthesizer. The company's Othello One GSM/GPRS direct-conversion transceiver chip incorporates the functions of the original Othello chip set on a single chip, and adds the power management and RF power detection required in a GSM handset. More recently, the company announced a direct-conversion IC developed for Mitsubishi Electric Corporation for W-CDMA 3G handsets.
Motorola began using its near zero IF architecture in 1992, with a second conversion down to DC (0 frequency). Concurrently, it was developing a direct conversion IC, which first shipped in 1997 in some of the company's "family radio" band 450-MHz products. In 1995, the company introduced a direct conversion pager design. The company released a VLIF GSM chip set in September.
Additional products on the market come from Conexant, which is shipping its CX74017 direct conversion solution and Silicon Laboratories, which is offering its Aero line of near zero IF products.
A glimpse at tomorrow
How well positioned are these new architectures for next generation solutions?
"There have been many skeptics about direct conversion for cellular/PCS handsets because no one has produced a high yielding handset using the technology," notes Bruderle, "If the technology is proven to work, it promises to reduce cost and increase standby times for cellular handsets. Direct conversion is of interest today because of its significantly reduced component count. As handsets incorporate more features it will be necessary to simplify the overall design of the handset. Direct conversion offers this possibility, if it can be manufactured at high yields."
However, simply reducing component count, because it can involve increased system design complexity as well as cost, is not a simple panacea. Designers must carefully weigh all of the trade-offs of increased integration and ultimately choose the receiver that best fits their design needs.
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Janine Sullivan is the president of The Write Solution based in Parsippany, NJ. She was formerly the chief editor for Communications Products magazine. She holds a BA from the University of Delaware and an MA from Duquense University. She can be contacted at firstname.lastname@example.org.