connectivityZONE Products for the week of September 5, 2005
X-EMI Says…
X-EMI's Quad and Octal Buffers for PCI Express Applications
Use Spread-Spectrum Techniques to Solve EMI Problems at the Beginning of
the Design Cycle
X-EMI a fabless semiconductor company, has developed a new approach
to electromagnetic interference (EMI) reduction that ensures regulatory
compliance at first pass. Today the company unveiled its XM1004 and XM1008
buffers, designed for consumer and industrial environments where EMI is
a major concern, such as distributed clocks, plastic enclosures, and cabling.
The X-EMI components are designed into consumer and industrial electronics
products at the beginning of the design phase, so that electronic products
such as game consoles, PCs, set-top boxes, telecom equipment, printers,
copiers, and servers are FCC compliant with no re-design necessary.
The XM1004 and 1008 were demonstrated at the recent Intel Developer's Forum (IDF) where attendees were able to see how the XM1004 and 1008 fit seamlessly into existing applications. They demonstration also showed how designers can achieve better performance with the Extreme EMI Reduced Clock (XClk) turned on. Utilizing the XClk results in at least 25dB of EMI reduction and system cost savings - reduced board area, relaxed trace spacing, no tape, no shielding, no ferrite beads or cores - all with no added jitter or other design changes, even at industrial temperature ranges. Several unnamed Fortune 500 customers are currently sampling the products, and the company is now pre-qualifying other companies needing pre-production samples.
General Product and Technology Description
The XM1004 and XM1008 buffers act as both receivers and transmitters, and
can be used with the XM1001 stand-alone clock generator/clock transceiver
(the XM1001 was unveiled on August 9). Both are differential buffers that
provide four and eight SRC clocks, respectively, for PCI Express and SATA
devices. Each of these PCI-Express compatible parts and X-EMI's Extreme
EMI Reduction clocking technology provide at least 25dB reduction in radiated
EMI.
With these product announcements, X-EMI is the only EMI-reduction company to provide stand-alone integrated circuit (IC) EMI-reduction solutions that work without design gimmicks or clock dithering in both today's and next-generation electronics system that utilize PCI Express - and other interconnects - and that operate at faster system speeds. Unlike current EMI-reduction methodologies, the XM1004, 1008 and 1001 mitigate EMI by making less jitter, not more jitter.
X-EMI has developed its ICs using Optimized Spectral Diffusion, or OSD, which is an optimization of what the company terms Spectral Diffusion (SD). The SD technology works by multiplication of both source and destination signals with identical digital noise sources. The modified source signal is sent to the destination where the original signal is recovered. OSD applies sophisticated analytical algorithms to produce a very low peak EMI signal.
Larry Woodson, CEO of X-EMI, said that the XM1004 and XM1008 are key components for designers at the beginning of the design cycle. "It's important that designers struggling with EMI reduction and regulatory compliance know that they now, finally, have a solution that can be designed in at the beginning of the design process, without doing a re-design," said Woodson. "Our purpose is not to fix a product once it has failed FCC compliance - it is to achieve compliance for our customers at first pass.
"This small chip could save a multi-million dollar product launch, and spread across several electronics industries, that translates into billions of dollars saved." Noted author, Signal Integrity columnist, and EMI industry expert, Dr. Howard Johnson was the first expert to review the technology behind the X-EMI product family, and participated in a real-life demonstration of the flagship product, the XM1001. "I have often fantasized about clock transceiver technology that combines zero jitter and zero emissions. Today, X-EMI brought my dream one step closer to reality. OSD integrates the best aspects of spread spectrum modulation with reliable digital system design practice. The XM1001 produces clock signals that are full-sized, with great signal quality, and low jitter, but the radiation simply vanishes. If radiation from your clock was a problem before, it isn't a problem any more," said Johnson, following X-EMI's introduction of the XM1001 August 9. The XM1001 provides the technology base for the XM1004 and 1008.
Key Technical Features
The XM1001 is a stand-alone oscillator/frequency synthesizer with Extreme
EMI Reduction, a differential or single-ended input and output, PCI Express
and SATA support. It may be used as a receiver with all X-EMI Extreme EMI
Reduced Clock buffers (XClk).
Output Attributes:
Notable Specifications:
Features and Advantages:
Applications:
analogZONE Says . . .
If you've ever had to deal with EMI issues, you'll know that the folks at X-EMI have an interesting proposition if their radical clock buffers work as-advertised. For any seasoned EE, it's difficult to imagine being able to achieve 20+ dB worth of noise reduction on some of the noisiest traces in your system, but that's what they claim to do. But from what I have learned in an extensive interview with the company, I think they have a good shot at making good on those claims.
As we'll see X-EMI has applied some very novel spread-spectrum theory to the problem of reducing noise in high-speed clock lines. In essence their transceivers transform a repetitive clock into a low-level spread-spectrum signal which is passed across a standard PCB trace or cable and reconstructed at the other side while preserving phase stability and without spraying the enormous amounts of RF energy along the way.
While
their first product, the XM1001 (see Fig. 1), applies these principles to
any point-to-point serial connection (either single-ended or differential)
at up to 250 MHz, we'll focus on their XM1004 ad XM1008 clock buffers which
were specifically designed for PCI Express applications. The XM1008 sports
eight differential pairs (see Fig.
2), each of which can invoke X-EMI's spread-spectrum
"Xclock" mode independently, allowing you to run a conventional
clock signal on shorter, less EMI-prone runs if you like. The Xclock channels
are usually received at the end-point by an XM001 chip, but the multi-channel
chips can also be used for remote clock recovery and distribution. Another
nice feature to note is the drive strength control (8 levels worth) which
allows you to fine tune the signal level on a per-channel basis to further
minimize EMI. 
Operating Theory
Since such steep reduction in radiated emissions is a very tall claim, I grilled X-EMI as thoroughly as I could about the technology they use to achieve it. As I understand it they use a much deeper spreading technique than the token dithering used in the optional spread-spectrum mode called out in the PCIe spec. Instead of the 30 kHz worth of spreading used by PICe, X-EMI uses CDMA techniques to the spread clock energy across the entire 100-MHz clock band, actually using a two-part technique to spread the clock signal.
First, the clock signal is modulated by a noise source -- in this case, the output of a random number generator (RNG) -- which creates aperiodic phase reversals that further spread the signal. The resulting series of symbols is encoded as a CDMA signal using a custom spreading code they developed which they claim is especially good at keeping energy peaks to a minimum while ensuring a robust signal recovery under noisy conditions.
Since the RNG is used to modulate e tclock and produce CDMA chips, the signal is recovered at the back end by applying the same RNG sequence. Of course this means that the RNGs on each end of the chain must be synchronized. This is accomplished by synchronizing the receive PLL at start-up using a proprietary bit sequence that provides a fixed point in time to start the RNG. The system also uses the same synch sequence to occasionally re-lock the receiver during operation. This and a few other clever design tricks eliminates need for the RAKE receivers usually used in CDMA systems as well as much faster lock than they deliver.
The
result is an EMI level that's 26 dB below normal PCIe clock noise (see Figs.
3
& 4).
While this makes sense based on what I know about CDMA, I'm still having
a tough time getting my head around how they manage to deliver such rock-solid
jitter variation at only 45 ps from cycle-to-cycle. Although some of it
is still not clear to me, it appears that the clever way they synch the
RNG is in good part responsible for this tight timing spec.
But however the spread-spectrum PLL manages to
do it the tight jitter variation it produces allows both the 4-port and
8-port transceivers to fall well within the maximum 600 ppm accuracy tolerance
Intel allows between all reference clocks (at the chip) within a PCIe system.
While this is pretty easy for any PCIe silicon to meet in its standard operating
mode, it can get a little dicey when the conventional spread-spectrum clock
is used because it introduces significant modulation-induced phase error.
X-EMI says that while there's no problem when running a simple point-to-point clock line in Intel's spread-spectrum mode, using two clock sources in the same system can cause clock jitter issues. This is mostly thanks to the 25x clock multiplication that PCI Express uses at the end point to derive a 2.5 GHz system clock from the bus's 100 MHz signal. According to X-EMI, this can make the phase interpolator schemes (a kind of DPLL) often used for CDR in PCIe chips susceptible to "jitter peaking" issues that amplify phase noise problems directly in the area of interest used by PCIe. This can be solved, but requires careful design: both at the chip and board levels. Jitter peaking issues can also lead to interoperability problems between chip vendors -- especially when spread-spectrum mode is invoked. That's why the PCIe committee highly recommends that all clocks come from the same source when running in spread-spectrum mode. While it keeps all chips in the system operating in synch, using a single clock source can create layout problems in larger, more complex PCIe designs, and boards which bump up against the 15" trace limit for clock signals.
Given all these issues, it would make much more sense to use X-EMI's parts to get better noise suppression while reducing the chance of having timing issues at the same time. It makes even more sense when you discover that their parts are pin-compatible with several commonly-used clock distribution parts from Cypress and ICS.
Although these parts are specifically intended for use in PCIe applications, the MX100's more general-purpose architecture makes it very handy for other uses like automotive busses or single-ended to differential conversion.
The parts are sampling with a price of $2.75. Volume
production of the XM1001 is targeted for Q4 2005 when volume pricing will
be announced.
Data Sheet XM1001
Data Sheet Demo Board
|
| ||||
