The present invention generally relates to EMC (Electro-Magnetic Compatibility) and in particular to a method of reducing emission of electromagnetic radiation.

Electronic equipment used in home, office or central office must meet regulatory standards for Electro-Magnetic Compatibility (EMC), in the country where the equipment is sold and used. Exemplary standards include FCC Part 15 in the USA and EN55022 in the EU.

EMC generally pursues two different issues, emission and immunity. Emission issues are related to the unwanted generation of electromagnetic energy by some source, and to the countermeasures which should be taken in order to reduce such generation and to avoid the escape of any remaining energies into the external environment. Immunity or susceptibility issues, in contrast, refer to the correct operation of electrical equipment, often referred to as the victim, in the presence of unplanned electromagnetic disturbances. Before EMC-approval different tests targeting both emission and immunity issues are performed. Radiated emission tests measure electromagnetic output from the product, that is, both intentional and undesired electromagnetic radiation. EMC measurement standards commonly set limits on how much power a product may emit over a given set of frequencies. Radiating too much at a given frequency may have adverse effect on nearby electronic equipment or radio transmissions. Immunity tests ensure that the product will not malfunction when exposed to reasonable amounts of electromagnetic noise or interference (EMI) from nearby equipment.

Any equipment exceeding a limit on radiated emission at a given frequency may not be sold and used where the limit applies. For some types of electronic circuits, in particular when equipment comprises a large number or circuits operating at a same fixed frequency, these limits may be very difficult to meet.

One method of reducing radiated emissions is to enclose the equipment in grounded metal chassis; however breaches in the chassis may allow electromagnetic emissions to escape or leak out. This is a particular problem for equipment that has removable and replaceable parts. An example of such and equipment is shown in Fig. 1 , showing a highly scalable and flexible, chassis based platform for high- definition video conferencing and voice communication 1. Ten different modules 2 or "blades" may be plugged into the chassis, where the blades may be ISDN gateways, MCUs, Telepresence servers, and supervisor modules. The chassis shown in Fig.1 further comprise a backplane having 180 connections for high speed communication between the blades, wherein each of the connections operate at 6.4 Gb/s.

Electromagnetic radiation at high frequencies, such as in the GHz range (1 Gbps serial communication yields 1 GHz electromagnetic radiation), has a short wavelength, typically in the cm-range ( e.g. 6.4 GHz gives a wavelength of approximately 4.5 cm), hence the radiation are able to pass through any small slots or gaps in the chassis. In addition to gaps or small slots almost impossible to avoid in products manufactured from sheet metal, holes in the chassis for cooling etc. make it almost impossible to rely on a grounded metal chassis to avoid excessive electromagnetic radiation when communicating on very high frequencies.

A second method is to slow down the speed of the communication, i.e. reduce the clocking frequency of the communication. For real-time, processing intensive applications, such as high definition video conferencing, this is clearly not a viable solution.

A third common method for reducing emission of electromagnetic radiation at a given frequency is spread spectrum clock generation (SSCG). Clock driven systems have a narrow frequency spectrum due to the periodicity of the clock. A perfect clock signal would have all its energy concentrated at a single frequency and its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed regulatory limits. SSCG modulates the frequency of the clock within a device when transmitting so that the bandwidth of the emissions are increased and therefore an average, or quasi-peak, receiver measurement centered at a given frequency is reduced, i.e. there is _. a reduction in spectral density. However, in some systems altering the reference clock has a detrimental effect on the stability of transceivers in the system. Further, in a system as shown in Fig. l , having several blades, additional and complex hardware would be required to synchronize the reference clock modulation between the blades.

Further, SSCG does not reduce the total energy radiated by a system and therefore does not necessarily make the system less likely to cause interference. SSCG merely take advantage of the EMC testing procedures, wherein the measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into narrow frequency bands. A clock driven system would typically radiate all of the systems energy into one frequency, and its harmonics, thus the measuring receivers would register a large peak at the monitored frequency band, thereby increasing the likelihood for exceeding statutory limits. SSCG distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits.

Thus, it is a need in the art for a method allowing high speed back plane

communication reducing emission of electromagnetic radiation without the additional hardware and complexity of SSCG.

The invention is indicated by the appended claims. According to a first aspect of the current invention a method for reducing

electromagnetic radiation from a high-speed interconnect backplane having connections to at least to two communication modules, wherein each

communication module comprising two Multi-Gigabit Transceivers (MGT) comprises transmitting data to the backplane from a first commutation module simultaneously on a first pair of operating frequencies, wherein a first operating frequency and a second operating frequency are selected from a first range of operating frequencies, and further selected so that a -3dB point of a first

electromagnetic emission spectrum peak originating from transmission of data at the first operating frequency is outside a -3dB point of a second electromagnetic emission spectrum peak originating from transmission of data at the second operating frequency; and receiving from the backplane at a second communication module data from the first communication module data on the first pair of operating frequencies. According to second aspect of the present invention the electromagnetic radiation from the backplane is further reduced by transmitting data to the backplane from the first commutation module simultaneously on a second pair of operating frequencies, wherein a third operating frequency and a second operating frequency are selected from a second range of operating frequencies, the second range of operating frequencies being different from the first range of operating frequencies, and further selected so that a -3dB point of a third electromagnetic emission spectrum peak originating from transmission of data at the third operating frequency is outside a -3dB point of a fourth electromagnetic emission spectrum peak originating from transmission of data at the fourth operating frequency; and receiving from the backplane at a third communication module data from the first communication module data on the second pair of operating frequencies.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. Figure 1 shows a flexible, chassis based platform for high-definition video conferencing and voice communication;

Figure 2 is schematic block diagram of an exemplary embodiment of the present invention;

Figure 3 A is a table comprising operational frequencies of an exemplary

embodiment of the present invention; Figure 3B shows an exemplary electromagnetic radiation spectrum of an exemplary embodiment of the present invention, and

Figure 4 shows an exemplary configuration of an exemplary hardware configuration according the present invention.

The following description is presented to enable a person of ordinary skill in the art to make and use the various aspects and examples of the invention. Descriptions of specific devices, techniques, and applications are provided only as examples.

Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention.

In the following detailed description a device according to the present invention is referred to as a Static Discrete Spread Spectrum system (SDSS). A SDSS operates on multiple pairs of operating frequencies. Each pair of operating frequencies is selected according to two conditions. First, the first operating frequency and the second operating frequency is close enough in frequency that transmitters and receivers using the first operating frequency of the pair can communicate with transmitters and receivers using the second frequency of the pair. Secondly, the first and second operating frequencies is at least separated in frequency so that an electromagnetic (EM)-emission spectrum peak originating from transmission of data at the first operating frequency is outside the -3dB point of an EM-emission spectrum peak originating from transmission of data at the second frequency. A second pair of operating frequencies is selected so that transmitters and receivers using the second pair of operating frequencies cannot communicate with the transmitters and receivers using the first pair of operating frequencies.

Figure 2 show an exemplary hardware module 2, in the following referred to as a blade, according to the present invention. The blade 2 comprise of a blade controller 3, two Multi-gigabit transceivers (MGTs) 5,6, a plurality of internal links 1 1 connecting the MGTs, four reference clocks, or oscillator modules, 7-10 and a plurality of connections 4 for multi-gigabit per second speed communication between blades.

An MGT transmit parallel data as stream of serial bits, and convert the serial bits it receives to parallel data. The most basic performance metric of an MGT is its serial bit rate, or operating frequency, which is the number of serial bits it can transmit or receive in 1 second. An MGT typically run at operating frequencies of 1 GHz or more, corresponding to line rates of 1 Gigabit/second or more. An MGT typically comprise of a Serializer/Deserializer (SerDes), an

encoder/decoder, a transmit buffer, a receive buffer and a clock data recovery (CDR). When an MGT receives serial data, the MGT use the same serial clock that serialized the data to de-serialize the data. A receiving MGT recover the clock signal from the data directly, using transitions in the data to adjust the rate of their local serial clock so it is locked to the rate used by a transmitting MGT.

There is always a small frequency difference between reference clock sources, even if they are nominally the same frequency. As a result, in systems where each MGT uses its own reference clock, each MGT uses a slightly different frequency for it transmitting data. The small frequency differences are handled by using clock correction. In clock correction, each MGT includes an asynchronous FIFO.

Received (RX) data are written to the FIFO using the parallel clock from the CDR, and read from the FIFO using the parallel clock from the rest of the system (the local clock). Since the CDR clock and the local clock are not exactly the same, the FIFO will eventually overflow or underflow unless it is corrected. To allow correction, each MGT periodically transmits one or more special characters, or pads, which the receiver is allowed to remove or replicate in the FIFO as necessary. By removing characters when the FIFO is too full and replicating characters when the FIFO is too empty, the receiver can prevent overflow/underflow. However, when the frequency difference between the local clock and the CRD clock is too large the clock correction eventually fails. The range of CRD clock frequencies for which an MGT is able to perform clock correction is in the following referred to as the operating frequency range of the MGT, and the local clock frequency of the MGT is referred to as the centre operating frequency (COF) of the MGT.

In an embodiment of the present invention, the ability of an MGT to receive data on a range of operating frequencies is utilized to transmit data on two different transmitting frequencies to a same receiver having a centre operating frequency (COF), wherein the first transmitting frequencies is lower (lower operating frequency (LOF)) than the COF and the second transmitting frequency is higher (higher operating frequency (HOF)) than the COF, and the first and second transmitting frequencies both are within the operating frequency range of the MGT. The MGT uses clock correction as described above to handle the LOF to HOF frequency differences.

According to an embodiment of the present invention more padding is added to the transmitted data on the H frequency than the L frequency.

Transmission of data at either operating frequencies causes emission of

electromagnetic radiation at the operating frequencies and their harmonics, giving rise to peaks in an electromagnetic spectrum corresponding to the different operating frequencies. MGTs transmit scrambled data continuously, i.e. a data packet is indistinguishable from an idle packet, thus the amount of energy emitted at a certain frequency, i.e. the height of a peak in the electromagnetic spectrum, is a function of the number of transceivers operating at that frequency. According to one aspect of the present invention the peak radiated electromagnetic energy for transmitting data to a receiver having a operating frequency range is reduced by transmitting the data on two different operating frequencies, a low operating frequency (LOF) and a high operating frequency (HOF), wherein the both LOF and the HOF is within an operating frequency range of the receiver, and the LOF and the HOF are at least separated in frequency so that an electromagnetic (EM)-emission spectrum peak originating from transmission of data at the LOF is outside the -3dB point of an EM-emission spectrum peak originating from

transmission of data at HOF. According to another aspect of the current invention, in a system comprising at least one first receiver having a first operating frequency range and at least one second receiver having second operating frequency range, the second operating frequency range being different than the first operating frequency range, further reducing the peak radiated electromagnetic energy by transmitting data on four different operating frequencies, wherein a first low operating frequency and a first high operating frequency is within the first operating frequency range of the at least one first receiver, and the first low operating frequency and the first high operating frequency are at least separated in frequency so that an EM-emission spectrum peak originating from transmission of data at the first low operating frequency is outside the -3dB point of an EM-emission spectrum peak originating from transmission of data at the first high operating frequency; and a second low operating frequency and a second high operating frequency is within the second operating frequency range of the at least one second receiver, and the second low operating frequency and the second high operating frequency are at least separated in frequency so that an EM- emission spectrum peak originating from transmission of data at the second low operating frequency is outside the -3dB point of an EM-emission spectrum peak originating from transmission of data at the second high operating frequency. Again, with reference to Figure 2; Reference clocks RefClkJPlL 7 and RefClk PlH 8 constitutes a first reference clock frequency pair having a centre clock frequency RefClk_Pl . Correspondingly, reference clocks RefClk_P2L 9 and RefClk_P2H 10 constitutes a second reference clock frequency pair having a centre frequency

RefClk_P2. One reference clock from each of the reference clock frequency pairs is connected to each of the MGTs, e.g. RefClk PIL and RefClk_P2L to the first MGT 5, and RefClk PlH and RefClk_P2H to the second MGT 6. Using the four different reference clocks the two MGTs transmit data at four different operating frequencies, or more specifically two pairs of operating frequencies. The first pair of operating frequencies corresponds to the first reference clock frequency pair, wherein

COFJP1 , LOF_P l and HOF_Pl corresponds operating frequencies obtained using reference clock frequencies RefClk Pl , RefClk PIL and RefClk P l H respectively. The second pair of operating frequencies corresponds to the second reference clock frequency pair, wherein COF P2, LOF_P2 and HOF_P2 corresponds operating frequencies obtained using reference clock frequencies RefClk_P2, RefClk_P2L and RefClk_P2H respectively.

In an exemplary embodiment of the present invention there are ten blades, the backplane has 180 connections and each connection is operating on approximately 6.4 GHz. Figure 3A lists the four different reference clock frequencies,

RefClk PIL, RefClk PlH, RefClk_P2L and RefClk_P2H and their corresponding operation frequencies, LOF Pl , HOF Pl , LOF P2 and HOF P2 respectively. These values are illustratively represented in Figure 3B, showing the four EM- spectrum peaks origination from transmission of data on the four operating frequencies. As can be seen in Figure 3B there are two pairs of EM-spectrum peaks, where the peak centre - peak centre distance is 3.2 MHz (equals a bit rate of 3.2 Mbps). A measuring receiver used by an EMC testing laboratory in the 6-7 GHz range typically has a measurement bandwidth of approximately 1 MHz, and as shown in Figure 3B, the -3dB point of the first peak is clearly separated from the - 3dB point neighboring peak.

In a chassis comprising of at least one transmitting blade and at least one receiving blade the lowest possible radiated EM-power is obtained by the blade controller 3 instructing the MGTs of a transmitting blade to transmit using reference clock RefClk Pl or reference clock RefClk_P2 based on the positions of both the transmitting blade and a destination blade in the chassis. Then if the blade is instructed to use reference clock RefClk Pl the blade start to transmit data at operating frequencies LOF_Pl and HOF P l, or if the blade is instructed to use reference clock RefClk_P2 the blade start to transmit data at operating frequencies LOF P2 and HOF P2. The blade controller's selection of reference clock RefClk Pl or reference clock RefClk_P2 based on the positions of both the transmitting blade and a destination blade in the chassis is based on a pre-configuration of the blade controller. The pre- configuration of the blade controller being the configuration yielding the lowest possible radiated EM-power typically resulting from results from computer simulations and/or EM radiation measurements. One exemplary pre-configuration is shown in Figure 4, also with reference to Figure 3, where 52 links are centered on RefClk_P2 and 48 links are centered on RefClk_Pl , yielding the lowest possible radiated EM-power for this particular hardware configuration.

Other features and advantages will be apparent to those skilled in the art. The foregoing system overview represents some exemplary implementations, but other implementations will be apparent to those skilled in the art, and all such alternatives are deemed equivalent and within the spirit and scope of the present invention, only as limited by the claims.