Abstract:
A transceiver for the transmission and reception of high speed optical signals contains a detuning structure which reduces the gain for quarter wave radiation of Electro Magnetic Radiation (EMI). A conductive housing couples light energy from a source or a detector to an external fiber having a metallic ferrule. The conductive housing also is referenced to the chassis enclosure of the transceiver thereby attenuating the quarter wave radiator.

Description:
FIELD OF THE INVENTION 
     The current invention is directed to a device which reduces the electromagnetic interference produced by an optical transceiver used in optical data links between data processing equipment. 
     BACKGROUND OF THE INVENTION 
     Optical transceivers are commonly used in communications equipment. A transceiver comprises an optical transmitter and an optical receiver, both housed in a common enclosure. The transmitter converts an electrical input into a temporally modulated optical signal coupled to an optical fiber. The receiver accepts an optical signal having a modulation function, and converts it to an electrical voltage. Some versions of this receiver also recover a clock to in addition to the recovered data. The transmitter and receiver generally operate at the same data rate. In the prior art, typical data rates are 155 Mbps (million bits per second) for oc-3 rates, 622 Mbps for oc-12 rates, and 2400 MBps for oc-48 rates. U.S. Pat. No. 4,979,787 by Lictenberger discloses an optical interface for receiving from and transmitting to an optical fiber. U.S. Pat. No. 5,113,466 by Acarlar et al, U.S. Pat. No. 5,011,246 by Corradetti et al, disclose optical transceivers, but do not speak to the problem of preventing EMI emissions. U.S. Pat. No. 5,337,396 describes a conductive plastic housing for a transmitter or a receiver which provides electrical contact to the printed circuit board ground pins, but does not address the problem of making a shielding contact to an enclosure, or reducing EMI coupling to the optical cables. 
     OBJECTS OF THE INVENTION 
     A first object of the invention is to reduce the EMI (Electro-Magnetic Interference) emissions of a transmitter by providing a grounded reference for unbalanced transmitter currents. A second object of the invention is to reduce the EMI emissions of a receiver by minimizing unbalanced clock currents flowing in the recovered clock output pins. A third object of the invention is to reduce the EMI emissions of a receiver by detuning the structure housing the receiver. A fourth object of the invention is to provide a chassis reference to an internal receive ferrule and an internal transmit ferrule through a first extension shield which is connected electrically to a collar shield, which is in contact with the chassis reference. 
     SUMMARY OF THE INVENTION 
     A prior art oc-12 transceiver conducts differential and common mode currents at a fundamental rate of 622 Mhz, and a prior art oc-48 transceiver conducts these currents at a fundamental rate of 2400 Mhz. A prior art transceiver has a physical length roughly equal to that of a quarter wave antenna for approximately 3 Ghz electromagnetic waves. A fundamental rate 622 Mhz square wave signal from an oc-12 data link has a 5th harmonic in this same 3 Ghz range. Hence, the 5th harmonic of a 622 Mhz transmit or receive electrical signal will excite a quarter wave antenna response in the 3 Ghz range from the prior art transceiver, affording gain and a nearby aperture for radiation of this signal. The present invention provides for the reduction of EMI by using pre-existing internal optical elements as shield elements, and by detuning the quarter wave antenna property of the transceiver. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIG. 1 is a front view of a prior art transceiver mounted on a printed circuit board. 
     FIG. 2 is a section view of the transceiver of FIG. 1 including a fiber-optic cable assembly. 
     FIG. 3 is a section view of the transceiver of FIG. 1 including a fiber-optic cable assembly. 
     FIG. 4 is a detailed section view of the transceiver of FIG.  1 . 
     FIG. 5 is the schematic and block diagram of the individual elements of the transceiver comprising a transmitter converting an electrical signal to a modulated light source and a receiver converting a modulated light input into a data signal accompanied by a recovered clock signal. 
     FIG. 6 is the side view of a transceiver with a detuning element. 
     FIG. 7 is an isometric view of the detuning elements of FIG.  6 . 
     FIG. 8 is an isometric view of an alternate detuning device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the front view of a prior art transceiver  10  mounted to a printed circuit board  16 . Transceiver apertures  12  and  14  accept optical connectors for the respective reception and transmission of light. For reference, sections A—A and B—B enable the understanding of internal structures found in the transceiver, front panel sheet metal chassis, and cable assembly elements. 
     FIG. 2 shows the section A—A view of FIG.  1 . Transceiver  10  is mounted on circuit board  16 , which is secured to a chassis  30  using grounded standoff spacer  47 . Chassis  30  provides mechanical support and EMI shielding for the electrical signals of circuit board  16 . Chassis  30  also has an aperture  31  in front of each transceiver  10  to allow cables  22  to plug into the transceiver  10 . Each cable assembly comprises an optical cable  20  molded into an optical termination  22  which has a strain relief part, an optical access part, and a locking part, as will be described later. Associated with the transceiver  10  are an electrical to optical (E/O) converter, or optical to electrical (O/E) converter device  34 , which is in close proximity to the optical fiber of cable terminator  22 . This alignment of the optical fiber to the E/O or O/E converter is achieved in area  38 . O/E or E/O converter electrical signals are disposed via converter leads  36  to printed wires on the internal circuit board  44 , and after further processing, are delivered to the system circuit board via interface pins  40 . Since the interface pins  40  are typically the only electrical reference to ground, the electrically unconstrained dimension l  46  is capable of radiating as an antenna if unbalanced currents appear on terminals  40 . As will be described later, these unbalanced currents are intrinsically present in differential circuits driven by asymmetrical drivers. 
     FIG. 3 shows section B—B of the transceiver of FIG.  1 . Transceiver  10  has a transmitter driver  60 , and receiver electronics  66 . Transmit optical cable  52  passes through aperture  31   a  in chassis  30 , and is optically coupled to the electrical to optical converter  54 . High speed electrical signals are carried via E/O converter leads  58  from the transmitter driver electronics  60 , which typically comprises a high speed power amplifier driven by a differential input signal  62 . The conductor signals  62  are formed on circuit board  44 , and transmit driver  60  typically comprises integrated circuits and other electronic components mounted on the circuit board  44 . Typical transmit driver integrated circuits  60  include part number VSC7923 by Vitesse Semiconductor Corporation of Camarillo, Calif. Receive cable  70  passes through an aperture  31   b  in chassis  30  to optical to electrical converter  50 , and wires  64  carry high speed signals to receive electronics  66 , which comprise a limiting amplifier and an optional clock extraction circuit. The receive electronics  66  produce two pairs of differential signals  67  provided as printed wiring on circuit board  44 , comprising recovered data and recovered clock, which are delivered to connector pins  68 , as will be described later. Typical oc-12 receiver integrated circuits include VSC7911 for a limiting amplifier, available from Vitesse Semiconductor, and S3027 clock recovery circuit available from Advanced Micro Circuit Corporation of San Diego, Calif. 
     FIG. 4 shows the detail of the relationship between the fiber and electrical/optical converter. Optical cable  72  having a continuous fiber  76  is attached to a strain relief housing  74  through cylindrical crimp ferrule  80  and terminating ferrule  78 . Fiber  76  proceeds through each of these elements  80  and  78  uninterrupted until the far side  88  of terminating ferrule  78 . The terminating ferrule  78  is typically a precision ground zirconium cylinder having an axial aperture for the optical fiber  76 , and this ferrule  78  has very tight and reliable tolerances, thereby affording a precision fit with either the transmit or receive receptacle housing  82 , and achieving precision optical alignment with lens  84  and electrical/optical converter  86 , which is either a laser diode or Light Emitting Diode (LED) for the case of element  86  being an E/O converter in a transmitter, or a photodetector diode for the case of element  86  being an O/E converter in a receiver. Converter leads  36  provide an electrical connection to internal circuit board  44 , as was described earlier. In practice, reliable fiber links require an integral and repeatable mechanical alignment between removable fiber  76  and fixed housing  82 , which is accomplished by polishing the fiber end  88 , and ensuring a precise mechanical relationship between precision ferrule  78  and crimp ferrule  80 . This is often accomplished in cable terminations by using a brass crimp piece  80 . In this manner, light delivered in the optical fiber is maximally coupled between the fiber  76  and the electrical/optical converter  86  by controlling the fiber position at face  88 . While the overall arrangement of elements of FIG. 4 ensures the reliable electrical and optical operation of the transceiver, the electromagnetic interference minimizing properties are compromised, particularly at higher frequencies. The precision receptacle  82  is typically machined from metal, and when mated with the cable precision ferrule  78 , is in close proximity with brass crimp ferrule  80 . Furthermore, roughly half of crimp ferrule  80  extends beyond the chassis  30 , and the functional purpose of chassis  30  is the containment of EMI. When a cable is plugged into a port, this containment is breached, as any EMI present in the enclosure is conducted via the conductive ferrule  80  outside the enclosure. Since the cables typically used for the transport of optical signals are a pre-existing part of the building wiring, they generally are used as-is. Over time, as data rates on these optical cables have increased from oc-3 (155 Mbps) to oc-12 (622 Mbps) to oc-48 (2.4 Gbps), these same optical cables are carrying faster optical signals. Accordingly, the electronic technologies inside the transceivers receiving and generating these optical signals have gone to higher internal switching speeds, as measured by the well known electrical parameter rise time. For example, the rise time of CMOS (Complimentary Metal Oxide Semiconductor) oc-3 circuits is on the order of 1000 pS (Pico-seconds, or 10 −12  seconds), while PECL (Positive Emitter Coupled Logic) rise times used in oc-12 circuits are reduced to 100 pS, and GaAs (Gallium Arsinide) technologies used for oc-48 have rise times on the order of 50 pS. A frequently used guideline from the book “High-Speed Digital Design” by Johnson and Graham is that the most of the frequency energy in a signal is below a knee frequency related to rise time by the expression:        Fknee   =     0.5     T                 r                              
     Where 
     F knee =break point in frequency spectrum 
     Tr=rise time of signal 
     By application of this formula, we can see that the knee points for the above CMOS oc-3 signal is 500 Mhz, while the PECL oc-12 signal has a knee frequency of 5000 Mhz, while the GaAs signal has a knee frequency of 10 Ghz. The bandwidth capabilities of the optical cable are sufficient for the increased signaling speeds, however the crimp ferrule  80  internal to the cable has become the source of radiation of signals, particularly as they are used in higher speed interfaces. 
     FIG. 5 shows the electrical elements of the prior art transceiver. Transmitter  90  has differential input signals  96 + and  96 − delivered from a serializer  91  to a termination resistor  98  and to power amplifier  92 , which drives a modulation current through diode  94  via leads  36   a . In the case of single mode lasers, diode  94  is often a Fabry-Perot laser diode, and in the case of a multi-mode optical source, it may be a Light Emitting Diode (LED). The laser diode or LED  94  is secured into a housing  82  with a lens  84  for the maximum delivery of energy to the fiber as described earlier in FIG.  4 . Receiver  102  comprises photodiode  104 , which converts input light energy to a current flowing through load resistor  106 , and amplifier  108  applies a variable gain amplification to ensure a constant amplitude output which is fed to clock and data recovery unit  110 . Output drivers  112  and  114  deliver recovered clock differential data to biasing resistors  120  and  122 , as well as termination resistor  130  in deserializer  93 , whose output is not shown for clarity. A typical integrated circuit combining a transmit serializer  91  and receive deserializer  93  is S3028 by Applied Micro Circuits Corporation (AMCC). Output drivers  116  and  118  deliver recovered data to biasing resistors  124  and  126 , as well as termination resistor  128 . There are several sources of EMI from these circuits. For the case of the transmitter, laser diode  94  is driven single ended by driver  92 , which means that while the input signals  96 + and  96 − are complimentary and symmetrical with respect to ground, the drive voltages in the leads  36   a  are neither symmetrical nor balanced with respect to ground. This transmitter diode is represented by device  86  of FIG. 4, and these unbalanced currents easily couple from the laser diode  86  to the housing  82  to the conductive crimp ferrule  80  of the cable termination  74 , all of which are sequentially coupled to each other through proximal capacitance. For the case of the receiver, EMI generation is caused by imbalances in the symmetrical outputs of the clock applied across resistor  130 . When receive signal is applied to photodiode  104 , the miniscule unbalanced voltages in leads  36   b  are insufficient to generate measurable EMI. However, during clock recovery, large currents flow through leads  40  into termination resistor  130 . Intrinsic imbalances in these currents excite the physical length l  46  of FIG.  2  and cause the receiver receptacle housing  82  of FIG. 4 to carry a magnified version of this signal, particularly if the signal harmonic wavelength is near the physical quarter wavelength dimension l  46 . In the case of oc-12 and oc-48 transceivers, where length l  46  is approximately 1.25 inches which has a quarter wavelength of 3 Ghz, harmonics in the 3 Ghz to 4 Ghz range will experience transmission gain. As before, crimp ferrule  80  affords emission via aperture  31   b  through chassis  30 , and excessive EMI radiation on recovered clock harmonics from 3 Ghz to 4 Ghz will be found on oc-12 and oc-48 transceivers, particularly when cable  74  is installed allowing coupling to ferrule  80 . 
     Examining the sources of these imbalances in FIG. 5, the PECL output transistors  112  and  114  may not remain linearly biased, and in this case the rising edge rate would be governed by the familiar transistor equation:        Zo   =     KT     q        (   Ie   )                                
     Where 
     Zo=output impedance 
     KT/q=0.026 V at room temperature T=25° C. 
     Ie is the instantaneous emitter current. 
     Examining the source of intrinsic imbalance, we can see that when Q 112  is high and Q 114  is low, more Ie is flowing in Q 112 , and less is flowing in Q 114 , so the output impedance of Q 112  is lower than the output impedance of Q 114 , which implies that while the rising edge of Q 112  or Q 114  is coincident with the falling edge of complimentary Q 114  or Q 112  respectively, the rising edge provided by each transistor will always be faster than the falling edge of the complimentary transistor. If the bias current were insufficient to handle the delivery of current to load resistor  128 , the output transistor would go into a non-linear state, and the output impedance of the stage would discontinuously become that of the bias resistor R 120  or R 122 . The difference in edge rates would thereafter cause the production of asymmetric currents, and this would result in the further production of EMI. The data outputs typically produce less EMI than the clock outputs, since the clock is typically a 50% duty cycle decomposing into odd harmonics of the fundamental, while the data pattern has a more random distribution of edge transitions, and hence contains more broadly distributed spectral energy. 
     FIG. 6 shows the present invention. Electrical to optical converter or optical to electrical converter  140  is placed in a metallic or conductive housing  142  which has ingress access  164  for an optical ferrule similar to  78  of FIG. 4. A first shield extension  144  and  154  connect the conductive housing  142  to a collar shield  146 , which comprises a conductive sheet formed around the housing of the transceiver. The collar shield  146  is formed with spring fingers which makes contact to the chassis  30 . The complete structure forms a reentrant detuning device comprising the front panel  30 , collar shield  146 , first shield extensions  144  and  154 , and conductive housing  142 , which also provides electrical continuity to O/E converter or E/O converter  140 . In this manner, unbalanced voltages from the transmitter or receiver sections as described earlier are shunted to the front panel chassis  30 , and the conductive housing  142  is held at a fixed potential, thereby minimizing radiation through housing  142 . When a cable is installed, no resonant structures are present to conduct EMI outside the enclosure, even though conductive ferrule  80  is still present in the attached cable. The quarter wavelength structure formed by l  46  is now chassis referenced, resulting in the detuning of the antenna. The structure is now terminated at both ends, and while a new mode of excitation may occur based on the two ends constrained to ground and the midspan of l  46  having modal radiation, the end near ferrule  80  is now at chassis potential, so minimal radiation occurs. The collar shield may be formed in the housing through the use of a conductive plastic, or through the application of a conductive epoxy such as . Similarly, the extension shields which connect the converter housings to the collar shield may also be formed using sheet foil conductors, conductive plastics, or conductive housings. The conductive properties of the material in general must be assured in the radiation range of the optical signal, which generally spans a range from the fundamental frequency through the 7th harmonic. For oc-12, which operates at a fundamental rate of 622 Mhz, this implies a frequency range from 622 Mhz through 4.2 Ghz. In this frequency range, the RF impedance should be less than 2 ohms from ferrule to shield. For a conductive epoxy connection having a length of 0.1 cm and a cross sectional area of 0.1 cm by 0.1 cm, this translates to a bulk resistivity of 0.2 ohm-cm, and would be the same for shield material formed from a conductive epoxy enclosure. The inter-shield connections  148 ,  150 , and  152  may be accomplished several different ways. Resilient fingers may be formed into the conductive extension or collar shields, or the inter-shield electrical connections may be formed through the application of a brazed, welded, or conductive adhesive. 
     FIG. 7 shows an isometric view of the detuning device of FIG.  6 . Conductive enclosure  30  having aperture  31  has a conductive collar shield  146  making contact via a plurality of contact fingers  170 , which form the electrical connection  152  as described earlier. The transmitter of FIG. 7 comprises electrical to optical converter  184 , which makes electrical contact with a transmit ferrule  182 , also referred to as a first transmit cylindrical housing  182 . First transmit cylindrical housing  182  has provisions for coupling optical energy from electrical to optical converter  184  on one side and to a second transmit cylindrical housing  172  on the other side, which couples optical energy to an optical cable  171 . First shield extension  144  is made from a conductive material, and has a plurality of fingers  174  for making contact to the inside of collar shield  146 , shown as connection  148  of FIG.  6 . First shield extension  144  makes contact with transmit ferrule  182 , and with receive ferrule  178 , which form the connection shown as  150  of FIG.  6 . In a similar manner, the receive ferrule  178 , also referred to as the first receive cylindrical housing  178  makes electrical contact with the receive optical to electrical converter  180  at connection  150 , as well as first shield extension  144  at connection  148 . The first receive cylindrical housing  178  aligns and receives an optical signal from mating second receive cylindrical ferrule  175 , which couples optical signal from optical fiber  177 . The elements chassis  30 , collar shield  146  including fingers  170 , first extension shield  144 , first transmit cylindrical housing  182 , first receive cylindrical housing  178 , and the housings of transmit electrical to optical converter  184  and receive optical to electrical converter  180  are electrically conductive, and electrically coupled to each other according to the method shown in FIGS. 6 and 7. 
     FIG. 8 shows an alternate means of making the connections shown in FIG.  6 . The enclosure  30 , collar shield  146  with fingers  170 , first transmit ferrule  182 , transmit electrical to optical converter  184 , first receive transmit ferrule  178 , and receive optical to electrical converter  180  perform the same functions described earlier. Transceiver conductive baseplate  196  is electrically coupled to first transmit ferrule  182  and to first receive ferrule  178  with contact ferrule  194  and  192 , respectively. Contact ferrules  194  and  192  may be formed from sheet metal, or they may be formed from a conductive epoxy, as long as they make a high frequency connection between baseplate  196  and first ferrules  182  and  178 . The bottom of baseplate  196  makes electrical contact with collar shield  146  on the inside bottom surface of collar shield  146 .