Patent Publication Number: US-2004052537-A1

Title: Thermal noise reduction technique for optical receivers using identical amplifier circuits

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to broadband communications systems, such as cable television networks, and more specifically to an optical receiver that is suitable for use in the broadband communications system, the optical receiver including a technique that reduces both the thermal noise and the input RF signal losses that are inherently generated in the optical receiver.  
       BACKGROUND OF THE INVENTION  
       [0002]FIG. 1 is a block diagram illustrating an example of a conventional ring-type broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) network. It will be appreciated that other networks exist, such as a star-type network. These networks may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The broadband signals transmitted over the networks include multiple information signals, such as video, voice, audio, and data, each having different frequencies. Headend equipment included in a headend facility  105  receives incoming information signals from a variety of sources, such as off-air signal source, a microwave signal source, a local origination source, and a satellite signal source and/or produces original information signals at the facility  105 . The headend  105  processes these signals from the sources and generates forward, or downstream, broadcast signals that are delivered to a plurality of subscriber equipment  110 . The broadcast signals can be digital or analog signals and are initially transported via optical fiber  115  using any chosen transport method, such as SONET, gigabit (G) Ethernet, 10 G Ethernet, or other proprietary digital transport methods. The broadcast signals are typically provided in a forward bandwidth, which may range, for example, from 45 MHz to 870 MHz. The information signals may be divided into channels of a specified bandwidth, e.g., 6 MHz, that conveys the information. The information is in the form of carrier signals that transmit the conventional television signals including video, color, and audio components of the channel. Also transmitted in the forward bandwidth may be telephony, or voice, signals and data signals.  
       [0003] Optical transmitters (not shown), which are generally located in the headend facility  105 , convert the electrical broadcast signals into optical broadcast signals. In most networks, the first communication medium  115  is a long haul segment that transports the signals typically having a wavelength in the 1550 nanometer (nm) range. The first communication medium  115  carries the broadcast optical signal to hubs  120 . The hubs  120  may include routers or switches to facilitate routing the information signals to the correct destination location (e.g., subscriber locations or network paths) using associated header information. The optical signals are subsequently transmitted over a second communication medium  125 . In most networks, the second communication medium  125  is an optical fiber that is typically designed for shorter distances, and which transports the optical signals over a second optical wavelength, for example, in the 1310 nm range.  
       [0004] From the hub  120 , the signals are transmitted to an optical node  130  including an optical receiver and a reverse optical transmitter (not shown). The optical receiver converts the optical signals to electrical, or radio frequency (RF), signals for transmission through a distribution network. The RF signals are then transmitted along a third communication medium  135 , such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers  140  positioned along the communication medium  135 . Taps (not shown) further split the forward RF signals in order to provide the broadcast RF signals to subscriber equipment  110 , such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one subscriber location  110  is shown for simplicity, however, each distribution branch may have as few as 500 or as many as 1000 subscriber locations. Additionally, those skilled in the art will appreciate that most networks include several different branches connecting the headend facility  105  with several additional hubs, optical nodes, amplifiers, and subscriber equipment. Moreover, a fiber-to-the-home (FTTH) network  145  may be included in the system. In this case, optical fiber is pulled to the curb or directly to the subscriber location and the optical signals are not transmitted through a conventional RF distribution network.  
       [0005] In a two-way network, the subscriber equipment  110  generates reverse RF signals, which may be generated for a variety of purposes, including video signals, e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream in a typical United States range from 5 MHz to 40 MHz through the reverse path to the headend facility  105 . The reverse RF signals from various subscriber locations are combined via the taps and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment  110 . The combined reverse electrical signals are amplified by one or more of the distribution amplifiers  140  and generally converted to optical signals by the reverse optical transmitter included in the optical node  130  before being transported through the hub ring and provided to the headend facility  105 .  
       [0006] Along with the desired information signals, noise signals are also present within the communications system. Noise signals can enter the system via faulty coaxial connectors, for example, or they can be inherently generated within the communications equipment, such as amplifiers, optical transmitters, or optical receivers. The noise signals are amplified via various communications equipment and are aggregated with other noise signals and transported along with the information signals to the headend facility  105 . Disadvantageously, the noise signals may interfere with the signal processing causing errors or poor service quality.  
       [0007] As a result, system operators need to focus on noise reduction techniques. Thus, the present invention is directed towards reducing the noise that is inherent in optical receivers. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008]FIG. 1 is a block diagram illustrating an example of a conventional ring-type broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) network.  
     [0009]FIG. 2 is a schematic of a conventional optical receiver  200  that is suitable for use in the headend facility  105  and in the nodes  130  for receiving optical signals from an optical transmitter and for providing electrical signals.  
     [0010]FIG. 3 illustrates a second embodiment of a conventional bias circuit  305  that is suitable for use in a conventional optical receiver  300 .  
     [0011]FIG. 4 is a schematic of an optical receiver including a noise reduction technique in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
     [0012] The present invention will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which an exemplary embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein; rather, the embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. For example, the present invention is explained relative to an optical receiver that is suitable for use in a communications system; however, the present invention can also be used in other communications equipment that needs to reduce noise, which is inherently generated in the electrical circuitry, commonly referred to as thermal noise. The present invention is described more fully hereinbelow.  
     [0013] Specifically, the present invention is directed towards a thermal noise reduction technique that is suitable for use in an optical receiver. The optical receiver includes a photodiode, e.g., a PIN diode, for converting received optical signals into electrical signals. The optical receiver further includes an amplification circuit including push-pull transimpedance amplifiers that amplify the electrical signal for further transmission through the communications system. Notably, the optical receiver in accordance with the present invention includes a technique for reducing the conventionally inherent, i.e., thermal, noise signals that are generated in conventional optical receivers.  
     [0014]FIG. 2 is a schematic of a conventional optical receiver  200  that is suitable for use in the headend facility  105  and in the nodes  130  for receiving optical signals from an optical transmitter and for providing electrical signals. Included in the optical receiver  200  is a photodiode  205  for receiving the optical signals and for providing electrical signals in accordance therewith. Two identical push-pull transimpedance amplifiers  210  and  215  included in an amplification circuit  240  amplify the electrical signals prior to combining the electrical signals into a single RF electrical signal. Two 12 volt (V) power supplies  216 ,  217  each power one of the amplifiers  210 ,  215 . Finally, a balanced-to-unbalanced electrical transformer, i.e., balun  220 , or other combining means is typically used to provide the combined RF electrical signal. It will be appreciated that the amplification circuit  240  can be discrete components that are assembled on the printed circuit board, or preferably, can be included in a monolithic Gallium Arsenide (GaAs) chip or Silicon Germanium (Si—Ge) microelectronic monolithic circuit to name but a couple examples.  
     [0015] Complicated bias circuits are also included in conventional optical receivers that are used in conjunction with the photodiode  205  and the transimpedance amplifiers  210 ,  215  in order to simultaneously apply the bias necessary to utilize photodiode  205  while keeping the bias voltage from appearing at the inputs of the transimpedance amplifiers  210 ,  215 , thereby disrupting their proper operation. Disadvantageously, such bias control circuits reduce the RF signal coupled from the photodiode to the transimpedance amplifiers. Some bias control circuits are designed to minimize this negative effect, however, it is impossible to totally eliminate the problem. In addition to signal loss, the bias circuits, through resistances intrinsic to their design, generate thermal noise, which is also known as Johnson noise. This reduction of RF signal along with an increase in thermal noise that is generated in the bias circuitry together act to reduce the ratio of signal (or carrier level) to noise, or CNR (carrier to noise ratio). Since high CNR values are necessary in optical and electrical distribution networks for efficient distribution of high quality signals, any reduction in CNR is detrimental to proper system operation.  
     [0016] One example of a conventional bias circuit  225  is shown in FIG. 1. The bias circuit  225  includes high impedance resistors  230 ,  235 , for example, 1 kilo ohm (KΩ), that are connected in series on either side of the photodiode  205  and are supplied a current and voltage with a 12 V power supply. Due to the high resistive values, however, thermal noise is introduced into the circuit. Accordingly, the thermal noise is subsequently amplified via the amplification circuit  240 , thereby resulting in amplified thermal noise signals being transmitted along with the information signals at the RF output port  245 .  
     [0017]FIG. 3 illustrates a second embodiment of a conventional bias circuit  305  that is suitable for use in a conventional optical receiver  300 . A magnetic transformer  310  configured as a 4:1 impedance transformer network is used along with a 12 V power supply to bias the photodiode  205 . Accordingly, thermal noise is also generated in this bias circuit  305  due to the resistance generated by the coils of the magnetic transformer  310 . Bypass capacitors  315 ,  320  are used to provide the low impedance path to ground that is required.  
     [0018] It will be appreciated that communications equipment having resistive networks intrinsically generate thermal noise. The thermal noise voltage that is produced by components containing a resistance is determined by the formula: V th ={square root}(4 kTBR), where k=Boltzmann&#39;s constant (1.38×10 −23  joules/°K.), T=Absolute temperature (°K.), B=Noise bandwidth (Hz), R=Resistance (Ω), and V th  is the Root-Mean-Square (RMS) voltage present across the component. Thus, it is seen that the noise voltage increases in proportion to the square root of the component&#39;s resistance, making high resistance devices undesirable sources of thermal noise. The thermal noise current that is produced by components containing a resistance is determined by the formula: I th ={square root}(4 kTB/R), where I th  is the RMS current flowing through the component. Thus, it will be appreciated that the noise current increases in inverse proportion to the square root of the component&#39;s resistance. Additionally, thermal noise is uniformly present throughout the bandwidth, for example, from 5 MHz to 40 MHz or from 45 MHz to 870 MHz. Typically, care is taken in the design of communications equipment to ensure proper processing despite received noise levels or the equipment is designed to limit the amount of transmitted noise.  
     [0019]FIG. 4 is a schematic of an optical receiver including a noise reduction technique in accordance with the present invention. The photodiode  205  receives the optical signals and converts them into electrical signals. An amplification circuit  405  amplifies the electrical signals to provide amplified RF signals to the RF output port  245 . In accordance with the present invention, however, the conventional bias circuits  225 ,  305  are not included. Advantageously, by utilizing the noise reduction technique of the present invention, the photodiode  205  of the optical receiver  400  no longer requires a conventional bias circuit.  
     [0020] The direct current (DC) voltage required to bias each of the push-pull amplifier circuits  210 ,  215  is, for example, 12 V. Additionally, the DC voltage required to bias the photodiode  205  is also typically 12 V. Accordingly, a common 24 V DC power supply  410  is used to bias the identical amplifier circuits  210 ,  215  by rewiring the amplifiers  210 ,  215  in DC bias series in order to use the common current supplied by the 24 V power supply  410 . The open arrows denoted on FIG. 4 show the two amplifier circuits  210 ,  215  receiving the DC bias current in series. Additionally, the photodiode  205  is biased using the difference of the potential voltage between the two amplifier stages  210 ,  215 , i.e., 12 V.  
     [0021] As mentioned, the amplifier circuits  210 ,  215  are identical and are preferably constructed as an amplification circuit that is assembled on a monolithic GaAs or Si—Ge chip. Accordingly, this construction allows the amplifier circuits  210 ,  215  to share the common series current from the 24 V power supply  410 . Additionally, on-chip  415  and off-chip  420  capacitors decouple the RF signals, which are denoted as the closed arrows on FIG. 4, equally between the individual amplifier circuits  210 ,  215 . The capacitors  415 ,  420 , having a higher potential than ground, are connected to the source of one amplifier that is not connected to ground. Amplifier  210  of FIG. 4 is illustrated as being coupled to the capacitors  415 ,  420 . Alternatively, the amplifiers  210 ,  215  can be biased with a negative voltage and, therefore, inverted. Amplifier  215  of FIG. 4 would then be coupled to the capacitors  415 ,  420 . It will be appreciated that the capacitors do not have to be included on the amplification circuit chip  405 , but can be positioned off the chip. More specifically, a small valued capacitor, such as a 100 pico Farad (pF) capacitor, is placed on the chip  405  and a larger valued capacitor, such as a 0.1 micro Farad (μF) is placed off the chip  405  due to its large physical size. It will be appreciated, however, that the capacitors  415 ,  420  can either be on or off the chip  405 .  
     [0022] In summary, the requirement for a bias circuit is removed from the optical receiver  400  of the present invention. Accordingly, the RF output signal does not include any internally generated bias circuit thermal noise signals that were once present. Nor does it introduce undesirable RF losses into the input signal path. Significantly, this decreases the thermal noise throughout the communications system and aids in the proper processing of the received signals.