Patent Application
20050231288
The present invention relates generally to the field of communications system, and more specifically towards an equalizer and an amplification circuit that are included in integrated circuits that are suitable for use in the broadband communications system.
Broadband communications systems include a transmitter that provides signals to a transmission channel, which may be, for example, optical fiber and/or coaxial cables. It is well known that every transmission channel introduces some amount of loss or attenuation so that the signal power progressively decreases with increasing distance. By way of example, inherent in the coaxial cable are losses due to conductor resistances, absorptive losses in the insulating material, and signal leakage between the braids of the outer shield. These losses, for example, are frequency dependent (i.e., there is more loss at higher frequencies). A receiver at an opposite end of the transmission channel receives the transmitted signals and then typically amplifies the signals to compensate for the transmission loss, and typically provides an amplification response shape that is the inverse of the losses.
Furthermore, additional unwanted effects are also introduced to the signal during signal transmission. These unwanted effects, such as distortion, interference, and noise, are serious considerations when designing products that operate in the communications system. More specifically, distortion is caused by an imperfect response of the system to the desired signal itself. Consequently, distortion may become higher when equalizers are placed at the output side of a device. On the other hand, noise, which is random and unpredictable electrical signals produced by internal and external processes in the system, may become excessive when an equalizer is placed at the input of a device. It will be appreciated that the presence of a significant amount of noise may seriously corrupt the signal and reduce its ability for amplification. Filtering reduces noise, but typically noise constitutes a system limitation.
In order to mitigate the effects of distortion and losses while maintaining a suitable level of signal to noise ratio, highly linear transmitters and receivers are typically desired. Equalizers are capable of frequency shaping the loss and potentially improving distortion to tolerable levels. Conventionally, equalizers are designed using discrete components comprised of inductors, capacitors, resistors, diodes, and transistors, to name a few, in order to achieve varying values. The values are chosen based on the distance of the receiver from the transmitter, which is a factor when determining signal loss. The equalizers are then included in a transmitter and/or receiver in order to improve linearity of the signal.
Referring to the second example 130, an input signal having an input power level is first provided to the equalizer 125 that attempts to initially correct for the loss (i.e., the cable tilt) caused by the system. Similarly, the equalizer 125 attenuates the power. The amplification chain 105 then amplifies the signal with the amplifier stages 115, 120 to provide an output signal having an output power level.
The gain and noise figures for the two examples 110, 130, respectively, can be calculated using example values as shown in the following tables. Since the equalizer is frequency dependent, two frequency points are shown.
Example at 50 MHz with a 5 dB up-tilt network as described:
Example at 1000 MHz with a 5 dB up-tilt network as described:
The gain and noise figures illustrated above are then linearized in order to add the cumulative effect of the noise through the amplification chain 105 and the equalizer 125. The formulas used for linearizing the gain and noise figure and adding the cumulative effect of the noise are as follows:
Gainn=10((Gainn(dB)/(10))
NFn=10((NFn(dB)/(10))
NFTOT=NF1+(NF2−1)/(Gain1)+(NF3−1)/((Gain2)(Gain1)) CSOtotal=20*Log (10{circumflex over ( )}(CSO1/20)+10{circumflex over ( )}(CSO2/20)) CTBtotal=10*Log (10{circumflex over ( )}(CTB1/10)+10{circumflex over ( )}(CTB2/10))
The linearized noise figure and the accumulated totals for the two examples 110, 130 are shown in the table below.
Example at 50 MHz with a 5 dB up-tilt network as described:
Example at 1000 MHz with a 5 dB up-tilt network as described:
Example with 5 dB up-tilted network as described
The final noise figure (dB) and distortion values (i.e., composite triple beat (CTB) and composite second order (CSO)) can also be calculated using the values above, and are illustrated in the following table:
Example with a 5 dB up-tilt:
Notably, the noise figure in Example 2 is worse than the noise figure of Example I due to the placement of the equalizer 125. Interestingly, however, the distortion figures in Example 2 are better than Example 1. As can be seen, output specifications (e.g., output power, distortion levels, and noise figures) need to be understood prior to designing a product. For example, an equalizer may produce better distortion levels, but disadvantageously provide worse noise figures. Therefore, what is needed is an optimum product that is designed for mitigating the effects of the signal loss while maintaining good distortion levels.
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 embodiments set forth herein; rather, the embodiments are 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. The present invention is described more fully hereinbelow.
The present invention is suitable for use in a broadband communications system that requires amplification and equalization of transmitted signals due to loss in the transmission channel. More specifically, the present invention comprises an equalizer and an amplification chain that are included within an integrated circuit. The equalizer may be a fixed value or alternatively it may be a tunable value. Importantly, the present invention does not contribute greatly to the noise figure and has improved distortion levels when compared to conventional equalizers and their placement relative to the amplification chain.
Example at 50 MHz with a 5 dB up-tilt network:
Example at 1000 MHz with a 5 dB up-tilt network:
The linearized noise figure and the accumulated totals for the two examples 110, 130 and the present invention are shown in the table below.
Example at 50 MHz with a 5 dB up-tilt network:
Example at 1000 MHz with a 5 dB up-tilt network:
Example with 5 dB up-tilted network as described
The final noise figure (dB) and distortion values (i.e., composite triple beat (CTB) and composite second order (CSO)) can also be calculated using the values above, and are illustrated in the following table:
Example with a 5 dB up-tilt network:
Notably, in accordance with the present invention, by placing the equalizer 215 between the amplifier stages 210, 220 and subsequently packaging in an integrated circuit, the distortion and noise figures are noticeably improved. It will be appreciated that the equalizer 215 and amplifier stages 210, 220 can alternatively be individually packaged as an integrated circuit and 15 then placed in the arrangement as shown in
It will be appreciated that the amplifier stages 210, 220 can be either single-ended or differential amplifiers. In the case of single-ended amplifiers 210, 220, a single-ended equalizer having either an up-tilt or a down-tilt, depending upon the design of the system, is placed between the amplifiers 210, 220.
In the case of differential amplifier stages 210, 220, a differential equalizer having either an up-tilt or a down-tilt is placed between the amplifier stages 210, 220.
It will be appreciated that modifications can be made to the embodiment of the present invention that is still within the scope of the invention. Additionally, the present invention can be implemented using hardware and/or software that are within the scope of one skilled in the art. The embodiments of the description have been presented for clarification purposes; however, the invention is defined by the following claims.
