a. The Field of the Invention
This invention relates to the field of network communications. In particular, the invention relates to a network where devices communicate using sinusoidal waves.
b. Background Information
Ethernet is an industry standard (e.g., IEEE 802.3 specification) method of communicating between various devices in a local area network. For example, a computer includes a network interface card (NIC) that formats ethernet data for transmission onto a network cable. The network cable carries the ethernet formatted packets out to the rest of the network. The data signal is generated to comply with a particular specification for that type of ethernet communications. For example, the ethernet data signal might be generated to comply with the ANSI/IEEE standard 802.3 ethernet voltage template. This voltage template applies to five and ten MHz ethernet data communications. Complying with the voltage template ensures that the NIC will not damage other devices connected to the network cable, and ensures that other devices will be able to properly receive and decode the signals from the NIC.
As part of the FCC's (and other national regulatory bodies) electromagnetic interference regulations, the NIC must not emit an amount of electromagnetic radiation above a preset limit. The FCC (or for Europe, the CISPR-B) specification, for example, limits the radiation from the ethernet NIC. Importantly, any multiples of the fundamental frequency are to be less than twenty-seven DB lower than the fundamental frequency, per the 802.3 specifications. For example, when driving an all-ones Manchester encoded signal, any harmonic measured shall be at least twenty-seven dB below the fundamental. One problem with generating the ethernet signal to comply with the ethernet voltage template is that high frequency noise may also be generated. This high frequency noise typically appears as an unacceptably high electromagnetic radiation signal at the connection between the NIC and the network cable. These high frequency signals can cause the network interface card to not comply with the FCC electromagnetic interference requirements.
One prior art NIC is shown in FIG. 1. This NIC is available from 3COM Corporation of Santa Clara, Calif. The NIC 100 is for generating the transmit signal 130 which corresponds to an ethernet transmit signal. The transmit signal is generated to support Manchester encoding of the ethernet data. The transmit signal 130 is approximately a square wave signal. The prior art system of FIG. 1 uses a square wave in combination with a pre-emphasis wave. Each of these waves are ten MHz and staggered five MHz but the combination provides a partially shaped wave form as shown in the transmit signal 130. The partially shaped wave form helps to meet the voltage template of the transmit signal 130.
This paragraph describes the operation of the NIC 100. The NIC 100 includes an ethernet controller 101, some filtering circuits, and a transformer. The ethernet controller 101 generates ethernet data signals and includes a number of digital output signals. These output signals include the twisted pair pre-emphasis minus 102, the twisted pair data plus 104, the twisted pair pre-emphasis plus 106, and the twisted pair data minus 108. The twisted pair data plus 104 and the twisted pair data minus 108 output signals that correspond to the positive portion and the negative portion of the transmit signal 130. The twisted pair pre-emphasis minus 102 and the twisted pair pre-emphasis plus 106 help shape the transmit signal 130 to better comply with the voltage template of the IEEE 802.3 specification. The circuitry between the outputs of the ethernet controller 101 and the transformer 120 helps combine the outputs into the one transmit signal 130. The transformer 120 includes a seven pole filter which shapes the transmit signal 130 and reduces the high frequency components in the transmit signal 130.
The NIC 100 operates quite well for many applications. However, the external circuitry on the NIC 100 adds to the costs of manufacturing the NIC 100. Therefore, it is desirable to generate the transmit signal with few external components. Additionally, the electromagnetic interference should be kept to a minimum.