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Splatterless Transmitter Disable Switching

IP.com Disclosure Number: IPCOM000034439D
Original Publication Date: 1989-Feb-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 3 page(s) / 32K

Publishing Venue

IBM

Related People

Kaltenback, MA: AUTHOR [+2]

Abstract

This article describes a transmitter switching mechanism which avoids the formation of spectrum splatter. The broadband local area network (LAN) that employs a radio frequency (RF) transmission technique that modulates data in the form of packets onto a RF carrier. The RF modulated data is then transmitted through a CATV type of cable system to establish a two-way path. The modem on the LAN has both a modulator and demodulator (transmitter and a receiver). The control lines that interface the modem to the communication controller are the transmitter data line (TXD), the request to send line (RTS), the receive data line (RXD) and the data carrier detect line (DCD). The modulation technique chosen to describe the switching mechanism is frequency shift keying (FSK).

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Splatterless Transmitter Disable Switching

This article describes a transmitter switching mechanism which avoids the formation of spectrum splatter. The broadband local area network (LAN) that employs a radio frequency (RF) transmission technique that modulates data in the form of packets onto a RF carrier. The RF modulated data is then transmitted through a CATV type of cable system to establish a two-way path. The modem on the LAN has both a modulator and demodulator (transmitter and a receiver). The control lines that interface the modem to the communication controller are the transmitter data line (TXD), the request to send line (RTS), the receive data line (RXD) and the data carrier detect line (DCD). The modulation technique chosen to describe the switching mechanism is frequency shift keying (FSK). This is the process of taking binary digital data and modulating it into two discrete frequencies (mark and space). The transmitter has a RTS line which is responsible for turning the transmitter carrier on and off. The TXD line enters a voltage-controlled oscillator (VCO) after receiving binary shaping to set the mark and space frequencies. The resultant FSK-modulated signal leaves the VCO where it is spectrally filtered to remain inside a single 6 MHz channel by a surface acoustic wave (SAW) filter. The SAW filter signal is then fed into a power amplifier (PA) that is keyed on and off by a PA enable line. When the TXD is modulated, and the RTS line is asserted "on", the modulated data travels from the VCO to the PA and out onto the cable network. When the RTS line is not asserted, the VCO is disabled and the PA is disabled preventing any signal from the transmitter going out the modem. In addition to the static on and off cases mentioned above, there is the dynamic case of switching the transmitter on and off. When switching the modem on or off, there is the possibility of generating spectrum splatter, which occurs when any amplifier or oscillator is turned off and is transitioned from an active linear mode of operation, to an inactive, off mode of operation. The period of transistion causes non-linearities in amplification. If there is an RF signal present in the amplifier at the time of transition, there will be a spectrum splatter. Splatter is the resultant amplitude modulation distortion created from a phase-modulated signal when non-linearities are introduced. The mathematical representation or spectrum splatter may be represented by the fourier transform of the...