Browse Prior Art Database

Low Cost RF Modem

IP.com Disclosure Number: IPCOM000061655D
Original Publication Date: 1986-Sep-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 3 page(s) / 83K

Publishing Venue

IBM

Related People

Appel, WD: AUTHOR [+2]

Abstract

A technique is described whereby a low cost radio frequency (RF) modem is designed for connection to standard cable television services. The modem is designed in such a way so as to eliminate the need for adjustments either during or after manufacture of the device by duplicating "1" and "0" channels and comparing their outputs. RF modems which can transmit binary information are currently used to transmit many channels of data simultaneously utilizing one or two coaxial cable systems. This reduces the cost of installing a single cable for each connection, as is done in baseband schemes, such as RS232, RS422, and current loop connections. However, RF modems in use today are comparatively complex, expensive and require special manufacturing and testing skills to build. The modem, as shown in the block diagram of Fig.

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Low Cost RF Modem

A technique is described whereby a low cost radio frequency (RF) modem is designed for connection to standard cable television services. The modem is designed in such a way so as to eliminate the need for adjustments either during or after manufacture of the device by duplicating "1" and "0" channels and comparing their outputs. RF modems which can transmit binary information are currently used to transmit many channels of data simultaneously utilizing one or two coaxial cable systems. This reduces the cost of installing a single cable for each connection, as is done in baseband schemes, such as RS232, RS422, and current loop connections. However, RF modems in use today are comparatively complex, expensive and require special manufacturing and testing skills to build. The modem, as shown in the block diagram of Fig. 1, uses low cost components to modulate and demodulate an RF carrier when the modem is installed between the television cable receive line 10 and transmit line 11. Digital modulator 12 and digital demodulator 13 are used in contrast to typical analog schemes. To avoid changes in calibration which could be caused by variations in components, the "1" and "0" channels are duplicated in the receiver and their outputs compared. Using this concept greatly increases the overall dynamic signal range and also provides greater noise immunity than if fixed reference comparisons were used. Due to this redundancy of the "1" and "0" channels, the local oscillator 14 can be shared by both the transmitter and receiver. All frequency synthesis and detection are accomplished by using fixed components. No tunable elements are used. The modem spans sixty-four channels which are 180 KHz wide. High reliability is achieved by duplicating the frequency synthesizers 15, as shown in Fig. 2, and operating them at a frequency offset equal to the intermediate frequency (IF). This offset frequency is then used by both the transmitter and the receiver. A single TTL (transistor-transistor logic)-type multiplexer 16 is used for modulation due to the high ratio of IF-to-bit rate. High-order harmonics are removed utilizing simple L/C filters. Output amplifier 17 uses the 733 video amplifier as an output driver due to its high bandwidth, adjustable gain settings and low cost. In the receiver section, a 733 video amplifier is also used as the input amplifier 18. Attenuating directional couplers 19 and 20 enhance the network integrity by providing a means of balancing signal strengt...