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WIDEBAND DATA RECEIVER

IP.com Disclosure Number: IPCOM000006990D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2002-Feb-14
Document File: 2 page(s) / 109K

Publishing Venue

Motorola

Related People

Jeff Dutkofski: AUTHOR [+3]

Abstract

Abstract: Low cost receiver integrated circuits used for narrow band voice applications do not typi- cally have adequate bandwidths for data applications approaching 1Mbps. In an FSK data system, it is necessary to have a wide receiver ~bandwidth and frequency response down to 0 Hz in order to accu- rately recover the received data pattern. This inven- tion uses an amplifier that provides a "virtual ground" at its input to cancel out any parasitic capacitance present in the integrated circuit that may limit the receiver bandwidth. The DC operating point of the circuit is stabilized by using low frequency feedback to the current amplifier's reference inputs.

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MOTOl?~LA INC. Technical Developments Volume 20 October 1993

WIDEBAND DATA RECEIVER

by Jeff Dutkofski, Russ Kahl and Mike Retzer

Abstract: Low cost receiver integrated circuits

used for narrow band voice applications do not typi- cally have adequate bandwidths for data applications approaching 1Mbps. In an FSK data system, it is necessary to have a wide receiver ~bandwidth and frequency response down to 0 Hz in order to accu- rately recover the received data pattern. This inven- tion uses an amplifier that provides a "virtual ground" at its input to cancel out any parasitic capacitance present in the integrated circuit that may limit the receiver bandwidth. The DC operating point of the circuit is stabilized by using low frequency feedback to the current amplifier's reference inputs.

  Low cost receiver integrated circuits, such as the Signetics NE615, typically provide differential out- puts for "data" applications. These outputs are implemented as current sources, with internal resis- tors to ground accomplishing the current to voltage conversion for the desired voltage output. These inter- nal resistors are typically fairly large in value (&5KQ), such that any parasitic capacitance to ground at the outputs will severely degrade the cir- cuit bandwidth. For example, only 29pF ofparasitic capacitance will limit the bandwidth to <lOOKHz using the 55KQ typical internal resistor value. This is within the range of parasitic values typically encountered in practical packaging and layouts. This bandwidth is not nearly sulflcient to allow data rates approaching 1Mbps. This situation is diagrammed in Figure 1.

  A typical solution to this problem is put forth in the manufacturer's data sheet. An external resistor to ground is placed at the output of the receiver IC. This increases the receiver bandwidth by reducing the overall RC time constant. The problem with this approach is that this also reduces the available volt- age signal swing. In many applications this is not a problem; the signal voltage can be recovered atler the receiver IC by using an AC coupled amplifier. However, an AC coupled amplifier will not provide the frequency response down to 0 Hz that is needed

0 MoBrOla, 1°C. 1993

for wideband data applications. Providing sufficient gain for signal recovery al...