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RSSI CONTROLLED I.F. LIMITER WITH IMPROVED INTERMODULATION PERFORMANCE

IP.com Disclosure Number: IPCOM000005811D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2001-Nov-08
Document File: 2 page(s) / 73K

Publishing Venue

Motorola

Related People

George Anderson: AUTHOR [+2]

Abstract

Large signals present in the receiver first i.f. stage may require limiting to avoid overdriving the i.f. amplifiers. In portable receivers this is usually done by placing a diode, typically a PIN or Schottky diode, in shunt with the crystal filters The diode conducts when the incoming i.f. signal is large enough to forward bias it, thereby limiting the signal (Figure 1).

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MOlOROLA Technical Developments Volume 9 August 1989

RSSI CONTROLLED I.F. LIMITER WITH IMPROVED INTERMODULATION PERFORMANCE

by George Anderson and Ed Callaway

   Large signals present in the receiver first i.f. stage may require limiting to avoid overdriving the i.f. amplifiers. In portable receivers this is usually done by placing a diode, typically a PIN or Schottky diode, in shunt with the crystal filters The diode conducts when the incoming i.f. signal is large enough to forward bias it, thereby limiting the signal (Figure 1).

   The disadvantage of this method of i.f. limiting is that it is nonlinear, producing a clipped or otherwise distorted waveform that, among other possible complications, degrades receiver intermodulation performance. This reduction must be compensated for by improved IM performance of the rest of the receiver, principally the cf. preamp and mixer. Since to a first approximation IM performance in active devices is proportional to cur- rent drain, this is not an attractive solution. The proposed circuit (Figure 2) uses the Received Signal Strength Indicator(RSSI) voltage from the i.f. IC to forward bias the shunt diode, rather than the incoming i.f. signal itself, when strong signals are present.

   With a d.c. voltage controlling a shunt PIN diode, a controlled conductance to ground is created, thus reducing the amplitude of the if. signal while preserving the shape of the if. waveform. This preservation of circuit lineari- ty while providin...