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A LOW POWER RADIO FREQUENCY ELECTRONIC RELAY

IP.com Disclosure Number: IPCOM000005874D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-13
Document File: 2 page(s) / 113K

Publishing Venue

Motorola

Related People

Andrew Lundholm: AUTHOR

Abstract

A typical circuit used to direct RF energy amongst selectable output ports implements the usage of PIN diodes. The diodes are selectively turned on/off, behaving much like switches. When arranged in a matrix, many useful switch- ing characteristics can be achieved. A major drawback of using PIN diodes, however, is that the diodes are switched on by forward biasing, requiring a current flow through the device. The intended application for this circuit is in battery operated equipment making minimal current drain very advantageous, The current consumed in the biasing of these diodes cannot be tolerated in battery powered equipment and an alternative low loss network is necessary.

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MOlOROLA Technical Developments Volume 10 March 1990

A LOW POWER RADIO FREQUENCY ELECTRONIC RELAY

by Andrew Lundholm

STATEMENT OF PROBLEM

   A typical circuit used to direct RF energy amongst selectable output ports implements the usage of PIN diodes. The diodes are selectively turned on/off, behaving much like switches. When arranged in a matrix, many useful switch- ing characteristics can be achieved. A major drawback of using PIN diodes, however, is that the diodes are switched on by forward biasing, requiring a current flow through the device. The intended application for this circuit is in battery operated equipment making minimal current drain very advantageous, The current consumed in the biasing of these diodes cannot be tolerated in battery powered equipment and an alternative low loss network is necessary.

SOLUTION

   The solution to the low loss RF switching network involves the principles of the tapped capacitor tuned circuit. A typical tapped capacitor network is shown in Figure 1. In theory, by changing either of the capacitors C, or C2 the im- pedance transformation can be controlled. The capacitors transform R Lw higher in resistance to an equivalent resis- tant REO presents at the input. The principle of controlling the matching network between the source and load is used as the mechanism to control the transmission of RF energy through the network,

If a high "Cl" resonant condition exists, certain approximations are made. These approximations are related to the following equations:

(C,+ CJ * R c: LOAD

J&F) c=cl+c,

W,=2*n*f= Cl * c*

RED=

   This model served as the starting point for further design. If C1 can vary then the equivalent impedance (REQ) across the tuned circuit will also vary. Two easily verified assumptions are now made: (1) the input/output ports are matched to their respective source/load impedances, and (2) the entire circuit is low loss. With these assumptions in place, selectively changing the output impedance of the network (by varying C2 is accomplished through the use of a reverse biased varactor. The capacitance of such a device is inversely proportional to the reverse biased voltage ap- plied to it.

   To fully perform the desired functional characteristics, the common port must have the ability to transmit energy through the network to either of at least two outputs. When connected to one of the output ports, the input shall also be disconnected from e undesired output port(s). The desired characteristics are implemented through the circuit as shown in Figure 2.

   The transistor's sole function is to provide the necessary logic inversions required to turn on one port and turn off the other. The actual devices used are not unique as the control functions could be implemented...