Abstract:
An antenna switch using two low-cost relays substantially improves isolation of signals from a transmitter to a receiver by connecting an impedance across the input of a receiver during transmit. During receive, the transmitter is disconnected from the antenna while the receiver is connected.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to RF antenna switches. In particular, this invention relates to relays used as RF antenna switches at frequencies up to 800 MHz. 
     An RF relay used as an antenna switch is typically a single pole, double throw relay, having the common contact switched between two contacts by an armature. When the armature is in a first position, one switch contact is connected to the common contact. With the armature in the other position, the other switch contact is connected to the common contact. To use a single pole double throw relay as an antenna switch, therefore, the antenna would normally be connected to the common node and the transmitter and receiver connected to the appropriate switch contacts. 
     During transmit mode, RF energy from the transmitter can leak to the receiver by virtue of stray capacitance existing in the relay. As the reactance of stray capacitance between the receiver and transmitter decreases, and as the frequency of the RF energy increases, the signal leaked to the receiver from the transmitter increases possibly reaching levels sufficient to damage the receiver&#39;s components. The level of signal coupled to the receiver from the transmitter in an antenna switch is often expressed in decibels as the isolation loss. The higher the isolation loss of an antenna switch, the greater the isolation of the transmitter signal from the receiver. 
     Many physical characteristics affect isolation loss in a relay. As the physical size of RF relays used for antenna switches decreases the coupling between a transmitter and a receiver increases, decreasing the isolation loss of the relay. Other factors such as dielectric selection, spacing between the contacts, and configuration of electrodes used to fabricate the relay can also affect isolation loss. 
     In many circuits, miniaturization is important. Small relays are frequently used to switch transmitters and receivers between the antenna. There are many compact RF relays suitable for printed circuit applications that have relatively high isolation losses. The principal drawback of these prior art RF relays is their cost, as they are substantially more expensive than other types of relays. A low cost relay having improved isolation loss when used at even high frequencies, and that is compact and usable with printed circuit board applications would be an improvement over the prior art. 
     SUMMARY OF THE INVENTION 
     There is provided herein an improved RF antenna switch using a low cost, single pole, double throw relay having substantially improved isolation loss. A unique circuit topology using two single-pole, double-throw relays and an additional impedance shunting the receiver input during transmit substantially improves isolation loss of an antenna switch using such relays. 
     In the transmit position, the common node of a first relay connects the antenna to the transmitter; other contacts of a second relay connect a small resistance in parallel with the input of the receiver to ground potential, thereby increasing the isolation loss of the relay. In the receive position, the relay is wired to connect the input of the receiver directly to the antenna, leaving the output of the transmitter open-circuited. 
     The small resistance across the input of the receiver during transmit shunts signals coupled to the receiver by stray capacitance to ground. This small resistance couples RF energy to ground that would otherwise be input into the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram of an improved antenna switch in the transmit position. 
     FIG. 2 shows the improved antenna switch in the receive position. 
     FIG. 3 shows an equivalent diagram of the circuit shown in FIG. 1. 
     FIG. 4 shows a simplified diagram of the circuit of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown an improved antenna switch (1). The antenna switch (1) connects either a transmitter (300) or a receiver (200) to an antenna (400) using relays. A transmitter/receiver controller (500) that controls a power supply (600) might be used to control current to the windings (110) of the antenna switch (1) comprised of relays. Current in the windings (110) of the relays controls whether the transmitter or receiver is connected to the antenna. 
     In FIG. 1, the antenna switch (1) is in the transmit position. Signals from the transmitter (300) are conducted through two relay contacts (70 and 40) to the antenna (400). In this position, the antenna switch (1) connects the input of the receiver (200) to ground (5) through a resistance (R 90 ) electrically parallel to the input impedance of the receiver (200). The resistance (R 90 ) in parallel to the receiver&#39;s input increases the isolation loss of the antenna switch (1). (A reactive element, such as a capacitor (C 100 ) might be used in combination with the external resistor (R 90 ) for impedance matching purposes.) 
     In FIG. 2, the antenna switch (1) is in the receive position. The antenna (400) is connected to the receiver (200) by the connection existing between relay contacts (60 and 20). In this position the transmitter is connected to a relay contact (70) that is an open circuit. When in the receive position the output of the transmitter is simultaneously disabled (C 100  or other reactive elements used for impedance matching when the receiver is disabled would remain coupled to R 90  when the antenna switch is in the receive position.). 
     Achieving improved isolation loss can be understood by an analysis of the equivalent circuit diagram shown in FIG. 3. The stray capacitance (C 75 ), which affects isolation loss, is shown between two of the relay contacts (70 and 80). A voltage source (V 300 ), representing the output voltage of the transmitter (300) and its output impedance (R 300 ), is also shown. The antenna impedance is represented by a fifty ohm resistance (R 400 ). The input impedance of the receiver (200) is shown as a resistor (R 200 ). The resistance at the lower contact (30) of the antenna switch (1) is shown as a resistor (R 90 ). 
     In FIG. 4, the circuit of FIG. 3 is simplified. The parallel resistances of the receiver&#39;s input impedance (R 200 ) and the antenna switch resistance (R 90 ), are shown as a single resistor (R 290 ) 
     To measure isolation loss, the voltage appearing at the receiver&#39;s input (the voltage at contact 80) may be calculated using the equivalent circuit shown in FIG. 4. In this figure the voltage appearing at the input of the receiver (200) would be the voltage appearing at contact 80, (V 80 ) which is the node between the capacitor (C 75 ) and R 290 . The voltage at this node 80 can be determined using a simple voltage divider equation using the voltage at the transmitter&#39;s output, the node at contact 70, as the input voltage to a divider comprised of the stray capacitance (C 75 ) and (R 290 ). 
     The voltage at the receiver (200) is: ##EQU1## where V 70  is the voltage at node 70, which is the output voltage of the transmitter, where V 80  is the voltage at node 80, which is the input terminal to the receiver, where jXC 75  is the imaginary portion of a complex number representing the impedance of the capacitor at a particular frequency and where R 290  is a real number, the parallel resistance of the input impedance of the receiver and the resistance (R 90 ) connected across the input of the receiver. 
     Dividing this equation by the voltage at node 70, V 70  yields a voltage transfer function well known to those skilled in the art: ##EQU2## where V 70  and V 80  are as defined above. The isolation loss expressed in decibels is: ##EQU3## 
     Since R 290  is the parallel combination of the external resistance (R 90 ) and the input impedance of the receiver (R 200 ), decreasing the external resistance (R 90 ) decreases R 290 . It can be seen from this equation that decreasing the value of R 90  substantially increases the isolation loss of the circuit shown in FIG. 1 and FIG. 2. As the value of R 90  decreases, the isolation loss of the circuit increases substantially. 
     In at least one alternate embodiment, a reactive element which is preferably a capacitor, (C 100 ) as shown in FIG. 2 can optionally be used across the external resistor (R90) for impedance matching purposes. 
     Using the circuit disclosed above, low cost, two form, C relays, as they are know in the art, can be used with substantially higher isolation losses when used at frequencies as high as 800 MHz.