High Frequency Absorptive Switch Architecture
An absorptive switch architecture suitable for use in high frequency RF applications. A switching circuit includes a common terminal and one or more ports, any of which may be selectively coupled to the common terminal by closing an associated path switch; non-selected, unused ports are isolated from the common terminal by opening an associated path switch. Between each path switch and a port are associated shunt switches for selectively coupling an associated signal path to circuit ground. Between each path switch and a port is an associated absorptive switch module. Each absorptive switch module includes a resistor coupled in parallel with a switch. The combination of the resistor and the switch of the absorptive switch module is placed in series with a corresponding signal path from each port to the common terminal, rather than in a shunt configuration.
1 . A switching circuit having at least one port coupled to an absorptive switch module in series with at least an RF common terminal, wherein the absorptive switch module has a first relatively small resistor in parallel with a second relatively large resistor in a first state and of the second relatively large resistor in parallel with a capacitor in a second state.
2 . The switching circuit of claim 1 , wherein the first relatively small resistor has a resistance equal to an “ON” FET resistance, the capacitor has a capacitance equal to C off of an “OFF” FET, and the second relatively large resistor has a resistance equal to a characteristic impedance of the switching circuit.
3 . The switching circuit of claim 2 , further including at least one shunt switch having characteristics of a distributed inductance-resistance (LR) network coupled from circuit ground to a signal path in an “ON” state and having characteristics of a distributed inductance-capacitance (LC) network coupled from circuit ground to the signal path in a second state,
wherein the impedance of the distributed LR network is Z LR =(Rb +jXb), where Rb and +jXb are the series equivalent resistance and reactance of the at least one shunt switch in the “ON” state and the distributed interconnect between the shunt switches and ground, and
wherein the impedance of the absorptive switch module when in the “OFF” state is Z Term =(Ra −jXa), where Ra and −jXa are series equivalent resistance and reactance of the parallel combination of the relatively large resistor and C off .
4 . The switching circuit of claim 3 , wherein Ra+Rb is essentially equal to the characteristic impedance of the switching circuit.
5 . The switching circuit of claim 4 , wherein the characteristic impedance is 50 ohms.
6 . The switching circuit of claim 3 , wherein Z term +Z LR =Z 0 , and wherein Z 0 is the characteristic impedance of the switching circuit.
7 . The switching circuit of claim 2 , wherein the absorptive switch module comprises a FET having a body and a source, and the absorptive switch module further includes a body diode coupled between body and the source of the FET.
8 . A switching circuit including:
(a) a common terminal;
(b) at least one port;
(c) an absorptive switch module having a first and second terminal, the first terminal coupled to a port, the absorptive switch module including a first switch and a resistor coupled in parallel between the first and second terminal;
(d) a path switch having a first terminal coupled to the common port and a second terminal coupled to the second terminal of the absorptive switch module; and
(e) at least one shunt switch having a first terminal coupled to the second terminal of the path switch and a second terminal coupled to ground,
wherein the absorptive switch module has an impedance equal to a termination resistance in a first state and equal to a relatively small resistance, Ron, in parallel with the termination resistance in a second state.
9 . The switching circuit of claim 8 , wherein the first switch is a field effect transistor (FET).
10 . The switching circuit of claim 9 , wherein the termination resistance has a first terminal coupled to a source terminal of the FET and a second terminal coupled to a drain of the FET.
11 . The switching circuit of claim 9 , wherein the impedance of the absorptive switch module is equal to the termination resistance in parallel with a capacitance Coff in the first state.
12 . The switching circuit of claim 11 , wherein the impedance from the absorptive switch module to ground through the at least one shunt switch, with the at least one shunt switch in an “ON” state, is inductive and matched to C off .
13 . The switching circuit of claim 12 , further including:
(a) at least a second port;
(b) at least a second absorptive switch module having a first and second terminal, the first terminal coupled to the second port;
(c) at least a second path switch having a first terminal coupled to the common terminal and a second terminal coupled to the second terminal of the second absorptive switch module; and
(d) at least a second shunt switch having a first terminal coupled to the second terminal of the path switch and a second terminal coupled to ground,
wherein the second absorptive switch module has an impedance equal to a second termination resistance in parallel with a capacitance C off2 in the first state of the second absorptive switch module; and
wherein the impedance from the second absorptive switch module to ground through the second shunt switch, with the second shunt switch in an “ON” state, is inductive and matched to C off2 .
14 . The switching circuit of claim 8 , wherein the termination resistance has an impedance of approximately 50 Ohms.
15 . The switching circuit of claim 8 , wherein the impedance from the absorptive switch module to ground through the at least one shunt switch, with the shunt switch in an “ON” state, is inductive and matched to C off for frequencies from 1 to 40 GHz.
16 . The switching circuit of claim 15 , wherein C off has an impedance of approximately 67 femtofarads.
17 . The switching circuit of claim 15 , wherein the resistance R on is approximately 3 Ohms.
18 . The switching circuit of claim 15 , wherein the first terminal of the shunt switch is a source of FET and the second terminal of a drain of the FET.
19 . The switching circuit of claim 8 , wherein the shunt switch is a FET.
20 . The switching circuit of claim 8 , wherein the switching circuit is a radio frequency (RF) switching circuit.
21 . The switching circuit of claim 8 , wherein the absorptive switch module has a capacitive reactance that increases with increasing frequency, and wherein the conductive path has an inductive reactance from the absorptive switch module to ground through the shunt switch that increases with increasing frequency.
22 . The switching circuit of claim 21 , wherein the increase in the capacitive reactance of the absorptive switch is essentially equal to the increase in the inductive reactance of the path through the shunt switch with increasing frequency.
23 . The switching circuit of claim 21 , wherein the impedance of the absorptive switch module functions as a negative reactance that substantially offsets a positive reactance of the at least one shunt switch for signal frequencies within a range of about 1 to 40 GHz.
24 . An RF switching circuit embodied in an integrated circuit, including:
(a) a common terminal;
(b) at first RF port;
(c) a path switch means having a first and second terminal, the first terminal coupled to the common terminal for imposing a relatively high resistance to RF signals in a first state and for imposing a relatively low resistance to RF signals in a second state;
(d) an absorptive switch module means having a first terminal coupled to the second terminal of the path switch means and a second terminal coupled to the first RF port, the absorptive switch module means for imposing a resistance R term in parallel with a capacitance, C off in a first state and imposing a resistance R on in parallel with the resistance R term in a second state; and
(e) a shunt switch means having a first terminal coupled to the second terminal of the path switch means and having a second terminal coupled to ground, for providing a low resistance path to ground in a first state and a high resistance path to ground in a second state.
25 . The RF switching circuit of claim 24 , wherein the path switch means is in the first state when the absorptive switch module means is in the second state, thereby creating a relatively low impedance signal path from the common terminal to the first RF port.
26 . The RF switching circuit of claim 25 , wherein the path switch means is in the second state when the absorptive switch module means is in the first state, thereby terminating the first RF port in a resistance equal to R term .
27 . The RF switching circuit of claim 26 , wherein the shunt switch means is in the first state when the path switch means is in the second state.
28 . The RF switching circuit of claim 27 , wherein the shunt switch means is an FET.
29 . The RF switching circuit of claim 27 , wherein a conductive path from the first terminal of the absorptive switch module means to ground through the shunt switch means with the shunt switch means in the first state has an impedance that is inductive.
30 . The RF switching circuit of claim 29 , wherein the conductive path from the first RF port to the second terminal of the absorptive switch module means has an impedance that is capacitive and an inductive reactance of the conductive path from the first terminal of the absorptive switch module means to ground through the shunt switch means with the shunt switch means in the first state is equal to the capacitive reactance of the conductive path from the first RF port to the second terminal of the absorptive switch module means, such that the impedance from the first RF port to ground through the absorptive switch module means in the first state and the shunt switch means in the first state is essentially purely resistive.
31 . The RF switch circuit of claim 30 , further including:
(a) a second port;
(b) a second path switch means having a first and second terminal, the first terminal coupled to the common terminal for imposing a relatively high resistance to RF signals in a first state and for imposing a relatively low resistance to RF signals in a second state;
(c) at least a second absorptive switch module means having a first terminal coupled to the second terminal of the second path switch means and a second terminal coupled to the second RF port, the second absorptive switch module means for imposing a resistance R term in parallel with a capacitance, C off2 in a first state and imposing a resistance R on2 in parallel with the resistance R term in a second state.
32 . The RF switch circuit of claim 31 , wherein the second path switch means is in the first state when the second absorptive switch module means is in the second state, thereby creating a relatively low impedance signal path from the common terminal to the second RF port.
33 . The RF switching circuit of claim 32 , wherein the second path switch means is in the second state when the second absorptive switch module means is in the first state, thereby terminating the second RF port in a resistance equal to R term.
34 . The RF switching circuit of claim 33 , wherein the second shunt switch means is in the first state when the second path switch means is in the second state.
35 . The RF switching circuit of claim 34 , wherein the conductive path from the first terminal of the second absorptive switch module means to ground through the second shunt switch means with the second shunt switch means in the first state has an impedance that is inductive.
36 . The RF switching circuit of claim 35 , wherein a conductive path from the second RF port to the second terminal of the second absorptive switch module means has an impedance that is capacitive and an inductive reactance of the conductive path from the first terminal of the second absorptive switch module means to ground through the second shunt switch means with the second shunt switch means in the first state is equal to the capacitive reactance of the conductive path from the second RF port to the second terminal of the second absorptive switch module means, such that the impedance from the second RF port to ground through the second absorptive switch module means in the first state and the second shunt switch means in the first state is essentially purely resistive.
37 . The RF switching circuit of claim 30 , wherein the impedance from the first RF port to ground through the absorptive switch module means in the first state and the shunt switch means in the first state is essentially equal to a characteristic impedance of the RF switching circuit.
38 . The RF switching circuit of claim 37 , wherein an insertion loss at the first RF port looking into the port from outside the RF switching circuit is less than −4 dB from about 1 GHz to 40 GHz at the characteristic impedance.
39 . The RF switching circuit of claim 38 , wherein the first switch means is a FET.
40 . The RF switching circuit of claim 30 , wherein the absorptive switch module means includes:
(a) a first switch means;
(b) a termination resistance coupled in parallel with the first switch mean.
41 . A method for connecting a common terminal with at least one port, including:
(a) providing a common terminal;
(b) providing at least one port;
(c) providing a path switch having a first terminal and second terminal;
(d) providing a shunt switch having a first and second terminal;
(e) coupling the first terminal of the path switch to the common port;
(f) coupling the first terminal of the shunt switch to the second terminal of the port switch;
(g) coupling the second terminal of the shunt switch to ground;
(h) providing an absorptive switch module having a first terminal and a second terminal, the absorptive switch module including a first switch in parallel with a termination resistor coupled between the first and second terminal;
(i) coupling the first terminal of the absorptive switch to the second terminal of the path switch;
(j) coupling the second terminal of the first switch to the port;
(k) turning on the path switch;
(l) turning on the first switch within the absorptive switch module; and
(m) turning off the shunt switch.
42 . The method of claim 41 , further for isolating the port from the common terminal and terminating the port in a characteristic impedance, further including;
(a) turning off the path switch;
(b) turning off the first switch within the absorptive switch module; and
(c) turning on the shunt switch.
43 . The method of claim 42 , further including matching an off-capacitance, C off of the first switch to an inductance of the conductive path from the first terminal of the first switch to ground through the shunt switch with the shunt switch on and with the first switch off.