Systems and methods to switch radio frequency signals for greater isolation
In semiconductor switches, the isolation can be limited by the capacitive coupling between the switch input and the switch output. Ultra-high isolation can be achieved by adding a coupled transmission line to the semiconductor switch. The coupled transmission line introduces inductive coupling, which cancels at least a part of the capacitive coupling between the switch input and the switch output.
1. A switching circuit comprising:
a coupled transmission line including a first transmission line and a second transmission line; and
a plurality of transistors including first and second transistors electrically connected in series and forming a switch that is configured to switch a signal from a first port to a second port through the first transmission line and to disconnect the first port from a third port, the second transmission line inducing an inductive signal which cancels at least a portion of a capacitive signal generated by the signal capacitively coupling through the switch to the third port, the switch having a common terminal electrically connected to the first port, a first terminal electrically connected to the second port through the first transmission line, and a second terminal electrically connected to the third port through the second transmission line.
2. The switching circuit of claim 1 further comprising a third transistor electrically connected to the first terminal and a fourth transistor electrically connected to the second terminal.
3. The switching circuit of claim 2 wherein the first, second, third, and fourth transistors include field-effect transistors.
4. The switching circuit of claim 1 wherein at least one of the plurality of transistors includes four transistor devices electrically connected in series.
5. The switching circuit of claim 1 wherein the first port is in communication with a radio frequency antenna, the second port is configured to receive a radio frequency receive signal, and the third port is configured to receive a radio frequency transmit signal.
6. The switching circuit of claim 1 wherein the switch includes a single pole double throw switch.
7. A semiconductor die including the switching circuit of claim 1 .
8. A portable transceiver including the semiconductor die of claim 7 .
9. A switching module including the switching circuit of claim 1 .
10. A portable transceiver including the switching circuit of claim 1 .
11. The switching circuit of claim 1 wherein the coupled transmission line comprises one of stripline, a microstrip, and a coplanar wave guide.
12. A method to improve isolation of a switching circuit, the method comprising:
receiving a radio frequency signal at a first port in a switching circuit that includes a switch having first and second transistors electrically connected in series;
switching the switching circuit so as to electrically connect the first port to a second port in the switching circuit and to electrically disconnect the first port from a third port in the switching circuit, the switch having a common terminal electrically connected to the first port, a first terminal electrically connected to the second port through the first transmission line, and a second terminal electrically connected to the third port through the second transmission line; and
conducting the radio frequency signal from the first port to the second port through a first transmission line of a coupled transmission line.
13. The method of claim 12 further comprising generating a capacitive signal by capacitive coupling of the radio frequency signal through the switching circuit to the third port.
14. The method of claim 13 further comprising inducing an inductive signal in a second transmission line of the coupled transmission line.
15. The method of claim 14 wherein the inductive signal cancels at least a portion of the capacitive signal to provide isolation improvement.
16. The method of claim 12 wherein the switching circuit includes a single pole double throw switch.
17. The method of claim 16 wherein the single pole double throw switch further includes a third transistor electrically connected to the first terminal and a fourth transistor electrically connected to the second terminal.
18. The method of claim 12 wherein the switching circuit includes a plurality of field-effect transistors.
19. The method of claim 18 wherein at least one of the plurality of field-effect transistors includes four transistor devices electrically connected in series.
20. The method of claim 12 wherein the coupled transmission line comprises one of stripline, a microstrip, and a coplanar wave guide.