Switch topology for switching filters multiplexers

1. Radio frequency (RF) front end circuitry comprising:

a first filter circuit coupled to a first RF port, wherein the first filter circuit is configured to present a first filter capacitance to the first RF port; and

a switch device comprising a common port, the first RF port, a first switchable path operable to be opened and closed and connected in series between the common port and the first RF port, wherein the switch device is configured to:

present approximately the first filter capacitance of the first filter circuit at the common port when the first switchable path is closed; and

present approximately a first device capacitance at the common port when the first switchable path is open, wherein a total capacitance presented at the common port is maintained independent of the first switchable path being open or closed.

2. The RF front end circuitry of claim 1 wherein the switch device is further configured to:

not present approximately the first filter capacitance at the common port when the first switchable path is open; and

not present approximately the first device capacitance at the common port when the first switchable path is closed.

3. The RF front end circuitry of claim 1 wherein the switch device further comprises a first switch connected in series within the first switchable path and a first switchable shunt path, wherein:

the first switch is operable to be turned on so that the first switchable path is closed and is operable to be turned off so that the first switchable path is opened;

the first switchable shunt path is operable to be opened and closed, wherein the first switchable shunt path is connected in shunt to the first switchable path between the first switch and the first RF port; and

the switch device is configured to:

present approximately the first filter capacitance at the common port when the first switch is turned on and the first switchable shunt path is open; and

present approximately the first device capacitance at the common port when the first switch is turned off and the first switchable shunt path is closed.

4. The RF front end circuitry of claim 3 wherein the switch device further comprises a first capacitive element having a first capacitance wherein:

the first capacitive element is connected in parallel with the first switch; the first switch is operable to provide a first off switch capacitance when the first switch is turned off; and

wherein the first device capacitance is approximately equal to a combination of the first capacitance and the first off switch capacitance.

5. The RF front end circuitry of claim 3 wherein the first switch is operable to provide a first off switch capacitance when the first switch is turned off, wherein the first device capacitance is the first off switch capacitance.

6. The RF front end circuitry of claim 3 further comprising control circuitry configured to:

turn on the first switch to close the first switchable path and open the first switchable shunt path so that the switch device presents approximately the first filter capacitance at the common port; and

turn off the first switch to open the first switchable path and close the first switchable shunt path so that the switch device presents approximately the first device capacitance at the common port.

7. The RF front end circuitry of claim 1 wherein the first filter circuit is a multiplexer and wherein:

the multiplexer comprises a plurality of filters, RF ports, and a second common port;

each of the RF ports is connected to a different corresponding one of the plurality of filters;

each of the plurality of filters is connected to the second common port;

the second common port is connected to the first RF port; and

the first filter capacitance is provided at the common port.

8. The RF front end circuitry of claim 1 wherein the first filter circuit is a first filter connected to the first RF port and is configured to present the first filter capacitance at the first RF port.

9. The RF front end circuitry of claim 1 further comprising a second filter circuit wherein:

the switch device further comprises a second RF port and a second switchable path;

the second switchable path is operable to be opened and closed and is connected in series between the second RF port and the common port;

the second filter circuit is coupled to the second RF port, wherein the second filter circuit is configured to present approximately a second filter capacitance to the second RF port; and

the switch device is further configured to:

present approximately the second filter capacitance at the common port when the second switchable path is closed; and

present approximately a second device capacitance at the common port when the second switchable path is open, wherein the second device capacitance approximately equals the second filter capacitance.

10. The RF front end circuitry of claim 9 wherein the switch device is further configured to:

not present approximately the first filter capacitance at the common port when the first switchable path is open;

not present approximately the first device capacitance at the common port when the first switchable path is closed;

not present approximately the second filter capacitance at the common port when the second switchable path is open; and

not present approximately the second device capacitance at the common port when the second switchable path is closed.

11. The RF front end circuitry of claim 9 wherein the switch device further comprises a first switch, a second switch, a first switchable shunt path, and a second switchable shunt path and wherein:

the first switch is connected in series within the first switchable path, wherein the first switch is operable to be turned on so that the first switchable path is closed and is operable to be turned off so that the first switchable path is opened;

the first switchable shunt path is operable to be opened and closed, wherein the first switchable shunt path is connected in shunt to the first switchable path between the first switch and the first RF port;

the second switch is connected in series within the second switchable path, wherein the second switch is operable to be turned on so that the second switchable path is closed and is operable to be turned off so that the second switchable path is opened;

the second switchable shunt path is operable to be opened and closed, wherein the second switchable shunt path is connected in shunt to the second switchable path between the second switch and the second RF port; and

the switch device is configured to:

present approximately the first filter capacitance at the common port when the first switch is turned on and the first switchable shunt path is open;

present approximately the first device capacitance at the common port when the first switch is turned off and the first switchable shunt path is closed;

present approximately the second filter capacitance at the common port when the second switch is turned on and the second switchable shunt path is open; and

present approximately the second device capacitance at the common port when the second switch is turned off and the second switchable shunt path is closed.

12. The RF front end circuitry of claim 11 wherein the switch device further comprises a first capacitive element having a first capacitance and a second capacitive element having a second capacitance and wherein:

the first capacitive element is connected in parallel with the first switch; the first switch is operable to provide a first off switch capacitance when the first switch is turned off;

the first device capacitance is approximately equal to a combination of the first capacitance and the first off switch capacitance;

the second capacitive element is connected in parallel with the second switch;

the second switch is operable to provide a second off switch capacitance when the second switch is turned off; and

the second device capacitance is approximately equal to a combination of the second capacitance and the second off switch capacitance.

13. The RF front end circuitry of claim 11 wherein:

the first switch is operable to provide a first off switch capacitance when the first switch is turned off, wherein the first device capacitance is the first off switch capacitance; and

the second switch is operable to provide a second off switch capacitance when the second switch is turned off, wherein the second device capacitance is the second off switch capacitance.

14. The RF front end circuitry of claim 11 further comprising control circuitry configured to:

turn on the first switch to close the first switchable path and open the first switchable shunt path so that the switch device presents approximately the first filter capacitance at the common port;

turn off the first switch to open the first switchable path and close the first switchable shunt path so that the switch device presents approximately the first device capacitance at the common port;

turn on the second switch to close the second switchable path and open the second switchable shunt path so that the switch device presents approximately the second filter capacitance at the common port; and

turn off the second switch to open the second switchable path and close the second switchable shunt path so that the switch device presents approximately the second device capacitance at the common port.

15. The RF front end circuitry of claim 14 wherein the control circuitry is configured to operate the switch device such that any one from a first group and any one from a second group can be selected by the control circuitry and wherein:

the first group comprises:

turning on the first switch to close the first switchable path and opening the first switchable shunt path so that the switch device presents approximately the first filter capacitance at the common port; and

turning off the first switch to open the first switchable path and closing the first switchable shunt path so that the switch device presents approximately the first device capacitance at the common port;

the second group comprises:

turning on the second switch to close the second switchable path and opening the second switchable shunt path so that the switch device presents approximately the second filter capacitance at the common port; and

turning off the second switch to open the second switchable path and closing the second switchable shunt path so that the switch device presents approximately the second device capacitance at the common port; and

the switch device is configured such that the total capacitance presented at the common port is approximately unchanged for any selection by the control circuitry from the first group and the second group.

16. The RF front end circuitry of claim 15 further comprising an inductor coupled to the common port so that the inductor resonates with the total capacitance presented at the common port.

17. The RF front end circuitry of claim 9 further comprising a third filter circuit, wherein the switch device further comprises a third RF port and a third switchable path and wherein:

the third switchable path is operable to be opened and closed and is connected in series between the third RF port and the common port;

the third filter circuit is coupled to the third RF port, wherein the third filter circuit is configured to present approximately a third filter capacitance to the third RF port; and

the switch device is further configured to:

present approximately the third filter capacitance at the common port when the third switchable path is closed; and

present approximately a third device capacitance at the common port when the third switchable path is open, wherein the third device capacitance approximately equals the third filter capacitance.

18. The RF front end circuitry of claim 17 wherein the switch device is further configured to:

not present approximately the first filter capacitance at the common port when the first switchable path is open;

not present approximately the first device capacitance at the common port when the first switchable path is closed;

not present approximately the second filter capacitance at the common port when the second switchable path is open;

not present approximately the second device capacitance at the common port when the second switchable path is closed;

not present approximately the third filter capacitance at the common port when the third switchable path is open; and

not present approximately the third device capacitance at the common port when the third switchable path is closed.

19. The RF front end circuitry of claim 1 further comprising a second filter circuit connected directly to the common port.

20. Radio frequency (RF) front end circuitry comprising:

a filtering circuitry configured to present filter capacitances to RF ports such that a different one of the filter capacitances is presented to each of the RF ports;

a switch device having a common port, the RF ports, and switchable paths wherein each switchable path of the switchable paths is connected between the common port and a different corresponding one of the RF ports and is operable to be opened and closed;

a variable capacitive element operable to provide a variable capacitance, wherein the variable capacitive element is coupled to the common port; and

control circuitry configured to:

open and close different combinations of the switchable paths; and

adjust the variable capacitance of the variable capacitive element so that a total capacitance presented at the common port is maintained independent of the switchable paths being open or closed.