RF switch with improved isolation at target frequencies
A compact RF switch with improved isolation is presented. According to one aspect, the RF switch includes a basic single-pole single-throw (SPST) switch element that includes an inductor in parallel with a series FET transistor. An inductance of the inductor is selected to provide in combination with an off capacitance of the series FET transistor a resonance at a specific frequency of interest. The frequency of interest can be in-band or out-of-band, including the band's fundamental frequency or a harmonic thereof. According to another aspect, the inductor is conditionally coupled to the series FET transistor via a reduced size FET transistor. Complex RF switches can include a plurality of the SPST switch elements, each tuned to a same or different frequency of interest. According to yet another aspect, SPST switch elements in their OFF states can provide matching to an SPST element in the ON state.
1 . A reduced size radio frequency (RF) switch, comprising:
a first single-pole single-throw (SPST) switch element coupled between a common port of the RF switch and a first port of the RF switch, the first SPST switch element comprising:
a first series transistor coupled between the common port and the first port;
a first shunting transistor coupled between the first port and a reference ground; and
a first switchable circuit comprising a first inductor in series connection with a first transistor switch, the first switchable circuit coupled between the common port and the first port,
wherein
a size of the first transistor switch is smaller than about ⅕ a size of the first series or shunting transistor to provide an OFF capacitance of the first transistor switch that is negligible to RF performance of the first SPST switch element when the first transistor switch is OFF, and
the first transistor switch is configured to be OFF when the first series transistor is ON.
2 . The reduced size radio frequency (RF) switch of claim 1 , wherein:
the size of the first transistor switch is in a range from ⅕ to 1/20 the size of the first series or shunting transistor.
3 . The reduced size radio frequency (RF) switch of claim 1 , wherein:
the size of the first transistor switch is in a range from ⅕ to 1/1000 the size of the first series or shunting transistor.
4 . The radio frequency (RF) switch of claim 1 , wherein:
the first transistor switch is configured to be ON when the first series transistor is OFF.
5 . The reduced size radio frequency (RF) switch of claim 1 , wherein:
an inductance of the first inductor is selected so that in combination with the OFF capacitance of the first series transistor form a first resonant circuit that resonates at a first frequency of interest.
6 . The reduced size radio frequency (RF) switch of claim 5 , wherein:
an effect of an ON resistance and/or the OFF capacitance of the first transistor switch on a frequency response of the first resonant circuit is negligible.
7 . The reduced size radio frequency (RF) switch of claim 5 , wherein:
the first frequency of interest is within a frequency band of operation of the RF switch.
8 . The reduced size radio frequency (RF) switch of claim 7 , wherein:
the first frequency of interest is a center frequency of the frequency band of operation.
9 . The reduced size radio frequency (RF) switch of claim 5 , wherein:
the first frequency of interest is outside a frequency band of operation of the RF switch.
10 . The reduced size radio frequency (RF) switch of claim 9 , wherein:
the first frequency of interest is a harmonic of a center frequency of the frequency band of operation of the RF switch.
11 . The reduced size radio frequency (RF) switch of claim 1 , wherein:
source and drain nodes of the first series transistor are coupled between the common port and the first port, and
source and drain nodes of the first shunting transistor are coupled between the first port and the reference ground.
12 . The reduced size radio frequency (RF) switch of claim 5 , further comprising:
a second single-pole single-throw (SPST) switch element coupled between the common port of the RF switch and a second port of the RF switch, the second SPST switch element comprising:
a second series transistor coupled between the common port and the second port;
a second shunting transistor coupled between the second port and the reference ground; and
a second switchable circuit comprising a second inductor in series connection with a second transistor switch, the second switchable circuit coupled between the common port and the second port,
wherein a size of the second transistor switch is smaller than about ⅕ a size of the second series or shunting transistor.
13 . The reduced size radio frequency (RF) switch of claim 12 , wherein:
an inductance of the second inductor is selected so that in combination with an OFF capacitance of the second series transistor form a second resonant circuit that resonates at a second frequency of interest.
14 . The reduced size radio frequency (RF) switch of claim 13 , wherein:
the first frequency of interest is equal to the second frequency of interest.
15 . The reduced size radio frequency (RF) switch of claim 13 , wherein:
the first frequency of interest is different from the second frequency of interest.
16 . The reduced size radio frequency (RF) switch of claim 13 , wherein:
the first frequency of interest is equal to a center frequency or a harmonic of the center frequency of a second RF channel processed through the second SPST switch element, and
the second frequency of interest is equal to a center frequency or a harmonic of the center frequency of a first RF channel processed through the first SPST switch element.
17 . The reduced size radio frequency (RF) switch of claim 1 , wherein:
the first series transistor, the first shunting transistor, or the first transistor switch is a metal-oxide-semiconductor (MOS) field effect transistor (FET).
18 . The reduced size radio frequency (RF) switch of claim 17 , wherein:
the first series transistor, the first shunting transistor or the first transistor switch is fabricated using one of: a) silicon-on-insulator (SOI) technology, or b) silicon-on-sapphire technology (SOS).
19 . A radio frequency (RF) frontend system comprising:
the reduced size RF switch of claim 12 ; and
an antenna coupled to the common port of the reduced size RF switch,
wherein the RF frontend system is configured to receive or transmit through the antenna:
a first RF signal coupled to the first port of the reduced size RF switch, and
second RF signal coupled to the second port of the reduced size RF switch.
20 . The radio frequency (RF) frontend system of claim 19 , wherein:
an inductance of the second inductor is selected such that during processing of the first RF signal, an impedance matching to the antenna is provided by an OFF-state impedance of the second SPST switch element, and
an inductance of the first inductor is selected such that during processing of the second RF signal, an impedance matching to the antenna is provided by an OFF-state impedance of the first SPST switch element.