Patent Application
20240364330
The instant patent application is related to and claims priority from the co-pending India provisional patent application entitled, “IMPROVING ISOLATION AND INSERTION LOSS IN RF SWITCH”, Serial No.: 202341030083, Filed: 26 Apr. 2023, Attorney docket no.: AURA-346-INPR, which is incorporated in its entirety herewith to the extent not inconsistent with the description herein.
Embodiments of the present disclosure relate generally to electronic switches, and more specifically to impedance design in a resonant switch.
A switch is operable to either block or pass-through an input signal from one port of the switch to another port, as is well known in the relevant arts. A resonant switch contains additional impedances to shape either the voltage or the current waveform of the input signal passing through the resonant switch, which ensures that the input signal is passed through with minimal distortion. Generally, at least some of the impedances operate at resonance with respect to the frequency band of the input signal under corresponding operation conditions (e.g., transmission or reception of the input signal by a device using the switch), and hence the switch is termed a ‘resonant switch’.
Additionally, the impedances (or at least some of them) may also serve the purpose of impedance-transformation or impedance-matching, for example, to ensure that when the resonant switch is used in a larger circuit/device the impedance at a port due to an external circuit is transformed to a desired value when viewed from another port. Such transformation or matching may be for the purpose of minimizing transmission-line reflections and consequent distortion and/or attenuation of the signal(s) passing through the switch from one port to another.
Resonant switches find use in blocks such as RF (Radio Frequency) transceivers.
Aspects of the present disclosure are directed to design of such impedances in resonant switches.
Example embodiments of the present disclosure will be described with reference to the accompanying drawings briefly described below.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.
A resonant switch provided according to an aspect of the present disclosure contains three ports (a first port, a second port and a third port) and includes multiple switching elements coupled in series between a first port and a second port of the resonant switch. Each switching element has a parasitic capacitance across it, and a resistance in the ON-state. The multiple switching elements include a first set of switching elements and a second set of switching elements. The first set of switching elements and the second set of switching elements are connected in series at a junction. The resonant switch further includes a capacitor connected between the junction and a constant reference potential.
In an embodiment, the resonant switch (switch) is used in a transceiver, and the first port, the second port and the third port are respectively connected to a termination resistor, an antenna (via a circulator) and a low-noise amplifier (LNA) of a receiver in the transceiver. The connection of the capacitor at the junction of the first set of switching elements and the second set of switching elements improves the isolation between the first port and the third port, and also reduces the insertion-loss between the first port and the second port.
Several aspects of the present disclosure are described below with reference to examples for illustration. However, one skilled in the relevant art will recognize that the disclosure can be practiced without one or more of the specific details or with other methods, components, materials and so forth. In other instances, well known structures, materials, or operations are not shown in detail to avoid obscuring the features of the disclosure. Furthermore, the features/aspects described can be practiced in various combinations, though only some of the combinations are described herein for conciseness.
Circulator 130 is shown as a 3-port device. The ports are marked by numerals 1, 2 and 3 in
TX 110 may contain several other blocks internally (which are not shown) in addition to PA 111. The corresponding blocks generate information (in analog or digital form), which is modulated on one or more carrier waves. PA 111 amplifies the modulated carriers and provides the amplified signal at port 1 of circulator. The information may originate from a storage block in TX 110, or in an external device/system or from a user input, which may provide the information to TX 110 via suitable input terminals (not shown).
Antenna 120 transmits signals received via port 2 from TX 110 on a wireless medium. Antenna 120 receives signals (including information-modulated carrier signals from other transceivers/transmitters) from the wireless medium and provides the received signals on port 2 of circulator 130, with the received signals exiting on port 3 of circulator 130. Antenna 120 may be implemented based on the specific details of the frequency-bands to be supported by TX 110 and RX 160 and other technical considerations specific to the environment of operation, etc.
In an embodiment of the present disclosure, the frequency bands of ‘transmit signals’ (received from TX 110) and ‘receive signals’ (passed on to RX 160) overlap at least partially, such that transceiver 100 operates in a time-division-multiplexed (TDM) fashion. Thus, ‘transmit intervals’ and ‘receive intervals’ are multiplexed in time in the embodiment. However, it is to be understood that the use of switch 140 is not limited to TDM environments. As an example, switch 140 can be used in frequency-division multiplexed (FDM) environments in which ‘transmit signals’ and ‘receive signals’ occupy non-overlapping frequency bands, but the guard band separating the ‘transmit band’ and ‘receive band’ is very narrow and inexpensive filters are used for filtering the ‘transmit signals’ and ‘receive signals’, and TDM is also additionally employed.
Switch 140 is shown having three ports named ANT, TERM and RX. The ANT port is connected to port 3 of circulator, the TERM port is connected to a termination element, namely resistor 150, the RX port is connected to LNA 161 of RX 160. Resistor 150 is implemented to have a resistance substantially equal to the resistance of the path (transmission line) from antenna 120 to the TERM port via port 3 of circulator 130 and ANT port of switch 140. Switch 140 operates to connect the ANT port to the TERM port during transmit intervals, and the ANT port to the RX port in receive intervals.
In transmit intervals, reflections of transmit signals (from PA 111) may occur from antenna 120 due, for example, to mismatches between impedances of the transmit path from PA 111 to antenna 120 (via ports 1 and 2 of circulator 130) and the antenna input impedance. The reflected signals travel to ANT port of switch 140 via port 3 of circulator 130. In transmit intervals, switch 140 operates (for example, based on a switch control signal, not shown, but which can be provided by a suitable circuit block in transceiver 100 or a larger system in which the blocks of
In receive intervals, switch 140 operates to connect ANT port to RX port, thereby passing received signals from antenna 120 to RX 160.
LNA 161 of RX 160 receives input signals from the wireless medium via antenna 120 and circulator 130, and operates to amplify the received signals. RX 160 may contain several other blocks internally (not shown) in addition to LNA 161. Together, such blocks operate to demodulate the amplified signals generated by LNA 161 to extract the information content (in analog or digital form). RX 160 may store the information in a storage block within RX 160 or provide the information to an external device/system or user.
Switch 140 generally needs to be implemented such that noise (e.g., in the form of thermal noise) generated by termination resistor 150 does not couple to RX port and thereby into LNA 161 and thus RX 160—or at least couples only minimally—at least during the receive intervals. That is, good isolation is a general requirement between the TERM port and RX port during receive intervals. Limited isolation between TERM port and RX port in receive intervals may cause the noise figure of LNA 161 to degrade (increase) due to the noise contributed by termination resistor 150, thereby degrading the performance of RX 160. As is well known in the relevant arts, Noise Figure is a measure of how much the Signal-to-Noise ratio of an input signal has degraded after going through an amplifier (such as LNA 161) and in general a receiver. It is generally desirable that the Noise Figure be as low as possible.
Another general requirement is zero or minimal insertion-loss due to switch 140 between ANT port and TERM port in transmit intervals.
A resonant switch (such as switch 140) implemented according to several aspects of the present disclosure has good isolation between its TERM port and RX port, and low insertion-loss between ANT port and TERM port. The implementation as well as operation of a resonant switch in embodiments of the present disclosure will be clearer in comparison with a prior implementation and operation of a resonant switch designed to operate as described above.
Switching element 250 is shown connected to inductor 220 at node 252. Ports 201, 202 and 203 respectively corresponding to ANT, TERM and RX ports of
In transmit intervals, MOSFETS 251 and 230 are each ON (switches 251 and 230 are ‘closed’). As a result, antenna port 201 (antenna) is connected to port 202 (termination), and port 203 (receive) is shorted to ground. A reflected signal from the antenna (such as in
In receive intervals, MOSFETS 251 and 230 are each ON (switches 251 and 230 shown in
In receive intervals, MOSFETS 251 and 230 are OFF (switches 251 and 230 are ‘open’). As a result, a signal received at antenna port 201 flows into the receiver via receive port 203. In transmit intervals, capacitors 210 and 240 along with inductor 220 form a parallel resonant LC network. The resonance created by such a network helps cancel the unwanted effects of components such as inductor 220 and capacitor 240 which are used in receive intervals for impedance matching/transformation. In receive intervals, in one embodiment, capacitor 240 and inductor 220 transform the input impedance of the LNA (which would be connected to receive port 203) to 50 ohms (for impedance-matching purposes) as seen from antenna port 201. Such transformation may be needed due to LNA's input impedance not being 50 ohms (or any other desired value) or because some components in the switch like transistor 230 cause the input impedance of the LNA it to be different from 50 ohms (or the desired value) as seen looking-in from the antenna port 201.
The inductance value of inductor 220 is determined as a trade-off between insertion loss due to switch 200 in transmit interval and bandwidth of the corresponding transmit signal path in switch 200 in transmit intervals and noise-figure of receiver connected to port 203. With inductor 220 so chosen, capacitance of Cant (capacitor 240) is determined such that the combination of Cant and Lrx transform LNA input impedance to 50-Ohm when viewed from antenna port 201. The capacitance of Cterm is determined such that the resonant frequency due to Cant, Cterm and Lrx is centered at the center of the transmit frequency band.
Thus, the component values of Lrx and Cant are chosen based on the above-noted impedance transformation of LNA's input impedance as seen from port-3 of circulator 130 or, more generally, as seen from antenna 120 as noted above. The capacitance of capacitor Cterm is chosen to ensure that the passive network in the transmit-path from port 401 to 402 (which includes capacitor 440, inductor 450, the parasitic capacitor(s), and Cterm itself) resonates at the desired frequency in transmit intervals of operation.
It is noted here that, typically, rather than use one switching element 250, a series connection of multiple ones of such switching elements (all identical to each other or different in terms of ON-resistance and OFF-capacitance) forming a switching-element stack may be used in place of switching element 250 in switch 200 to allow switch 200 to withstand higher breakdown voltages and therefore to operate with higher signal voltages, as is well known.
An example switching-element ‘stack’ (switch stack) is shown in
Prior switch 200, whether used with one switching element (e.g., 250) or multiple series-connected switching elements (such as stack 300 of
In receive intervals, the off-capacitance (Coff) degrades the isolation between termination port 202 and receive port 203. That is, Coff provides a path for noise in the termination resistor (that would be connected between node 202 and ground) to couple into the receiver (that would be connected to node 203). As a result, the noise figure of LNA/receiver is degraded. The drawbacks may become worse when the size of each series-connected switching elements is increased to reduce insertion loss, since larger size would cause the parasitic capacitance across each switching element to become larger.
Some possible techniques to improve isolation between ports 202 and 203 by reducing the effective parasitic OFF capacitance across the switching element(s) can be achieved by reducing the width of each MOSFET and/or increasing the number of series-connected switching elements without increasing the unit-size of each MOSFET. However, both these approaches will result in a higher insertion-loss, which is undesirable.
A resonant switch implemented according to aspects of the present disclosure overcomes one or more of the drawbacks noted above, and is described next.
Inductor 450 and capacitors 440 and 480 correspond respectively to inductor 220 and capacitors 210 and 240 of
With combined reference to
Switch stack 490 is shown containing four switching elements 490-1, 490-2, 490-3 and 490-4. Each switching element may be similar to the switching elements shown in
Although switch stack 490 is shown as having only four switching elements, stack 490 can also have more or fewer switching elements. The specific number of switching elements would typically be determined by consideration such as signal levels, breakdown voltages, etc., as is well known in the relevant arts. Further, the multiple switching elements may all be identical to each other or different in terms of characteristics such as, for example, transistor-size, ON-resistance, parasitic OFF-capacitance, etc. The control signal (e.g., gate drive signal for the MOSFETs in a stack) may be provided by an external component/block such as RX 160 (
In transmit intervals, switching element 460 as well as all the elements of stack 490 are closed (i.e., the corresponding transistors are ON), and the reflected signal from the antenna (such as in
Similar to as with prior switch 200, in transmit intervals, capacitors 440 and 480 along with inductor 460 form a parallel resonant LC (inductor-capacitor) network. In receive intervals, capacitor 440 and inductor 450 transform the input impedance of the LNA (which would be connected to receive port 403) to 50 ohms (for impedance-matching purposes) as seen from antenna port 401. The magnitudes of capacitances of capacitors 440 and 480, and inductor 450 may be determined in a manner similar to that described above with respect to capacitors 240 and 210 and inductor 220 of
According to an aspect of the present disclosure, capacitor 480 rather than being connected between the termination port 402 and ground, is instead connected between a node at a junction of a pair of switching elements in switch stack 490 and ground. In an embodiment, capacitor 480 is connected between the mid-point of the stack and ground. Assuming switch stack 490 has only four switching elements as shown in
Various advantages offered by switch 400 and further improvements are briefly noted next.
When used in place of switch 140 in transceiver 100 (
Denoting ‘Ceff-prior’ as the effective OFF-state capacitance between nodes 402 and 452 (
Equation 1 assumes that there are an equal number of switching elements on either side of capacitor 480. The reduction in the effective capacitance is due to the T-attenuator network formed by the parasitic OFF-capacitances 420 and capacitor 480. It may be observed that Ceff is lower than Ceff-prior by the factor [1+(0.25*(C480/Ceff-prior))]. Larger the ratio C480/Ceff-prior, lower is the effective capacitance Ceff. The smaller effective capacitance increases the isolation between ports 402 and 403, thereby resulting in a reduction in the noise-power that would couple from the termination resistor into LNA 161 and RX 160.
In transmit intervals, had capacitor 480 been connected between node 402 and ground (as in prior switch of
Based for example on objectives of the design and/or the operating environment, the optimal connection-point of capacitor 480 to stack 490 can be a switching-element-junction located either to the right of or to the left of the mid-point-junction. For example, if reducing insertion-loss and limiting the reflected current's flow through switch stack 490 is desired, capacitor 480 may be connection to the junction of the rightmost pair of switches.
For a given number of switching elements in stack 490, the effective capacitance ‘Ceff’ between nodes 402 and 452 depends on the junction to which capacitor 480 is connected, and is specified by the following Equation:
It may be observed from Equation 2 that when/if C480 equals 0, then Ceff equals Cp/N. Also, when ‘n’ is 0 or ‘N’, then too Ceff equals Cp/N. The effective capacitance Ceff is a minimum for n=N/2. Ceff also scales (changes proportionally) with the manufacturing technology or process, usually termed ‘process node’ (e.g., 25 nanometer process), usually specified in terms of the dimensions of the smallest feature (e.g., the channel width of a transistor) that can be fabricated using that manufacturing technology/process. The finer (smaller features) the process node, smaller would be the magnitude of ‘Ceff’. Hence, for the same value of C480, the isolation between nodes 402 and 452 would increase as the process node becomes finer.
To obtain further improvements, since techniques described above enable reduction of the current in the section of the switch stack to the left (as viewed in
Further, the improvements noted above can be combined with traditional approaches to increasing isolation such as by reducing the size of each switching element and/or by increasing the stack height, i.e., the number of switching elements in series in the stack. It may be appreciated that when capacitor 480 is connected to a switching-element-junction in stack 490 as described above, the factor by which the size/dimension of each switching element needs to reduced or the stack height needs to be increased to improve isolation would be lower than if capacitor 480 were to be connected to port 402. More generally, capacitor 480 can be connected at any available node, i.e., at either end nodes of stack 490 or at any junction of a pair of switching elements of stack 490. If capacitor 480 were connected at the leftmost node (i.e., at TERM port 402 in
References throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
While in the illustrations of
While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202341030083 | Apr 2023 | IN | national |
