Patent Application
20230393182
This disclosure relates to cryogenic electronic device testing.
Cryogenic electronics refers to an operation of electronic devices, circuits, and systems at cryogenic temperatures. Cryogenic electronics (also referred to as low-temperature electronics, or cold electronics) can be based on semiconductive devices, superconductive devices, or a combination of the two.
In an example, a system can include a channel select circuit that can be configured in a first state to electrically isolate a channel output circuit from a cryogenic device under test (DUT) and in a second state to electrically couple the channel output circuit to the cryogenic DUT, at least one resistor positioned along a transmission path that couples a pattern generator circuit to the channel output circuit that includes the channel select circuit and a controller. The controller can be configured to cause respective test current signals to be provided by the pattern generator circuit along the transmission path when the channel select circuit is in respective first and second states to establish respective first and second voltages across the at least one resistor, determine first and second impedance characteristics of the transmission path based on the established respective first and second voltages and an amount of current being provided by respective test current signals, and determine an impedance of the cryogenic DUT based on the first and second determined impedance characteristics of the transmission path.
In another example, a system can include a measuring circuit that can be configured to measure a first voltage established across a resistor along a transmission path based on a first current signal. The first current test signal can be provided along the transmission path when a channel output circuit is isolated from a cryogenic DUT. The transmission path can couple a pattern generator circuit for generating current signals to a channel output circuit. The measuring circuit can be configured to measure a second voltage established across the resistor based on a second current signal. The second current test signal can be provided along the transmission path when the channel output circuit is electrically coupled to cryogenic DUT. The measuring circuit can be configured to determine first and second average voltages based on the measurements of the first and second voltages, respectively. The system includes a controller that can be configured to determine first and second impedance characteristics of the transmission path based on respective first and second average voltages and an amount of current being provided by respective first and second current test signals, and determine an impedance of the cryogenic DUT based on the first and second determined impedance characteristics of the transmission path.
In a further example, a method can include causing a channel output circuit to be electrically isolated from a cryogenic DUT, controlling a test pattern generator circuit to provide a first current test signal along a transmission path so that the first current test signal flows through at least one resistor located in the transmission path to establish a first voltage across the at least one resistor, receiving a first determined average voltage based on measurements of the first voltage across the at least one resistor, determining first impedance characteristics of the transmission path based on the determined first average voltage and an amount of current being provided by the first current test signal, causing the channel output circuit to be electrically coupled to the cryogenic DUT such that the cryogenic DUT is part of the transmission path in response to determining the first impedance characteristics of the transmission path, controlling the test pattern generator circuit to provide a second current test signal along the transmission path so that the second current test signal flows through the cryogenic DUT and the at least one resistor to establish a first voltage across the at least one resistor, receiving a second determined average voltage based on measurements of the second voltage across the at least one resistor, determining second impedance characteristics of the transmission path based on the second average voltage and an amount of current being provided by the second current test signal, and determining impedance characteristics of the cryogenic DUT based on the first and second impedance characteristic of the transmission path.
For testing of cryogenic electronics, a cryogenic DUT is connected via transmission paths (e.g., wires, conductive traces, etc.) to a test pattern generator. To test or verify a performance or characteristics of the cryogenic DUT, test patterns are generated by the test pattern generator and provided via the transmission paths to the cryogenic DUT. Nonidealities and transmission mismatching, and noise can limit a performance of a cryogenic DUT testing system, such that it becomes difficult to accurately measure impedance characteristics of the cryogenic DUT.
Examples are described for measuring impedance characteristics of a cryogenic DUT. The term “impedance” as used herein refers to a phase shift or difference in a voltage or current operating angle. By way of example, the cryogenic DUT can be a Josephson Junction (JJ) device, however, in other examples, the cryogenic DUT can be another type of cryogenic device. In the examples herein, a cryogenic DUT testing system is configured to determine the impedance characteristics of the DUT by reducing or eliminating sources of DUT impedance error such as resulting from non-idealities in transmission paths and impedance mismatches between the transmission paths.
For example, to determine the impedance characteristics of the cryogenic DUT, a controller of the cryogenic DUT testing system can be configured to control a current test pattern generator circuit of the system to output a current test signal to a channel output circuit of the system. The controller can be configured to cause the channel output circuit to be electrically isolated from the cryogenic DUT. By electrically isolating the channel output circuit from the cryogenic DUT impedance characteristics of transmission paths of the system can be estimated or determined according to the examples herein. The transmission paths can be used to electrically couple the current test pattern generator circuit to the channel output circuit (or circuitry therein). The channel output circuit can include a resistor that can be positioned or located along or within a respective transmission path over which current test signals can be provided to the cryogenic DUT (e.g., when the channel output circuit is electrically coupled to the cryogenic DUT). In some instances, the resistor can be positioned along or within the respective transmission path between the current test pattern generator circuit and the channel output circuit.
The cryogenic DUT testing system includes a voltage sensing circuit and a voltage calculator circuit. The voltage sensing circuit can sense (e.g., measure) a voltage across the resistor to provide a sensed voltage signal. The voltage calculator circuit can be configured to sample the sensed voltage signal based on a sampling rate, which can be set by the controller. The voltage calculator circuit can be configured to determine an average voltage based on the samples of the sensed voltage signal. The voltage calculator circuit can be configured to output an average voltage signal indicative of the average voltage of the sensed voltage signal.
The controller can be configured to determine transmission path characteristics (e.g., an impedance) for the transmission paths based on the average voltage signal and an amount of current being outputted by the current test pattern generator circuit. Because a resistance of the resistor and a voltage across the resistor is known, and the amount of current being outputted by the current test pattern generator circuit is also known, the controller can be configured to determine first impedance characteristics of the transmission paths (e.g., while the cryogenic DUT is electrically isolated from the channel output circuit and thus the transmission paths). The first impedance characteristics of the transmission paths can be representative of alternating current (AC) or direct current (DC) impedance characteristics of the transmission paths based on whether the current test pattern generator circuit outputs an AC or DC current test signal.
In some examples, the controller can be configured to cause the channel output circuit to be in electrical connection with the cryogenic DUT. The impedance characteristics of transmission paths with the DUT in electrical connection (e.g., not electrically isolated from the channel output circuit and thus the transmission paths) can be calculated in a same or similar manner as described herein for calculating the first impedance characteristics of the transmission paths. Thus, the controller can be configured to determine second impedance characteristics of the transmission paths during which the cryogenic DUT is electrically connected to the channel output circuit. The controller can be configured to determine an impedance difference based on the first and second impedance characteristics of the transmission paths. The impedance difference can be representative of impedance characteristics (e.g., an impedance, such as an AC or DC impedance) of the cryogenic DUT.
Accordingly, the cryogenic DUT testing system described herein can accurately measure impedance characteristics of the cryogenic DUT (e.g., such as AC and/or DC impedance characteristics) by compensating for transmission path nonidealities and impedance mismatches between the transmission paths.
The current test pattern generator circuit 104 can generate one or more different test patterns. For example, the current test pattern generator circuit 104 can generate a differential current test signal 116 that can correspond to a respective test pattern or be part of the respective test pattern. Thus, the current test pattern generator circuit 104 in some instances can be implemented as a differential current source. In additional or alternative examples, the current test pattern generator circuit 104 can be implemented as a bipolar current source such that current can be sinked or sourced with respect to the cryogenic DUT 112. The current test pattern generator circuit 104 has a first output 118 and a second output 120. In some instances, the first output 118 provides a current test signal 122 and the second output 120 receives the current test signal 122. Thus, the first output 118 can output the current test signal 122 and the second output 120 can pull the current test signal 122. Because the system 100 is for testing cryogenic DUTs, the current test signal 122 can be referred to as a high-precision test current. A “high-precision” test current is a current signal in a relatively low current range, such as a micro-amperes or nano-amperes range. In some instances, the current test signal 122 is an AC current test signal and in other instances, the current test signal 122 is a DC current test signal. The AC and DC current test signals can be used to determine AC and DC impedance characteristics of transmission paths and the cryogenic DUT 112 as described herein.
The system 100 can further include a channel output circuit 124. The channel output circuit 124 is coupled between the current test pattern generator circuit 104 and the cryogenic DUT 112. The system 100 can include any number of channel output circuits similar to the channel output circuit 124, as shown in
In the example of
By way of further example, the processor 106 can execute the impedance test logic 110 to provide a reference signal 128. The reference signal 128 can directly or indirectly indicate an amount of current that is to be outputted by the current test pattern generator circuit 104. For example, the reference signal 128 can indicate a voltage reference and the current test pattern generator circuit can include voltage-to-current (VI) converter that can provide the current test signal 122 based on the reference signal 128. The current test pattern generator circuit 104 can provide the differential current test signal 116, which includes the current test signal 122, based on the reference signal 128. The system 100 includes a current sensing circuit 130 to measure an amount of current being outputted by the current test pattern generator circuit 104. The current sensing circuit 130 can sense (e.g., sample) the current test signal 122 to provide a sensed current test signal 132. In some examples, the current test pattern generator circuit 104 can communicate to the controller 102 the amount of current being outputted as the current test signal 122.
The controller 102 can execute the impedance test logic 110 to provide a channel control signal 134. The channel control signal 134 can control an electrical coupling of the channel output circuit 124 to the cryogenic DUT 112. The channel control signal 134 can be provided to a channel select circuit 136 to electrically isolate the channel output circuit 124 from the cryogenic DUT 112. By electrically isolating the channel output circuit 124 from the cryogenic DUT 112 impedance characteristics of transmission paths can be estimated or determined according to the examples described herein.
In some instances, the channel select circuit 136 is configured in a first state (e.g., DUT isolating state) in which the channel output circuit 124 is electrically isolated from the cryogenic DUT 112. In examples wherein the channel select circuit 136 is in the first state, the controller 102 can provide the channel control signal 134 to switch the channel select circuit 124 to a second state, thereby electrically coupling the channel output circuit 124 to the cryogenic DUT 112. In some instances, the channel select circuit 136 is implemented as a single pole double throw relay (SPDT).
As described herein, by electrically isolating the channel output circuit 124 from the cryogenic DUT 112 impedance characteristics of the transmission paths can be estimated or determined. The term “transmission path” as used herein can refer to a signal path (e.g., a wire, a trace, intermediate circuitry, and/or a combination thereof) along which a test signal or pattern can be provided. In the example of
By way of example, the channel select circuit 136 can include circuitry that can couple the first and second inputs 140 and 146 internally so that the first and second inputs 140 and 146 are shorted therein. In some instances, the first and second inputs 140 and 146 of the channel select circuit 136 are shorted in response to the channel control signal 134. For example, the channel select circuit 136 can include a switch that can be positioned between the first and second inputs 140 and 146. The switch can be actuated and thus closed in response to the channel control signal 134 to short the first and second inputs 140 and 146. When the switch is in an open state, the first and second inputs 140 and 146 can be electrically coupled to respective first and second outputs of the channel select circuit 136 so that the cryogenic DUT 112 can be provided test patterns. By shorting the first and second inputs 140 and 146, a closed loop circuit 148 is formed in which the current test signal 122 flows from the first output 118 of the current test pattern generator circuit 104 through the short formed between the first and second inputs 140 and 146 through the resistor 144 back to the second output 120 of the current test pattern generator circuit 104. A voltage can be established across the resistor 144 based on the current test signal 122.
The measuring circuit 126 includes a voltage sensing circuit 150 that can sense (e.g., measure) the voltage across the resistor 144 to provide a sensed voltage signal 152. The voltage sensing circuit 150 can provide the sensed voltage signal 152 with a sufficient gain that raises the sensed voltage signal 152 above a noise floor of a voltage calculator circuit 154. This is because the sensed voltage signal 152 is in a micro-volt range or nano-volt range as the voltage across the resistor 144 is based on a current that is in a micro-amperes or nano-amperes range. The voltage calculator circuit 154 can be configured to sample the sensed voltage signal 152 based on a sampling rate, which can be set by the controller 102. The voltage calculator circuit 154 can be configured to determine an average voltage based on the samples of the sensed voltage signal 152. The voltage calculator circuit 154 can be configured to output an average voltage signal 156 indicative of an average voltage of the sensed voltage signal 152. In some examples, the voltage calculator circuit 154 can be configured to output an RMS voltage based on the samples of the sensed voltage signal 152. A polarity of the average voltage signal 156 can indicate in which direction a current flows through the resistor 144. For example, if the current test signal 122 is provided from the second output 120 of the current test pattern generator circuit 104 to the resistor 144, the average voltage signal 156 can have a negative polarity.
The controller 102 can be configured to receive the average voltage signal 156. The impedance test logic 110 can cause the controller 102 to determine transmission path characteristics based on the average voltage signal 156 and an amount of current being outputted by the current test pattern generator circuit 104. The amount of current outputted by the current test pattern generator circuit 104 can be determined based on the sensed current test signal 132. For example, the processor 106 can execute the impedance characteristic calculator 114 to determine the transmission path characteristics for the first and second transmission paths 138 and 142.
Because a resistance of the resistor 144 and a voltage across the resistor 144 is known (based on the average voltage signal 156), and the amount of current outputted by the current test pattern generator circuit 104 is also known (based on the sensed current test signal 132), the impedance characteristic calculator 114 can be programmed to determine first impedance characteristics representative of an overall impedance of the first and second transmission paths 138 and 142. In examples wherein the current test signal 122 is the DC current test signal the first determined impedance characteristics can characterize an overall DC impedance of the first and second transmission paths 138 and 142. In examples wherein the current test signal 122 is the AC current test signal the first determined impedance characteristics can characterize an overall AC impedance of the first and second transmission paths 138 and 142.
In some examples, the controller 102 can be configured to cause the channel output circuit 124 to be in electrical connection with the cryogenic DUT 112. For example, the controller 102 can be configured to stop providing the channel control signal 134 to remove the short between the first and second inputs 140 and 146 of the channel select circuit 136. The channel select circuit 136 in some instances can be configured to couple the first and second inputs 140 and 146 to respective outputs 158 and 160 of the channel select circuit 136 in response to not receiving the channel control signal 134. In other examples, the controller 102 can be configured to provide a different or another channel control signal 134 to cause the channel select circuit 136 to couple the first and second inputs 140 and 146 to respective outputs 158 and 160 of the channel select circuit 136 therein. Once the first and second inputs 140 and 146 are coupled to respective outputs 158 and 160 to which corresponding inputs of the cryogenic DUT 112 are coupled, the cryogenic DUT 112 can be electrically coupled to the first and second transmission paths 138 and 142.
Overall impedance characteristics for the first and second transmission paths 138 and 142 with the cryogenic DUT 112 in electrical connection (e.g., not electrically isolated) can be calculated in a same or similar manner as described herein for calculating the first impedance characteristics for the first and second transmission paths 138 and 142. Thus, the controller 102 can be configured to determine second impedance characteristics representative of an overall impedance of the first and second transmission paths 138 and 142 during which the cryogenic DUT 112 is electrically connected to the channel output circuit 124. In some instances, the AC and DC impedance characteristics for the first and second transmission paths 138 and 142 can be determined in a same or similar manner as described herein. Because the cryogenic DUT 112 is electrically coupled to the channel output circuit 124, impedance characteristics of one of the first and second transmission paths 138 and 142 can change as the cryogenic DUT 112 can influence an impedance of at least one of the first and second transmission paths 138 and 142.
The impedance characteristic calculator 114 can be programmed to determine an impedance difference based on the first and second determined impedance characteristics for the first and second transmission paths 138 and 142. The impedance difference can be representative of an impedance of the cryogenic DUT 112. For example, the impedance characteristic calculator 114 can be programmed to subtract a second impedance from the first impedance to compute an impedance difference corresponding to the impedance of the cryogenic DUT 112. In some examples, the impedance characteristic calculator 114 can be programmed to compute a DUT current noise contribution and DUT voltage drop. For example, the DUT voltage drop can be computed based on a difference in the voltage across the resistor 144 when the cryogenic DUT 112 is electrically isolated from the channel output circuit 124 and when the cryogenic DUT 112 is electrically coupled to the channel output circuit 124. In additional or alternative examples, the controller 102 can be configured to determine the DUT current noise contribution by applying a Fast Fourier Transform (FFT) based on the average voltage signal 156. The controller 102 can be configured to compute the DUT current noise contribution of the cryogenic DUT 112 based on a difference in the DUT current noise contribution when the cryogenic DUT 112 is electrically coupled to the channel output circuit 124 and electrically isolated from the channel output circuit 124.
Thus, the controller 102 can be configured to measure DUT electrical attributes, such as current (e.g., based on the resistance of the resistor 144 and the average voltage signal 156), a phase (e.g., based on the current test signal 122 and the average voltage signal 156, and power (e.g., consumed by the cryogenic DUT 112, such as based on the current test signal 122 and the average voltage signal 156). Because these attributes can change depending on a DUTs performance, proper operating points of the DUT can be determined. Moreover, the measured DUT electrical attributes can be used for diagnosing operational issues of the cryogenic DUT 112.
Accordingly, the cryogenic DUT testing system 100 can accurately measure impedance characteristics of the cryogenic DUT 112 (e.g., such as AC and/or DC impedance characteristics) by compensating for transmission path nonidealities and impedance mismatches between the first and second transmission paths 138 and 142.
The system 200 includes a voltage source circuit 208. The voltage source circuit 208 can be configured to provide a differential temperature-compensated voltage 210, which can be used to provide a temperature compensated differential current test signal (e.g., in some instances corresponding to the differential current test signal 116, as shown in
The differential voltage generator 214 can be configured to receive the temperature compensation signal 212 and a reference signal 216 that can provide a reference voltage. In some examples, the reference signal 216 can correspond to the reference signal 128, as shown in
The system 200 can further include a current output circuit 220. The current output circuit 220 can include a VI converter 222. In some examples, the differential voltage generator 214 and the VI converter 222 can form a current test pattern generator circuit 224, such as the current test pattern generator circuit 104, as shown in
In the example of
By way of example, the current test pattern generator circuit 224 has a first output 230 and a second output 232. In some instances, the first output 230 provides a current test signal 234 and the second output 232 receives the current test signal 122. The current test signal 234 can be similar to the current test signal 122, as shown in
In further examples, the current output circuit 220 includes a differential amplifier 236 and an average voltage calculator circuit 238. The differential amplifier 236 can correspond to the voltage sensing circuit 150, as shown in
Continuing with the example of
The main controller 218 can be configured to provide a channel control signal 246. The channel control signal 246 can correspond to the channel control signal 134, as shown in
In some instances, the channel select circuit 248 is configured in a first state (e.g., DUT isolating state) in which the channel output circuit 226 is electrically isolated from the cryogenic DUT. In examples wherein the channel select circuit 248 is in the first state, the channel controller 228 can provide the channel control signal 246 to switch the channel select circuit 248 to a second state, thereby electrically coupling the channel output circuit 124 to the cryogenic DUT. In some instances, the channel select circuit 248 includes a relay 250, such as a SPDT relay.
In the example of
In some instances, the first and second inputs 256 and 262 of the relay 250 are shorted to form a short 263 in response to the channel control signal 246. By shorting the first and second inputs 256 and 262, the current test signal 234 flows from the first output 230 of the current test pattern generator circuit 224 through the first resistor 254 and the short 263 formed between the first and second inputs 256 and 262 through the second resistor 260 to the second output 232 of the current test pattern generator circuit 224. A respective voltage can be established across the first and second resistors 254 and 262 based on the current test signal 234. The differential amplifier 236 can be configured to sense (e.g., measure) the voltage across the first resistor 254 to provide a sensed voltage signal 264. While the example of
The differential amplifier 236 can provide the sensed voltage signal 264 with a sufficient gain that raises the differential amplifier 236 above a noise floor of the average voltage calculator circuit 238. This is because the sensed voltage signal 264 is in a micro-volt range or nano-volt range as the voltage across the first resistor 254 is based on a current that is in a micro-amperes or nano-amperes range. The average voltage calculator circuit 238 can be configured to sample the sensed voltage signal 264 based on a sampling rate, which can be set by the channel controller 228. The average voltage calculator circuit 238 can be configured to determine an average voltage based on the samples of the sensed voltage signal 264. The average voltage calculator circuit 238 can be configured to output an average voltage signal 266 indicative of an average voltage of the sensed voltage signal 264. A polarity of the average voltage signal 266 can indicate in which direction current flows through the first resistor 254. For example, if the current test signal 234 is provided from the second output 232 of the current test pattern generator circuit 224 and thus flows in an opposite direction if provided from the first output 230, the average voltage signal 266 can have a negative polarity.
The channel controller 228 can be configured to provide the average voltage signal 266 to the main controller 218. The main controller 218 can be configured to determine transmission path characteristics based on the average voltage signal 266 and an amount of current outputted by the current test pattern generator circuit 224. The amount of current outputted by the current test pattern generator circuit 224 can be determined based on the sensed current test signal 244. For example, the main controller 218 can execute a characteristic calculator (e.g., the characteristic calculator 114, as shown in
Because a resistance of the first resistor 254 and a voltage across the first resistor 254 is known (based on the average voltage signal 266), and the amount of current outputted by the current test pattern generator circuit 224 is also known (based on the sensed current test signal 244), the main controller 218 can be configured to determine first impedance characteristics representative of an overall impedance of the first and second transmission paths 252 and 258. In examples wherein the current test signal 234 is the DC current test signal the first determined impedance characteristics can characterize an overall DC impedance of the first and second transmission paths 252 and 258. In examples wherein the current test signal 234 is the AC current test signal the first determined impedance characteristics can characterize an overall AC impedance of the first and second transmission paths 252 and 258.
In some examples, the main controller 218 can be configured to cause the channel output circuit 226 to be in electrical connection with the cryogenic DUT 112. For example, the main controller 218 can communicate with the channel controller 228 to cause the channel controller 28 to stop providing the channel control signal 246 to remove the short 233 between the first and second inputs 256 and 262 of the relay 250. Overall impedance characteristics for the first and second transmission paths 252 and 258 with the DUT in electrical connection (e.g., not electrically isolated) can be calculated in a same or similar manner as described herein for calculating the first impedance characteristics for the first and second transmission paths 252 and 258. Thus, the main controller 218 can be configured to determine second impedance characteristics representative of an overall impedance of the first and second transmission paths 252 and 258 during which the cryogenic DUT is electrically connected to the channel output circuit 226. In some instances, the AC and DC impedance characteristics for the first and second transmission paths 252 and 258 can be determined in a same or similar manner as described herein. Because the cryogenic DUT is electrically coupled to the channel output circuit 226, impedance characteristics of one of the first and second transmission paths 252 and 258 can change as the cryogenic DUT can influence an impedance of at least one of the first and second transmission paths 252 and 258.
The main controller 218 can be configured to determine an impedance difference based on the first and second determined impedance characteristics for the first and second transmission paths 252 and 258. The impedance difference can be representative of an impedance of the cryogenic DUT. For example, the main controller 218 can be configured to subtract a second impedance from a first impedance to compute an impedance difference corresponding to the impedance of the cryogenic DUT.
Accordingly, the cryogenic DUT testing system 200 can accurately measure impedance characteristics of the cryogenic DUT (e.g., such as AC and/or DC impedance characteristics) by compensating for transmission path nonidealities and impedance mismatches between the first and second transmission paths 252 and 258.
In view of the foregoing structural and functional features described above, example methods will be better appreciated with reference to
At 406, measuring a first voltage across the at least one resistor based on the first current test signal. At 408, determining a first average voltage based on measurements of the first voltage across the resistor. At 410, determining first impedance characteristics of the transmission path based on the first average voltage and an amount of current being provided by the first current test signal. At 412, electrically coupling the channel output circuit to the cryogenic DUT such that the cryogenic DUT is part of the transmission path. At 414, providing a second current test signal (e.g., the current test signal 122, as shown in
At 510, causing the channel output circuit to be electrically coupled to the cryogenic DUT such that the cryogenic DUT is part of the transmission path. At 512, controlling the test pattern generator circuit to provide a second current test signal along the transmission path so that the second current test signal flows through the cryogenic DUT and the at least one resistor to establish a first voltage across the at least one resistor. At 514, receiving a second determined average voltage based on measurements of the second voltage across the at least one resistor. At 516, determining second impedance characteristics of the transmission path based on the second average voltage and an amount of current being provided by the second current test signal. At 518, determining impedance characteristics of the cryogenic DUT based on the first and second impedance characteristic of the transmission path.
What has been described above are examples. It is, of course, not possible to describe every conceivable combination of components or methodologies, but one of ordinary skill in the art will recognize that many further combinations and permutations are possible. Accordingly, the disclosure is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements.
