Methods for calibration of radio-frequency path loss in radio-frequency test equipment

Calibration equipment for calibrating multiple test stations in a test system is provided. Each test station may include a test unit, a test fixture, and a radio-frequency (RF) cable that connects the test unit to the test fixture. A control test setup may be used to calibrate uplink and downlink characteristics associated with each test station (e.g., to determine path loss associated with the RF cable and test fixture and variations associated with the test unit). The control test setup may calibrate each test station at desired frequencies to generate a test station error (offset) table. The test unit of each test station may be individually configured based on the test station error table so that offset is minimized among the different stations and so that the test stations may reliably measure hundreds or thousands of wireless electronic devices during product testing.

BACKGROUND

This relates generally to testing wireless electronic devices, and more particularly, to calibrating test equipment that is used to test wireless electronic devices.

Wireless electronic devices typically include transceiver circuitry, antenna circuitry, and other radio-frequency circuitry that provides wireless communications capabilities. During testing, wireless electronic devices under test (DUTs) can each exhibit different performance levels. For example, each wireless DUT can exhibit its own output power level, gain, frequency response, efficiency, linearity, dynamic range, etc.

The performance of a wireless DUT can be measured using a test instrument. A wireless DUT is typically connected to a test instrument using a radio-frequency cable. There is a path loss associated with a given radio-frequency cable. This path loss is defined as the power attenuation of an electromagnetic signal as it propagates through the radio-frequency cable.

Multiple test stations may be used to test multiple wireless DUTs in parallel, where each test station includes its own test instrument, radio-frequency cable, and test fixture connected in series. A DUT is connected to the test fixture of each test station during test operations. The radio-frequency path of each test station has its own unique path loss characteristic.

In an effort to take into account the radio-frequency cable path loss, a vector network analyzer (VNA) can be connected to each cable to determine its path loss. However, calibrating path loss using this approach requires a VNA and ignores path loss associated with the test fixtures and potential variations that may exist among the different test instruments in the different test stations. Failing to account for test fixture path loss and variations in the behavior of the different test instruments may result in inconsistent measurement data. For example, performance parameters measured using one test station may be offset with respect to the performance parameters measured using another test station.

It would therefore be desirable to be able to provide improved calibration techniques for calibrating wireless test equipment.

SUMMARY

Wireless electronic devices may be tested in a test system. Wireless electronic devices that are tested may be referred to as devices under test (DUTs).

A test system may include multiple test stations. Each test station may include a test unit, a test fixture, and a radio-frequency (RF) cable that connects the test fixture to the test unit. Because the path loss associated with the RF cable and test fixture and the variations associated with the test unit may vary from station to station, each test station may be calibrated using a control test setup (control test setup apparatus). Calibrating each test station using the control test setup aims to minimize the amount of offset (error) that may exist from one station to the next. Minimizing offset in this way may allow each test station in the test system to measure consistent and comparable data from numerous DUTs during product testing.

Uplink and downlink calibration processes may be performed using the control test setup. During uplink calibration, the control test setup may include a calibration plate connected to a signal generator. A power meter may be used to calibrate the calibration plate and the signal generator to determine a known output power level at an output of the calibration plate.

The output of the calibration plate may be connected to the test fixture of a given test station. The signal generator may output RF signals at desired frequencies, and the test unit may measure the output power levels of the RF signals. Uplink test station error values may be calculated based on the measured output power levels and the known output power level at each frequency. The uplink characteristics of multiple test stations may be calibrated using this approach.

During downlink calibration, the control test setup may include a calibration plate connected to a power meter. A signal generator may be used to calibrate the calibration plate and the power meter to determine path loss that is associated with the control test setup.

The calibration plate may be connected to the test fixture of a given test station. The test unit of the given test station may output RF signals at a requested output power level at desired frequencies, and the power meter may measure the output power of the RF signals received by the control test setup. Downlink test station error values may be calculated based on the requested output power level, the measured output power level, and the path loss of the control test setup at each frequency. The downlink characteristics of multiple test stations may be calibrated using this approach.

Uplink and downlink test station error values may be tabulated in a test station error table. The test station error table may include test station error (offset) values for each test station at the desired frequencies for both uplink and downlink operations.

Each test station may be connected to computing equipment. The computing equipment may include a database that is loaded with the test station error table. The computing equipment may automatically configure each test station with respective offset values so that the test stations in the test system exhibit minimal offset from one another. If desired, the offset values may be manually supplied by a user during product testing.

DETAILED DESCRIPTION

Wireless electronic devices include antenna and transceiver circuitry that supports wireless communications. Examples of wireless electronic devices include desktop computers, computer monitors, computer monitors containing embedded computers, wireless computer cards, wireless adapters, televisions, set-top boxes, gaming consoles, routers, or other electronic equipment. Examples of portable wireless electronic devices include laptop computers, tablet computers, handheld computers, cellular telephones, media players, and small devices such as wrist-watch devices, pendant devices, headphone and earpiece devices, and other miniature devices.

Wireless electronic devices may communicate with wireless equipment such as cellular telephone base stations, local area network equipment (e.g., IEEE 802.11 equipment), and other wireless network equipment. In uplink test station environments, the wireless electronic devices are sometimes referred to as devices under test (DUTs).

FIG. 1shows an illustrative device under test (DUT). As shown inFIG. 1, DUT10may include transceiver circuitry such as transceiver circuitry14mounted on a substrate such as substrate12. Substrate12may, for example, be a printed circuit board (PCB). An antenna such as antenna16may be formed on substrate12.

Antenna16may be fabricated by patterning a metal layer on a circuit board substrate or may be formed from a sheet of thin metal using a foil stamping process. Antennas such as planar inverted-F antennas (PIFAs) and antennas based on L-shaped resonating elements can be fabricated in this way. Antenna16may also be a PIFA antenna, an L-shaped resonating antenna, or other suitable type of antenna. If desired, more than one antenna16may be formed on substrate12.

A conductive path such as path18may be formed on substrate12to connect transceiver circuitry14to antenna resonating element16. Path18may sometimes be referred to as a transmission line. Transceiver circuitry14may send signals through path18(in the direction of arrow17) to antenna16for wireless transmission. Antenna16may receive wireless signals and send the received signals through trace18(in the direction of arrow19) to transceiver circuitry14. DUT10may be operated to simultaneously transmit and receive wireless signals, if desired.

DUT10may include a radio-frequency (RF) connector such as RF DUT connector20mounted on top of transmission line18(see, e.g.,FIG. 1). During normal operation, connector20is not connected to another mating connector. When connector20is in its unmated state, trace18will remain shorted (i.e., trace18will connect transceiver circuitry14to antenna16).

During test operations, connector20may mate with a corresponding mating connector in a test fixture. When connector20is in its mated state, trace18will be open-circuited (i.e., trace18no longer connects transceiver circuitry14to antenna16). Instead, transceiver circuitry14may send signals through a portion of trace18and through the mating connector to an external source and may receive signals that are conveyed through the mating connector from the external source. Generally, during test operations, antenna16is non-operational and does not receive nor transmit wireless signals.

During product testing, many wireless devices (e.g., hundreds, thousands or more of DUTs10) may be tested in a test system such as test system21, as shown inFIG. 2. Test system21may include multiple test stations such as test stations48. Each test station48may include a test fixture22, a radio-frequency (RF) cable30, and a test unit34.

Test fixture22may have features for engaging DUTs. For example, test fixture22may have a region such as recessed portion (slot)24in which a DUT10may be placed during testing. Text fixture22may have a connector26that is located in region24and a connector28that is attached outside region24(see, e.g.,FIG. 2). During testing, a DUT10may be inserted into region24so that connector20of DUT10mates with corresponding connector26of test fixture22.

Test unit (tester)34in each test station28may be a radio communications tester of the type that is sometimes referred to as a test box or a universal radio communications tester. Test unit34may, for example, be the CMU200 Universal Radio Communication Tester available from Rohde & Schwarz. Testers of this type may perform radio-frequency signaling tests for a variety of different radio-frequency communications bands and channels. If desired, any type of radio-frequency tester may be used in each test station48.

Test unit34may be operated directly or via computer control. When operated directly, a user may control a test unit34by supplying commands directly to the test unit using the user input interface of the test unit. For example, a user may press buttons in a control panel38on the test unit while viewing information that is displayed on a display36in the test unit. In computer controlled configurations, computing equipment such as computer50(e.g., software running autonomously or semi-autonomously on the computer) may communicate with the test unit (e.g., by sending and receiving data over a wired path52or a wireless path between the computer and the test unit).

Test unit34may have a test unit connector such as test unit connector40through which test signals may be conveyed, as shown inFIG. 2. Test unit34may be connected to test fixture22through radio-frequency (RF) cable30. RF cable30may include first and second ports that have first and second connectors30-1and30-2, respectively. First connector30-1of RF cable30may be connected to connector28of test fixture22(as shown by dotted line32) whereas second connector30-2of RF cable30may be connected to connector40of test unit34(as shown by dotted line42).

Test station48with test unit34connected to test fixture22using RF cable30in this way may be used to test a wireless electronic device. For example, a DUT10may be attached to test fixture22during testing so that DUT connector20is connected to connector26of test fixture22. When connected using this arrangement, wireless signals may be conveyed between transceiver circuitry14of DUT10and test unit34through RF cable30.

Test unit34may be used to characterize the uplink and downlink behavior of DUT10. During uplink characterization, DUT10may send wireless signals through RF cable30in the direction of arrow44, and test unit34may be used to measure an output power level, frequency response, gain, linearity, efficiency, and other performance parameters of DUT10at desired frequencies.

During downlink characterization, test unit34may be used to send wireless signals through RF cable30in the direction of arrow46. Measurements such as a received power level may then be taken at the DUT10(as an example). Test stations48may therefore be referred to as bidirectional, because test stations48can be used to measure both uplink and downlink characteristics.

Each test station48should be calibrated prior to testing to ensure that measurements taken across the different test stations are consistent with one another. Sources of error (offset) that may exist from one test station to another include path loss associated with each RF cable30and test fixture22and variations in each test unit34(e.g., process, voltage, and temperature (PVT) variations that may affect the operation of each tester).

Path loss can be defined as the attenuation in power density as wireless signals propagate through a particular medium. The path loss associated with cable30and test fixture22in each test station48may be unique, because it is challenging to manufacture test equipment (e.g., RF cables30, test fixtures22, etc.) that are exactly identical to one another. As a result, it may be necessary to take into account of what path loss is associated with each cable30and fixture22when calibrating test stations48.

Similarly, manufacturing identical test units34may be equally if not even more challenging. In addition to process variations (e.g., the testers include silicon parts that may suffer from random device parameter variations created during non-ideal semiconductor fabrication steps), operational variations such as voltage and temperature variations may also cause each test unit34to behave differently during actual testing conditions.

Calibration techniques that take into account the path loss of RF cables30and test fixtures22and the PVT variations in test units34and that aim to minimize offset that exist among the different test stations may therefore be desirable.

To calibrate the different test stations48in test system21, a control test setup apparatus (calibration equipment) may be used to calibrate each test station48and to determine the amount of offset (test station error) that is associated with each test station48.

For example, consider a scenario in which a control test setup is used to characterize test station errors during uplink operations. The control test setup may, as an example, output a precise known output power level of −30 dBm. Power in terms of dBm expresses power relative to 1 mW in units of decibels. The control test setup may be connected to a first test fixture22of a first test station48. A first test unit34of first test station48may be used to measure the output power level of the wireless signals that are received from the control test setup over RF cable30. First test unit34may measure a received power level of −32 dBm (as an example). The test station error of first test station48may therefore have a value of 2 dB (−30 minus −32).

To calibrate another test station, the control test setup may be connected to a second test fixture22of a second test station48. A second test unit34of second test station48may be used to measure the output power level of the wireless signals that are received from the control test setup over RF cable30. Second test unit34may measure a received power level of −33 dBm (as an example). The test station error of second test station48may therefore have a value of 3 dB (−30 minus −33).

The uplink test station error values of each test station48may be measured across different operating frequencies using this approach. The downlink test station error values may also be measured using the control test setup. Consider another scenario in which the control test setup includes a power meter that can be used to measure the power of incoming wireless signals during downlink operations. There may be a known fixed 0.4 dB loss associated with the control test setup (as an example).

The control test setup may be connected to a first test fixture22of a first test station48. A first test unit34of first test station48may be set to transmit wireless signals at requested power level of −30 dBm over RF cable30(as an example). The power meter of the control test setup may, for example, measure a received power level of −31.5 dBm. Taking into account the fixed loss associated with the control test setup, the test station error of first test station48may therefore have a value of 1.9 dB (−30 minus −31.5 plus 0.4).

To calibrate another test station, the control test setup may be connected to a second test fixture22of a second test station48. A second test unit34of second test station48may be set to transmit wireless signals at requested power level of −30 dBm over RF cable30(as an example). The power meter of the control test setup may, for example, measure a received power level of −32.7 dBm. Taking into account the fixed loss associated with the control test setup, the test station error of first test station48may therefore have a value of 3.1 dB (−30 minus −32.7 plus 0.4). Downlink test station error values may be measured at different operating frequencies, if desired.

Measuring uplink and downlink test station error values of each test station48at various frequencies in this way using the control test setup may generate test station error (offset) values that take into account the path loss associated with RF cables30and test fixtures22(and for connections formed with these components) and the variations associated with each test unit34during actual testing conditions.

The calculated test station error values may be used to individually calibrate each test station48so that there is minimal offset (e.g., difference in path loss and test unit variations) among the different test stations.

The test station error values may be tabulated in a test station error (offset) table. The test station error table may have uplink and downlink test station error values corresponding to different operating frequencies for each test station48. For example, the test station error table may display an uplink error value of 1 dB and a downlink error value of 1.5 dB for a given test station operating at 800 MHz whereas the test station error table may display an uplink error value of 1.8 dB and a downlink error value of 2.5 dB for the given test station operating at 1800 MHz.

As shown inFIG. 2, each test station48may be connected to computing equipment50through line52. Computing equipment50may have storage equipment such as database51. The test station error table may be stored on database51. Computing equipment50may automatically configure test unit34of each test station48based on the test station error table (e.g., computing equipment50may adjust each test unit34with a corresponding offset amount depending on its current operating frequency) so that DUT measurements taken across each test station48in test system21are consistent and comparable. If desired, the error values may be manually input by a user based on the test station error table by controlling an offset control knob in control panel38(as an example). In general, any suitable way of configuring test stations48to have minimal offset among one another may be used.

The control test setup may include a calibration plate such as calibration plate54, as shown inFIG. 3. Calibration plate54may have a first RF connector (a first port) such as connector54-1and a second RF connector (a second port) such as connector54-2. RF ports54-1and54-2may be shorted through conductive paths within calibration plate54.

Test equipment that may be connected to calibration plate54during calibration steps may include an RF power meter such as power meter56and an RF reference signal generator such as reference signal generator62, as shown respectively inFIGS. 4 and 5. Power meter56may be used to measure the power level of radio-frequency signals received through test probe60and radio-frequency cable58(e.g., RF cable58connects power meter main unit56to test probe60). Test probe60may have an RF connector62. Connector62of test probe60may mate with connector54-1or54-2of calibration plate54during calibration processes.

Power meter56may, for example, be the NRP Power Meter available from Rohde & Schwarz. Power meters of this type may perform radio-frequency power measurements for signals across different radio-frequency communications bands and channels. For example, this type of tester may be used in performing peak and peak-to-average power measurements for signals from steady state (i.e., direct current (DC) conditions) to 40 GHz. If desired, any type of radio-frequency power meter may be used.

RF reference signal generator62may be used to generate radio-frequency signals through cable64and connector66(e.g., RF cable64connects signal generator main unit62to connector66). Connector66of signal generator62may mate with connector54-1or54-2of calibration plate54during calibration processes.

Reference signal generator62may, for example, be the SMU200A available from Rohde & Schwarz. Signal generators of this type may generate multiple radio-frequency signals across different radio-frequency communications bands and channels. For example, this type of tester may be used to generate radio-frequency waveforms with frequencies up to 6 GHz. If desired, any type of radio-frequency signal generator may be used.

FIG. 6shows steps involved in calibrating the uplink characteristics of test system21using the control test setup. At step68, reference signal generator62may be connected to first connector54-1of calibration plate54(i.e., connector66of signal generator62is connected to connector54-1of calibration plate54).

At step70, calibration plate54and signal generator62may be calibrated using power meter56. Before connecting power meter56to another test equipment (e.g., when nothing is connected to power meter test probe60), power meter56may be zeroed out (step72). After zeroing power meter56, power meter56may then be connected to second connector54-2of calibration plate54(i.e., connector62of power meter56is connected to connector54-2of calibration plate54), as shown in step74.

Signal generator62may then be tuned to generate reference radio-frequency (RF) signals at desired frequencies (step76). At each frequency, signal generator62may be allowed to stabilize at a requested output power level. Thereafter, power meter56may be used to measure a reference power level PREF(f) as a function of frequency. Reference power level PREF(f) represents the power level of wireless signals (e.g., reference RF signals generated by signal generator62) output at port54-2of calibration plate54.

At step78, power meter56may be disconnected from calibration plate54(i.e., connector62of power meter56is disconnected from connector54-2of calibration plate54).

At step80, calibration plate54may be connected to test fixture22of a given test station48(i.e., connector54-2of calibration plate is connected to connector26of test fixture22). Test unit34of given test station48may be powered on and initialized (step82).

At step84, signal generator62may be tuned to generate RF signals at the desired frequencies. At each frequency, signal generator62will send RF signals through RF cable30to test unit34. Test unit34may be used to measure a test station power level PTS(f) as a function of frequency. Test station power level PTS(f) represents the power level of the RF signals that are received at test unit34.

At step86, the test station error may be calculated for each frequency. In particular, the test station error may be calculated by subtracting test station power level PTS(f) from reference power level PREF(f) at each frequency (e.g., calculate |PREF(f)−PTS(f)| at each frequency).

For example, given test station48may be calibrated at 1900 MHz. Reference power level PREF(1900 MHz) and test station power level PTS(1900 MHz) may be −31 dBm and −33 dBm, respectively. The test station error of given test station48at 1900 MHz is therefore 2 dB (|−31−(−33)|). Test station error values may be calculated in this way for any desired operating frequencies. Test station error data collected using this approach may be stored in database51of computing equipment50(e.g., test station error values may be stored in the test station error table that is stored in database51).

Uplink calibration processes may loop back to step80if additional test stations other than the given test station still need to be calibrated, as indicated by path88. Once all test stations48in test system21have been calibrated, test unit34of each test station48may be individually configured based on the test station error table so that uplink offset/error among the different test stations48is minimized.

FIG. 7shows steps involved in calibrating the downlink characteristics of test system21using the control test setup. At step90, power meter56may be zeroed out and connected to first connector54-1of calibration plate (i.e., connector62of power meter56may be connected to first connector54-1of calibration plate54). In downlink calibration processes, power meter56may remain connected to calibration plate54and may serve as a control measurement unit against which power measurements may be compared.

At step92, calibration plate54and power meter56may be calibrated using signal generator62to determine control test setup path loss power levels. Signal generator62may first be initialized (step94). Signal generator62may be connected to second connector54-2of calibration plate54(i.e., connector66of signal generator62is connected to second connector54-2of calibration plate54).

Signal generator62may be tuned to output signal generator RF signals at a signal generator power level PGEN(f) at desired frequencies (step96). At each frequency, signal generator62may be allowed to stabilize at the signal generator power level, and signal generator62may send the signal generator RF signals through calibration plate54to power meter56. Power unit56may be used to measure a reference (output) power level PREF(f) as a function of frequency.

At step98, the control test setup path loss associated with calibration plate54and power meter cable58may be calculated by subtracting reference power PREF(f) from requested output power level PGEN(f) (e.g., by calculating PCAL(f) as |PGEN(f)−PREF(f)| at each frequency). Control test setup path loss power level PCAL(f) may represent the amount of power attenuation experienced by RF signals that travel through calibration plate54and power meter cable64.

At step100, signal generator62may be disconnected from calibration plate54(i.e., connector66of signal generator62may be disconnected from second connector54-2of calibration plate54).

At step102, calibration plate54may be connected to test fixture22of a given test station48(i.e., connector54-2of calibration plate54may be connected to connector26of test fixture22). Test unit34of given test station48may be powered on and initialized (step104).

At step106, test unit34may be tuned to output signals at a requested (output) power level PREQ(f) at the desired frequencies. At each frequency, test unit34is allowed to stabilize at requested power level PREQ(f) and a power meter power level PPM(f) measured using power meter56may be recorded. Requested power level PREQ(f) may represent the requested power of the RF signals that is generated by the tester of given test station48whereas power meter power level PPM(f) represents the power level of RF signals that is received at power meter56through RF cable30, calibration plate54, and power meter cable58.

At step108, the downlink test station error may be calculated for each frequency. In particular, the downlink test station error may be calculated at each frequency by subtracting power meter power level PPM(f) from requested output power level PREQ(f) and then adding power difference PCAL(f) (e.g., the value |PREQ(f)−PPM(f)+PCAL(f)| may be calculated at each frequency).

Subtracting power meter power level PPM(f) from requested output power level PREQ(f) calculates the path loss and discrepancy between the desired (requested) output power of RF signals and the actual power of RF signals measured using the control test setup (e.g., calibration plate54and power meter56in the downlink setup). Adding PCAL(f) removes the path loss contribution of the control test setup, because proper calibration should only take into account path loss and variations that are associated with test unit34and RF cable30but not the path loss associated with the control test setup used for calibration purposes.

For example, given test station48may be calibrated at 2.4 GHz. Power level PREQ(2.4 GHz), PPM(2.4 GHz), and PCAL(2.4 GHz) may be −29 dBm, −31.5 dBm, and 0.8 dB, respectively. The test station error of given test station48at 2.4 GHz is therefore 3.3 dB (|−29−(−31.5)+0.8|). Test station error values may be calculated in this way for the desired frequencies. Test station error data collected using this approach may be stored in database51of computing equipment50(e.g., test station error values may be stored in the test station error table that is stored in database51).

Downlink calibration processes may loop back to step102if additional test stations other than the given test station still need to be calibrated, as indicated by path110. Once all test stations48in test system21have been calibrated, test unit34of each test station48may then be individually configured based on the test station error table so that downlink offset/error among the different test stations48is minimized. At this point, test stations48may be used to test hundreds of thousands of DUTs10and reliable (consistent and comparable) data may be collected across the different test stations for product testing purposes.