BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates in general to production testing of printed circuit boards, and more particularly to production testing of impedance of a radio frequency circuit incorporated on a printed circuit board.

Description of the Related Art

A radio frequency (RF) circuit may be incorporated on a printed circuit board (PCB) or the like of an RF transmitter, receiver or transceiver. The PCB mounts communication circuitry along with an antenna for sending and/or receiving RF signals. In many configurations, an RF front end and digital circuitry portions are incorporated on a separate RF integrated circuit (RFIC), which is mounted on the PCB. A crystal (XTAL) may also be provided and interfaced with the RFIC to form a crystal oscillator for generating a requisite frequency reference for enabling RF communications. Many such RF communications are in the 2.4 Gigahertz (GHz) frequency range, although alternative frequency ranges are contemplated. The RF circuitry may also include tuning circuitry or the like for matching impedances between the RFIC and the antenna. The tuning circuitry is referred to herein as matching circuitry since tuned for matching desired impedance level for RF communications.

Each PCB should be tested during the manufacturing process prior to shipping. It is noted that the RFIC may be separately tested prior to mounting on the RF PCB. Conventional testing of the RF PCB involved radiated testing, in which the RF matching circuitry and the antenna were the primary focus of radiated testing. In a conventional configuration, the RF PCB was mounted to the test equipment and placed into a radiated mode for sending and receiving RF signals with the tester through a transmission medium, such as the open air. The antenna, however, operated as a near-field mode and was very sensitive to the surroundings, which made the measurements less repeatable. The radiated testing was not sensitive to value changes of the matching circuits since it has minimal impact on transmit power or received signal strength indicator (RSSI) compared to part-to-part variations. Thus, radiated tests were not repeatable, did not reliably detect antenna issues, and also did not accurately detect matching problems of the matching circuits.

A vector network analyzer (VNA) or a scalar network analyzer (SNA) may be used for testing. Network analyzer equipment, however, is very expensive and not practicable for mass production testing of mass-produced RF PCBs (e.g., hundreds of thousands or even millions of units). In particular, a network analyzer is a general purpose piece of equipment that requires a slow and manual process for setup that is only suitable for laboratory work. In addition, the calibration of a network analyzer is not designed to remove measurement error on a mass production setup.

SUMMARY OF THE INVENTION

A test system for testing a radio frequency (RF) printed circuit board (PCB) according to one embodiment includes an RF probe for electrically interfacing an intermediate node of the RF PCB, an RF source providing at least one RF test signal, a reflectometer, and a test measurement system that makes a pass/fail determination of the RF PCB using a measured reflection coefficient. The RF PCB includes an RF integrated circuit (RFIC), an IC matching circuit coupled between the RFIC and the intermediate node, and an antenna matching circuit coupled between the intermediate node and an antenna. The reflectometer transfers the RF test signal to the intermediate node via the RF probe, detects a reflected RF signal from the RF probe, and outputs the measured reflection coefficient indicative of a comparison between the reflected RF signal and the RF test signal.

In one embodiment, the reflectometer includes a bidirectional coupler and a gain/phase detector. The bidirectional coupler delivers RF power from the RF source to the RF probe and that provides a forward power signal indicative of the RF test signal and a reflected power signal indicative of the reflected RF signal. The gain/phase detector provides the measured reflection coefficient as a magnitude value indicative of a power ratio of the reflected power signal and the forward power signal and a phase value indicative of a phase differential between the forward and reflected signals.

The test measurement system may use at least one error value to convert the measured reflection coefficient into an actual reflection coefficient in which it uses the actual reflection coefficient to make the pass/fail determination. The test measurement system may convert the actual reflection coefficient into an impedance vector including a resistance value and a reactance value. In this case, the test measurement system makes the pass/fail determination by comparing the resistance value with an allowable resistance range and by comparing the reactance value with an allowable reactance range.

The RF source may provide multiple forward RF signals at different frequencies for providing corresponding measurement reflection coefficients. In this case, the test measurement system may use at least one error value for each of the different frequencies to convert the measured reflection coefficients into actual reflection coefficients, and then may use the actual reflection coefficients to make the pass/fail determination.

The at least one error value may actually be multiple error values that are determined for each of the different frequencies. The error values may be determined by a calibration procedure based on measurements taken for each of multiple test PCBs with known load conditions. In one embodiment, the test PCBs may include a first test PCB with short circuit load, a second test PCB with an open circuit load, and a third test PCB with a predetermined resistive load. Each of the test PCBs may be measured by a network analyzer at each of a set of frequencies including the different frequencies to determine a corresponding set of reference reflection coefficients for each test PCBs. The test PCBs are also measured by the test system at a subset of the same set of frequencies to determine a corresponding set of test reflection coefficients for each test PCBs. In this case, multiple error values are determined for each of the different frequencies using the set of reference reflection coefficients and the set of test reflection coefficients.

The test measurement system may use at least one error value for each of the different frequencies to convert the measured reflection coefficients into corresponding actual reflection coefficients, in which case the test measurement system may convert each actual reflection coefficient into a corresponding impedance vector which is used to make the pass/fail determination. Each impedance vector may include a resistance component and a reactive component. The test measurement system makes the pass/fail determination by comparing each resistance component with a corresponding allowable resistance range and by comparing each reactance component with a corresponding allowable reactance range.

A method of testing the RF PCB according to one embodiment includes electrically interfacing the intermediate node of the RF PCB using an RF probe, applying at least one RF test signal to the intermediate node via the RF probe, detecting a reflected RF signal from the RF PCB, providing a measured reflection coefficient indicative of a comparison between the reflected RF signal and the RF test signal, and making a pass/fail determination of the RF PCB using the measured reflection coefficient.

The method may include providing a magnitude value indicative of a ratio of the reflected RF signal and the RF test signal and providing a phase value indicative of a phase between the RF test signal and the reflected RF signal. The method may include converting the measured reflection coefficient into an actual reflection coefficient using at least one error value, and using the actual reflection coefficient to make the pass/fail determination. The method may include converting the actual reflection coefficient into an impedance vector including a resistance value and a reactance value, and comparing the resistance value with an allowable resistance range and comparing the reactance value with an allowable reactance range for making the pass/fail determination.

The method may include applying multiple RF test signals at different frequencies for providing corresponding measurement reflection coefficients, converting the measured reflection coefficients into corresponding actual reflection coefficients using at least one error value for each of the different frequencies, and making the pass/fail determination using the actual reflection coefficients. The method may include performing a calibration procedure by taking measurements of each of multiple test PCBs with known load conditions to determine multiple error values for each of the different frequencies. The test PCBs may include, for example, first test PCB with a short circuit load, a second test PCB with an open circuit load, and a third test PCB with a predetermined resistive load.

The method may include measuring each test PCB at each of a set of frequencies including the different frequencies previously described using a network analyzer to determine a corresponding set of reference reflection coefficients for each test PCB, testing each test PCB to provide a corresponding set of test reflection coefficients for each test PCB, and determining the error values for each of the different frequencies using the set of reference reflection coefficients the set of test reflection coefficients.

The method may include applying multiple RF test signals at different frequencies for providing corresponding measurement reflection coefficients, converting the measured reflection coefficients into corresponding actual reflection coefficients using at least one error value for each of the different frequencies, converting each of the actual reflection coefficients into a corresponding one of multiple impedance vectors, and making the pass/fail determination using the impedance vectors. The method may include making the pass/fail determination by comparing each resistance component of the impedance vector with a corresponding allowable resistance range and by comparing each reactance component of the impedance vector with a corresponding allowable reactance range.

