In today’s highly competitive mobile phone market, product differentiation is becoming more challenging due to the increasing homogenization of devices. As a result, manufacturers are focusing not only on design innovation but also on ensuring top-notch product quality. Among various components, the RF hardware plays a crucial role, and precise RF testing during both R&D and production stages is essential for maintaining quality standards.
One of the key parameters in mobile phone transmitter testing is **transmitting power**. It must be high enough to ensure reliable communication, yet as low as possible under the same conditions to save energy and reduce interference. This means that transmit power needs to be accurately controlled based on real-time requirements. Similarly, **receive sensitivity** is a critical metric for receiver performance, reflecting how well the device can pick up weak signals. Accurate measurement of this parameter is vital for overall system performance.
A typical RF test setup for mobile phones, as shown in Figure 1, includes a comprehensive tester, a test fixture, and the device under test (DUT). The test fixture connects the DUT to the tester and introduces some insertion loss, which is generally consistent. The uncertainty in measurements from the comprehensive tester typically ranges between 0.5dB and 1dB, with repeatability better than 0.1dB. These values are statistical and depend on multiple instruments and environmental factors. For a specific instrument, the measurement uncertainty remains relatively constant. Both the insertion loss of the fixture and the instrument's uncertainty are considered system losses, which can be minimized through proper calibration.
Figure 1: Schematic diagram of a mobile phone RF test system
**Path Loss Calibration Scheme**
As illustrated in Figure 2, the internal structure of the comprehensive tester consists of two main modules: a signal source and a signal analyzer. A switch connects the RF port of the tester to the external test fixture. During transmit and receive tests, the signal paths differ, so it's necessary to calibrate the path loss separately for each scenario. In practice, engineers often use either a gold calibration method or a vector network analyzer for this purpose.
Figure 2: Schematic diagram of the internal structure of the comprehensive tester
**Gold Calibration**
The gold calibration method involves selecting a reference device—known as a "gold machine"—with stable transmission power and reception levels. This device is evaluated using other precision instruments, and its performance is used as a baseline. When calibrating the RF test system, the system measures the gold machine's transmit power and receive level, and the difference between the measured value and the reference is calculated to determine the system path loss. While this method is simple and fast, it has limitations in real-world applications.
First, the data from the gold machine is measured using a comprehensive tester, which itself has an uncertainty of about 0.5–1dB. This leads to inconsistencies between different gold machines, making the reference value less reliable. Second, repeated use of the gold machine can cause wear on the antenna test stand, leading to poor contact with the test fixture and introducing random errors. Finally, in some production environments, temperature and humidity fluctuations can affect the performance of the gold machine, further reducing measurement accuracy.
**Vector Network Analyzer Measurement**
An alternative approach is to use a vector network analyzer (VNA) to measure the S21 parameters of the RF cable in the test fixture. This value is then used as the system path loss. The VNA offers very low uncertainty, typically less than 0.05dB, ensuring high measurement accuracy. However, this method only accounts for the loss between the antenna test stand and the RF port of the comprehensive tester. It does not address the uncertainty of the tester itself, which may still impact the overall measurement results.
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