How to improve the RF measurement technology of RF test instruments?

All RF signal sources can generate continuous (CW) RF sine wave signals. Some signal generators can also generate analog modulated RF signals (such as AM signals or pulsed RF signals), and vector signal generators use IQ modulators to generate various analog or digital modulated signals.

Radio frequency signal sources can be further divided into many types, including fixed frequency CW sine wave output sources, swept frequency sources that scan the frequency band of non-fixed frequency CW sine waves, analog signal generators and vector signal generators with analog and digital modulation functions.

If the test needs an excitation signal, it needs an RF signal source. The key indicators of RF signal source are frequency and amplitude range, amplitude accuracy and modulation quality (for the signal source that generates modulation signal). Frequency tuning speed and amplitude stabilization time are also very important for reducing test time.

Vector signal generator is a kind of high-performance signal source, which is usually combined with arbitrary waveform generator to generate some modulation signals. The vector signal generator can generate any type of analog or digital modulation signal through any waveform generator. This kind of generator can generate a variety of baseband waveforms internally, and in some cases, it can also generate a baseband waveform externally and load it into the instrument. If the test specification requires the tested component, equipment or system to be tested according to the processing modulation method in the final use of the tested equipment, then a vector signal generator is usually needed in this case.

If the test specification requires receiver sensitivity test, bit error rate test, adjacent channel suppression, dual-tone intermodulation suppression or dual-tone intermodulation distortion test, then an RF signal source is needed. Dual tone intermodulation test and adjacent channel rejection test need two signal sources, and receiver sensitivity test and/or bit error rate test only need one RF signal source.

If the device under test is used in a mobile phone, the tester may need to test the modulation signal type according to the requirements of the mobile phone standard. Mobile phone power amplifier needs to be tested with modulation signal source (such as vector signal generator). Before selecting the vector signal generator, it is necessary to evaluate the switching speed of the signal generator between different modulation signals to ensure that it can provide the fastest test time.

Second, RF power meter-digital multimeter in RF field

Power is the most frequently measured quantity in RF field. The simplest way to measure power is to use a power meter, which is actually used to measure the power of RF signals. Broadband detectors are used in power meters to display absolute power in watts, dBm or db μ V. For most power meters, broadband detectors (or sensors) are radio frequency Schottky diodes or diode networks to realize the conversion from radio frequency to DC.

Power meter is the most accurate of all RF instruments for measuring power. High-end power meters (usually requiring external power sensors) can achieve measurement accuracy of 0. 1dB or higher. The power meter can measure the minimum power of -70dBm( 100pW). There are many types of sensors, ranging from high-power models and high-frequency (40GHz) models to high-bandwidth models for peak power measurement.

There are two kinds of power meters: single channel and dual channel. Each channel needs to be equipped with its own sensor. Dual-channel power meter can measure the input and output power of equipment, circuit or system, and calculate gain or loss. Some power meters can measure 200 to 1500 readings per second. However, some power meters can measure the peak power characteristics of various signals, including modulated signals and pulsed RF signals used in communication and some applications. The dual-channel power meter can also accurately measure the relative power. The power meter can also be designed into an exquisite shape to meet the needs of portable applications and make it more suitable for field testing. The main limitation of power meter is its amplitude measurement range. Frequency range is a compromise of measurement range. In addition, although the power meter can measure power very accurately, it cannot represent the frequency component of the signal.

Three, RF spectrum or RF signal analyzer-the oscilloscope of RF engineer.

Spectrum or vector signal analyzer uses narrowband detection technology to measure RF signals in frequency domain. Its main output shows the relationship between power spectrum and frequency, including absolute power and relative power. The analyzer can also output demodulated signals.

The accuracy of spectrum analyzer and vector signal analyzer is not as good as that of power meter, but the narrow-band detection technology adopted by RF analyzer enables it to measure power as low as-150dBm. The accuracy of RF analyzer is generally above 0.5dB.

The spectrum and vector signal analyzer can measure the signal frequency from 1kHz to 40GHz (or even higher). The wider the frequency range, the higher the cost of the analyzer. The most common analyzer frequency reaches 3GHz. The new communication standard working in the frequency range of 5.8GHz requires the bandwidth of analyzer to be higher than 6GHz.

Vector signal analyzer is a kind of spectrum analyzer with signal processing function. It can not only measure the amplitude of the signal, but also decompose the signal into in-phase and quadrature components. Vector signal analyzer can demodulate some modulated signals, such as those generated by mobile phones, wireless LAN devices and other devices based on some new communication standards. The vector signal analyzer can display constellation diagram, code domain diagram and calculation measures of modulation quality (such as error vector amplitude).

The traditional spectrum analyzer is a scanning tuner device, because the local oscillator in it scans a frequency range, and the narrow-band filter can get the power component of each unit frequency in that frequency range. The vector signal analyzer also scans a part of the frequency spectrum, but captures the data in a certain broadband and carries out fast Fourier transform to get the power component at the unit frequency. Therefore, the vector signal analyzer scans the spectrum much faster than the spectrum analyzer.

The key index to measure the performance of vector signal analyzer is its measurement bandwidth. Some new high-bandwidth communication standards, such as WLAN and WiMax, need to capture signals with 20MHz bandwidth. In order to capture and analyze these signals, the analyzer must have enough bandwidth to capture the whole signal. If testing high-bandwidth and digitally modulated signals, ensure that the measuring bandwidth of the analyzer can completely capture the measuring signals.

The spectrum analyzer can be used to check whether the transmitter under test has produced the correct power spectrum. If the design project needs to test some distortion components, such as harmonics or parasitic signals, it needs a spectrum analyzer or a vector signal analyzer. Similarly, if designers are concerned about the noise power of devices, they also need to use such an RF analyzer. Other examples that need spectrum analyzer or vector signal analyzer include: testing intermodulation distortion, third-order truncation, 1dB gain compression of power amplifier or power transistor, frequency response of devices, etc.

In order to test the transmitter or amplifier involving digital modulation signal, it is necessary to use vector signal analyzer to demodulate the modulation signal. Vector signal analyzer can measure how much modulation distortion a device produces. Demodulation is a complex and computationally intensive process. The vector signal analyzer, which can quickly demodulate and measure, can greatly shorten the test time and reduce the test cost.

Fourthly, network analyzer.

Besides spectrum analyzer and vector signal analyzer, the third analyzer is network analyzer. The network analyzer includes a built-in RF signal source and a broadband (or narrowband) detector for testing RF equipment. The network analyzer outputs the characteristics of explicit equipment in the form of x-y coordinate, polar coordinate or Smith chart.

In fact, the network analyzer measures the S parameters of the equipment. Vector network analyzer can provide amplitude and phase information, and can judge the transmission loss and gain of these devices with high accuracy in a wide frequency band. Through the vector network analyzer, we can also measure the return loss (reflection coefficient) and impedance matching, and measure the phase and group delay.

Network analyzer is mainly used to analyze filters, amplifiers and other components. It is worth noting that the network analysis uses unmodulated continuous wave, and the calibration of the analyzer is very important. The calibration of the network analyzer can be realized by using the calibration kit provided by the manufacturer. Because the network analyzer integrates the signal source and measurement function into one instrument, and the frequency range of the analyzer is very wide, the price of such instruments is relatively expensive.

Sometimes it is necessary to use the above four main RF test instruments at the same time, such as the test of power amplifier (PA). Signal source can provide input signal, and power meter or spectrum analyzer can measure output power. If accuracy is very important, such as measuring the maximum power, you need to use a power meter to measure the output. Input matching of power amplifier is a key parameter for RF transmitter designers. All the power supplied to the PA must be amplified without losing the actual power due to reflection. Therefore, PA manufacturers generally use network analyzer to measure the return loss of PA (that is, S 1 1).