Fundamentals of RF Measurement Technology for Non-RF Test Engineers

introduction

Currently, wireless communication products based on radio frequency principles are everywhere, and their number is growing at an alarming rate. From cellular phones and wireless PDAs to WiFi-enabled laptops, Bluetooth headsets, radio frequency identification tags, wireless medical devices and ZigBee sensors, the market size of radio frequency devices is expanding rapidly. This year alone, more than 850 million cellular phones will be manufactured and sold worldwide.

In order to conduct comprehensive production testing and improve testing capacity, test engineers must understand the basic principles of RF, know the content of the test, and know how to choose the most suitable instruments to complete these tests. The problem is that most engineers engaged in low-frequency applications (operating frequency below 1MHz) are not familiar with the application characteristics of high frequencies.

RF Terminology: The "Working Language" You Must Master

Forget voltage, RF engineers often use power

The strength of radio frequency signals varies greatly. As the signal propagates in free space, the unit power decreases in proportion to the square of the distance, and the change in power is usually expressed in decibels (dB).

Measuring power in decibels also greatly simplifies the calculation process.

Both the addition and subtraction of power and loss are done in decibels. Therefore, the multiplication operation is simplified to addition. The formal definition of dB is:

dB = 10 log (Pout/Pin)

The decibel dB is a relative value. Another related unit is the milliwatt decibel dBm, which is an absolute power relative to 1mW. Figure 1 shows the dBm value and its corresponding wattage, which also shows the reference range of transmitter transmission power for mobile phones and the minimum signal power that can be detected by a sensitive receiver. The equation given in Figure 2 defines the theoretical thermal noise of an RF signal at room temperature. Due to the transmission of RF signals through the air and interference from the atmosphere and other signals, the signal level reaching the receiver can become very low. Receivers often need to detect signals below 0.1pW (or signal levels below microvolts).

Noise Floor: Noise floor

The common problem is no longer the input impedance, but the impedance mismatch of the transmission line

At low frequencies, our goal in transmitting voltage across a circuit is to achieve minimal attenuation. The most efficient circuit is one with high input impedance and low output impedance. For RF applications, where the length of the cable may be only a quarter of the wavelength, we must think of the signal transmission as a wave. If the wave is blocked, part of the wave signal will be reflected. The goal of RF transmission is to deliver all the power to the load without loss. Any reflection of power means a loss of power delivered to the load. Therefore, mismatch is a critical parameter. Any impedance difference between the circuit element and the transmission line will cause reflections and power loss.

In RF applications, transmission lines are usually coaxial cables, which are external components relative to the circuit board and the microstrip lines within the circuit board. These components have their own characteristic impedance. The characteristic impedance of the transmission line depends on the geometry of the conductor, the properties of the conductor, and the insulation that wraps or isolates the conductor. For RF applications, the characteristic impedance of the transmission line and the input and output impedance of each component are usually 50 ohms or 75 ohms. The impedance of 50 ohms is used to optimize power transfer within the system, while the impedance of 75 ohms is used to achieve minimum attenuation, such as cable TV network systems. Most RF wireless transmission systems are designed and optimized for power transmission, so the characteristic impedance is 50 ohms.

To minimize reflections, RF cables and components in wireless test and measurement applications are designed based on a 50 ohm characteristic impedance. Conversely, when impedances are matched, optimal power transfer is achieved.
[page]

If a signal wave is transmitted from one characteristic impedance to another different characteristic impedance, it will cause signal reflection and reverse transmission. If the impedance is the same, no reflection will occur. When signal transmission occurs due to impedance discontinuity, signal wave transmission will occur in both directions of the transmission line. At the point where the two waves are in phase, the maximum voltage amplitude Vmax will appear; at the point where they are 180 degrees out of phase, Vmin will appear. The ratio of Vmax to Vmin is called the voltage standing wave ratio, or VSWR. VSWR is an indicator to measure whether the impedance of a connector or a cable is close to 50 ohms. Figure 3 shows the relationship between these three values ​​in an ideal full match (no reflection), an ideal open circuit (100% reflection), and an extreme case.

Return Loss: Return loss

Reflected Power: Reflected Power

Become familiar with new connectors, cables and components

Cables with BNC connectors usually begin to attenuate above 500MHz. In the RF world, cables are often equipped with N-type connectors and SMA connectors. N-type connectors are often used on test instruments because they are very durable, can handle high power, and work well at frequencies up to 18GHz. SMA connectors are much smaller than N-type connectors and have lower power than N connectors, but can be used well at frequencies above 18GHz.

All RF cables are coaxial. Coaxial RF cables can be inflexible (i.e. rigid), bendable to a certain degree (i.e. semi-rigid), or bendable. For RF, we have to be more careful with cables than at lower frequencies. Excessive bending of the cable and significant 90-degree bends can damage the cable and seriously degrade transmission performance.

At low frequencies, a good connection means that the wires are touching each other (simple continuity). At RF, impedance mismatch is a serious problem, which means that a good connection requires not only ensuring that the wires are touching each other, but also that the wires are

The connectors also need to be torqued together properly, so RF manufacturers often use 7 foot-pounds of torque to ensure good contact and minimal resistance (called insertion loss in RF terms) between the connectors.

Guaranteed 50 ohm transmission lines throughout the test system

Parallel connections or multiple signal paths in RF circuits are not as simple as they are in low-frequency circuits. It is critical to ensure that the impedance of the entire circuit path is matched and to reduce impedance discontinuities and signal reflections. RF switches are manufactured with precision machining to ensure that the impedance of the entire switch is 50 ohms. To achieve parallel paths, people use devices such as so-called splitters or separators to divide an input signal path into two or more output paths, each with 50 ohms. Combiners achieve the opposite effect and merge multiple input paths into a single output path. If you are new to RF testing, don't be intimidated by these complex situations. RF components cost much more than equivalent DC components.

What kind of RF instrumentation do you need to meet your testing needs?

Low-frequency test instruments are becoming more and more popular, and the types of RF test instruments are also increasing, with more and more applications, including various instruments from signal sources and power meters to spectrum and network analyzers. These instruments are used to generate RF signals and measure a large number of signal parameters.

RF Power Meter - Digital Multimeter for RF

Power is the most frequently measured quantity in the RF field. The simplest way to measure power is to use a power meter, which is actually used to

A wideband detector is used in power meters to measure the power of RF signals. The wideband detector is used in power meters to display the absolute power in watts, dBm, or dB μV. For most power meters, the wideband detector (or sensor) is an RF Schottky diode or diode network that performs the RF to DC conversion process.

Power meters are the most accurate of all RF instruments for measuring power. High-end power meters (usually require an external power sensor) can achieve measurement accuracy of 0.1dB or better. Power meters can measure power as low as -70dBm (100pW). Sensors come in a variety of models, from high power models, high frequency models (40GHz), to high bandwidth models for peak power measurement.

Power meters are available in single-channel and dual-channel versions. Each channel requires its own sensor. A two-channel power meter can measure the input and output power of a device, circuit, or system and calculate gain or loss.

Some power meters can achieve measurement speeds of 200 to 1500 readings per second. Some power meters can measure the peak power characteristics of a variety of signals, including modulated signals and pulsed RF signals used in communications and certain applications. Dual-channel power meters can also accurately measure relative power. Power meters can also be designed in a compact size for portable applications, making them more suitable for field testing needs.

The main limitation of a power meter is its amplitude measurement range. Frequency range is a trade-off with measurement range. Also, while a power meter can measure power very accurately, it cannot represent the frequency content of a signal.

[page]

RF Spectrum or RF Signal Analyzer – The RF Engineer’s Oscilloscope

A spectrum or vector signal analyzer measures RF signals in the frequency domain using narrowband detection techniques. Its primary output is the power spectrum versus frequency, both absolute and relative. This analyzer can also output a demodulated signal.

Spectrum analyzers and vector signal analyzers do not have the same accuracy as power meters, but the narrowband detection technology used in RF analyzers enables them to measure power as low as -150dBm. The accuracy of RF analyzers is generally better than ±0.5dB.

Spectrum and vector signal analyzers can measure signal frequencies from 1kHz to 40GHz (or even higher). The wider the frequency range, the more expensive the analyzer. The most common analyzers have frequencies up to 3GHz. New communication standards operating in the 5.8GHz frequency range require analyzers with bandwidths of more than 6GHz.

A vector signal analyzer is a spectrum analyzer with added signal processing capabilities that not only measure the amplitude of a signal, but also decompose the signal into its in-phase and quadrature components. A vector signal analyzer can demodulate certain modulated signals, such as those produced by mobile phones, wireless LAN devices, and devices based on some other emerging communications standards. A vector signal analyzer can display constellation diagrams, code domain diagrams, and calculated measures of modulation quality (such as error vector magnitude).

Traditional spectrum analyzers are scan-tuned devices because the local oscillator in them scans a frequency range, and the narrowband filter can obtain the power component at each unit frequency within the frequency range. Vector signal analyzers also scan a part of the spectrum, but they capture data within a certain broadband and perform fast Fourier transform to obtain the power component at the unit frequency. Therefore, the speed of scanning the spectrum by vector signal analyzers is much faster than that of spectrum analyzers.

The key indicator for evaluating the performance of a vector signal analyzer is its measurement bandwidth. Some new high-bandwidth communication standards, such as WLAN and WiMax, require capturing signals with a bandwidth of 20MHz. To capture and analyze these signals, the analyzer must have a large enough bandwidth to capture the entire signal. If testing high-bandwidth, digitally modulated signals, make sure the analyzer's measurement bandwidth can fully capture the measured signal.

A spectrum analyzer can be used to verify that the transmitter under test is producing the correct power spectrum. If the design project requires testing certain distortion components, such as harmonics or spurious signals, then a spectrum analyzer or vector signal analyzer is needed. Similarly, if the designer is concerned about the noise power of the device, such an RF analyzer is also needed. Other examples that require a spectrum analyzer or vector signal analyzer include: testing intermodulation distortion, third-order intercept, 1dB gain compression of power amplifiers or power transistors, and frequency response of devices.

Testing transmitters or amplifiers that involve digitally modulated signals requires the use of a vector signal analyzer to demodulate the modulated signal.

It can measure how much modulation distortion a device produces. The demodulation process is a complex and computationally intensive process. A vector signal analyzer that can quickly perform demodulation and measurement calculations can greatly shorten test time and reduce test costs.

RF signal source

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.

RF signal sources can be further divided into many categories, including fixed frequency CW sine wave output sources, swept frequency sources that sweep and output a frequency band of non-fixed frequency CW sine waves, analog signal generators, and vector signal generators that add analog and digital modulation capabilities.

If the test requires an excitation signal, then an RF signal source is required. The key specifications of an RF signal source are the frequency and amplitude range, amplitude accuracy, and modulation quality (for a signal source that generates a modulated signal). Frequency tuning speed and amplitude stabilization time are also critical to reducing test time.

A vector signal generator is a high-performance signal source that is usually combined with an arbitrary waveform generator to generate certain modulated signals. The arbitrary waveform generator can be used to generate any type of analog or digital modulated signal. This generator can generate a variety of baseband waveforms internally, and in some cases, it can also generate a certain baseband waveform externally and then load it into the instrument. If the test specification requires the measured

Vector signal generators are often used when components, devices or systems are to be tested in the same modulation as the device under test will process in its final use.

If the test specification requires receiver sensitivity testing, bit error rate testing, adjacent channel rejection, two-tone intermodulation rejection, or two-tone intermodulation distortion testing, then an RF signal source is also required. Two-tone intermodulation testing and adjacent channel rejection testing require two signal sources, while receiver sensitivity testing and/or bit error rate testing only require one RF signal source.

If the device under test is for a mobile phone, the tester may need to test the type of modulation signals required by the mobile phone standard. Mobile phone power amplifiers need to be tested in conjunction with a modulation signal source such as a vector signal generator. Before selecting a vector signal generator, evaluate how quickly the signal generator can switch between different modulation signals to ensure that it can provide the fastest test time.

Network Analyzer

In addition to spectrum analyzers and vector signal analyzers, the third type of analyzer is the network analyzer. A network analyzer contains a built-in RF signal source and a broadband (or narrowband) detector for testing RF devices. The network analyzer outputs the characteristics of the device in the form of xy coordinates, polar coordinates, or Smith charts.

Essentially, a network analyzer measures the S parameters of a device. A vector network analyzer can provide amplitude and phase information, and can determine the transmission loss and gain of these devices over a wide frequency band with high accuracy. A vector network analyzer can also measure return loss (reflection coefficient) and impedance matching, and perform phase and group delay measurements.

Network analyzers are mainly used to analyze components such as filters and amplifiers. It is worth noting that network analysis uses unmodulated continuous waves, so the calibration of the analyzer is very important. Calibration of the network analyzer can be achieved using the calibration kit provided by the manufacturer.

Because network analyzers integrate signal source and measurement functions in one instrument and have a wide frequency range, these instruments are relatively expensive.

[page]

Typical Applications

An example of an application that requires the simultaneous use of all four major RF test instruments is the testing of power amplifiers (PAs).

The signal source provides the input signal, and a power meter or spectrum analyzer measures the output power. If accuracy is critical, such as when measuring maximum power, then a power meter is needed for output measurement.

The input match of a PA is a critical parameter for designers working on RF transmitters. It is very important to amplify all the power supplied to the PA and not lose the actual power due to reflections. Therefore, PA manufacturers specify and measure the return loss (or S11) of the PA, which can be measured by a network analyzer. Alternatively, if only a scalar amplitude measurement is required, a signal source and a spectrum analyzer (or power meter) can be combined through a coupler to measure the amplitude of the reflected power. The only disadvantage of this method compared to using a network analyzer is that the configuration process is more complicated and requires the use of additional passive RF components. For scalar return loss measurements, a power meter can achieve more accurate power measurements.

The ability of a PA to deliver power to a load whose input impedance does not match the output impedance (typically 50 ohms) is an important measure of the PA's performance under real-world conditions, where the load (e.g., an antenna) may not have a characteristic input impedance of exactly 50 ohms. In this case, a non-50 ohm resistive load is switched to the PA's output. This load will force the PA to output a VSWR as high as 20:1 (a 50 ohm load would have a VSWR of nearly 1:1 with an ideal match). The PA must be able to work properly and deliver some power to the load in the presence of significant reflected power.

Certain output measurements require spectrum analysis. RF PAs used in broadcast or mobile phone applications (or other FCC-compliant applications) are required to not generate excess power in channels adjacent to the PA's operating channel. Measurements of adjacent channel power, intermodulation distortion, and harmonic distortion measure the power generated by the PA outside the actual transmission channel. For these measurements, dynamic range, the ability to measure small signals in the presence of large signals (such as the carrier), becomes an important specification for spectrum analyzers. For example, if a PA is specified to have an adjacent channel power (for a certain type of modulation scheme, or for a particular mobile phone standard) of 60dBc (decibels below carrier), then the dynamic range of the spectrum analyzer (under the required test conditions) must be at least 6dB greater than the minimum allowable power of the harmonic power, adjacent channel power, or intermodulation products.

The adjacent channel power must be measured with a modulated signal, which means that the adjacent channel performance of the signal source must be considered. The adjacent channel power output of the signal source must be at least 6dB less than the maximum allowable adjacent channel power produced by the power amplifier.

For harmonic measurements, the frequency range of the analyzer must be three times greater than the maximum operating frequency (3dB bandwidth frequency) of the PA to fully capture the third harmonic power of the maximum operating frequency. In addition, the dynamic range and background noise of the spectrum analyzer must be at least 6dB lower than the value to be measured to measure the third harmonic component well; it must have a reasonable signal-to-noise ratio to achieve accurate and reproducible measurements. Harmonic measurements show the amount of distortion generated by the PA. Excessive distortion will have a negative impact on modulation performance.

When signals of different frequencies or signal components of different frequencies are input to the PA, intermodulation distortion determines how much distortion the PA produces. Two signal sources are required to generate such a test signal. A dual-output signal source is not enough because there is not enough isolation between its two output signals. The signal source will generate its own intermodulation distortion, which will lead to excessively high amplifier distortion measurements, resulting in erroneous measurement results.

PAs designed for the mobile phone market and certain market areas (such as WLAN applications) are also often tested for modulation quality. In these application areas, more complex modulation techniques are generally used.

This type of test usually measures the error vector magnitude (EVM).

Conclusion

The above brief introduction to the main RF theories is intended to help readers review the relevant knowledge. This overview of RF test instruments will provide readers with some general guidance on how to choose the right test instruments for their test needs. In most cases, testers will use one or more of the following four types of test instruments: signal source, power meter, spectrum analyzer, and network analyzer.