Choosing a suitable analyzer.
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HEWLETT PACKARD”
CHOOSING A SUITABLE ANALYZER
One should feel sorry for the novice engineer who encounters a multitude of industrial measuring devices for the first time. Most often, engineering schools do not provide sufficient knowledge in the field of measuring equipment and control and measuring instruments. In addition, the existing technical jargon increases the number of names and purposes of spectral and circuit analyzers, causing confusion and confusing the mass of specialized measuring devices, each of which is designed to diagnose certain parameters. This publication is devoted to the systematization of numerous names and areas of application of the most common analyzers.
There are two main categories of analyzer instruments: network analyzers and signal analyzers. In this article, we will not consider digital analyzers (logical and numerical data analyzers), but will focus on RF and microwave analyzers.
NETWORK ANALYZERS
Network analyzers include vector and scalar analyzers, as well as impedance and LC/RAD meters. Network analyzers are designed to evaluate the impedance or dissipation characteristics of active and passive circuits such as amplifiers, frequency converters, antenna switches, filters, switching devices, attenuators, and many other components used in various circuits. Circuits may have a single port (input or output) or many ports. Generally speaking, if a designer can measure the input characteristics of each port, as well as the transmission characteristics from one port to each of the others, this information allows these components to be used in systems such as communications or radar systems. Typically, active components such as amplifiers are examined in their linear range, while nonlinear components most often require signal analyzers to determine the signal distortion parameters.
Vector network analyzers (VNAs) are the most powerful of their group, as they measure and display a full set of amplitude and phase characteristics of a circuit. These parameters include S-parameters, transfer functions, magnitude and phase, standing wave ratio (SWR), insertion loss or gain, attenuation, group delay, return loss or reflection coefficient. VNAs typically measure frequencies from 100 kHz to 110 GHz. VNAs include a sweep signal source (sometimes built-in), a signal analyzer to separate the forward and reverse test signals, and a highly sensitive dual-channel phase-coherent receiver equipped with a video display to show a vector diagram (such as a Smith chart) of the signal versus frequency in the range of interest. The parameters measured in the RF and microwave ranges are usually referred to as scattering characteristics (S-parameters), as is common in most CAD systems.
Current and voltage based representations do not work in the frequency range above 50 or 100 MHz. The S-parameter method is based on the well-known signal flow diagram method.
If a passive or active component in a circuit has been designed using a full picture of BAC measurements and the manufacturing process is controllable, scalar network analyzers can be used(SNA SNA) as less expensive measurement devices. They measure only the amplitude component of the 8-parameters (scatter characteristics), expressed in terms of gain, loss, SWR, or return loss. Although SNAs also require an external or built-in sweep source and a signal separator, they use simple amplitude detectors rather than complex and expensive dual-channel phase-coherent instruments. SNAs are most widely used on production lines, where amplitude characteristics can identify faulty components.
Automatic vector network analyzers (AVNAs) grew out of a desire to extend the measurement capabilities of VNAs with powerful data processing and computer-aided design (CAD) techniques. By integrating models derived from engineering calculations and CAD software with actual test data from the measurement system, a designer can first define design quantities using CAD, design the component, automatically generate test data, and then iterate through the design cycle until the desired performance is achieved. By coupling a VNA to a computer, a variety of specialized test systems can be built. Some examples include: antenna near-field performance test systems, radar transmit/receive (T/R) module test systems, and a dielectric test rig with a special attachment for testing arbitrary radar materials to determine their microwave performance.For the low RF range, where current and voltage based models are applied using calibration under real conditions in short and open circuits, it is common practice in most cases to evaluate the circuit using impedance analyzers and inductive, capacitive and active resistance measurements.This range is still dominated by bulk components such as resistors, capacitors and inductors. They are the preferred test devices for components such as transistors and other semiconductor devices. Most commercial models of this type provide the ability to test both wire-lead devices and surface-mount components. Such devices are also often the basis of integrated test stations, which investigate non-numerical characteristics of circuits.
Other interesting custom applications are possible with specialized fixtures for characterizing dielectric and magnetic materials. Since such materials produce measurable attenuation and phase shift values when interacting with an RF test signal, impedance analyzers can be configured to deeply characterize these materials. The measured properties include permittivity, loss tangent, permittivity, permeability, etc. Even liquids such as oil can be analyzed with appropriate fixtures.
It should be noted that network analyzers are also used for optical components (fiber optics). Here, scalar measurements are mainly used, since vector devices would require coherent signals in the optical wavelength range.
SIGNAL ANALYZERS
This category is the most numerous. The devices belonging to it perform the functions of spectrum analyzers, parametric modulation analyzers, distortion analyzers, signal analyzers based on dynamic fast Fourier transform (FFT), modulation analyzers, phase noise analyzers, pulse power analyzers and many others. In general, all signal analyzers are aimed at studying components and systems in terms of their signal transmission characteristics. Since a number of these characteristics are critical for the functioning of the entire system, the corresponding measuring instruments in most cases must provide increased accuracy and sensitivity when studying such subtle parameters as phase noise and fluctuation noise.
A spectrum analyzer is an oscilloscope for measuring frequency characteristics. For an RF/microwave engineer, these are as important as time characteristics. Perhaps every design workstation needs one. It is a superheterodyne receiver with its own sweep generator, providing a visual representation of the amplitude versus frequency over a wide dynamic range and a large set of convenient measurement devices such as markers, relative value calculation. Most modern spectrum analyzers have gone far beyond simple narrow-band filters, allowing detection of the full width of the modulation interval and providing the user with information (with fairly high accuracy) about other signal parameters such as the modulated signal envelope or the magnitude of interference.
Most feature-rich spectrum analyzers include a “personal software module” that allows the basic analyzer to be configured to perform specialized functions, such as RFI testing for cable TV, cellular, and digital communications. When performing such functions, the instrument’s display provides explanations relevant to the subject area being examined. For example, when testing subscriber TV components, the display shows the signal control limits established by the Federal Communications Commission (USA).
Modulation analyzers are tunable receivers that provide highly accurate signal modulation characteristics. In addition to amplitude, frequency, and phase modulation data, they provide accurate signal level determination, which is why they are also calledmeasuring receivers.This allows them to be used in metrology and standardization laboratories for functions such as calibrating signal generators.
Another device in this category is aimed at testing high-speed digital vector modulation. It is called a vector modulation analyzerand defines such parameters of modulation of digital radio device of microwave range as QPSK, 64QAM, eye diagrams, “star” diagrams and others. These types of modulation are used in new satellite systems with digital video channel.
Vector signal analyzers combine frequency and time measurements to characterize the most complex and time-varying system signals. Typical system signals include bursts, pulses, transients, frequency jumps, and analog-to-digital modulated signals. These instruments feature the familiar “waterfall” and spectrographic displays that provide high-resolution, high-dynamic-range, time-series spectral slices on the screen, while also providing high-speed data processing. Because these instruments process vector signal information, they are well suited for numerical analysis of modulation parameters based on indicator and star diagrams. In many cases, they enable carrier recovery for coherent analysis of real-world communications signals. Basic configuration devices cover a frequency range of up to 10 MHz, and precision downconverters extend the coverage to 1.8 or 2.65 GHz.
Automated spectrum analyzers, in addition to the described capabilities, increase the computing power of the device to perform a wide range of in-depth and specialized measurements. For example, spectrum monitoring system,coupled with broadband antennas, can provide a complete spectral response to the mountaintop environment where the new system's antennas are to be installed. Other automated signal analyzers monitor satellite transponder channels under load and overload-induced distortion. Another specialized measurement function is electromagnetic compatibility (EMC) testing for product qualification testing. With nearly every electronic product and data processing device now subject to RFI requirements, metrology labs need automated compliance testing equipment that can monitor performance, reproducibly, and analyze data in depth.
Microwave spectrum analyzers are also used in the optical spectrum using optical converters. They can provide many of the same spectral responses.
Dynamic FFT analyzers are powerful, unique measurement instruments that use a mathematical algorithm known as the Fourier transform. This means that a single analog signal or event can be analyzed to obtain complete frequency response information. FFT instruments use sampling techniques and powerful mathematical routines to analyze spectral parameters in many applications. These instruments are most effective at low frequencies, which is why they are most widely used in vibration and acoustics. Common applications include machine theory, structural analysis, seismology, engine vibration analysis, and most sound, ultrasound, and sonar measurements. FFT technology is used in the Vector Signal Analyzers mentioned above.
Parametric modulation analyzers (PAMs) differ from modulation analyzers in their architecture. Modulation parameters can be considered as a third dimension in addition to time and frequency parameters. Time parameters are the dependence of amplitude (current or voltage) on time, frequency parameters are the dependence of amplitude on frequency. Modulation parameters describe the dependence of frequency on time. These analyzers are based on complex data processing using electronic counters. These instruments make extremely fast sequential periodic measurements of signal characteristics. PAMs simplify the study of the step response of voltage-controlled oscillators and the frequency hopping characteristics of frequency-agile transmitters. They allow diagnostics of the degree of linearity of chirp pulses and phase switching in radar systems. These instruments provide powerful tools for analyzing signal synchronization instability in communication systems, disk drive read/write components, and mechanical systems. There are also applications in circuit synchronization devices and synchronized optical distribution (SDH) system analysis.
Pulse power analyzers are designed to fully characterize 13 pulse envelope parameters in applications such as radar and navigation systems. The seven timing parameters are rise and fall times, pulse width, PRI, pulse repetition frequency (PRF), duty cycle, and delay. The five amplitude parameters are peak and average power, peak-to-trough amplitude difference, and trailing edge overshoot. Pulse power analyzers detect the pulse envelope and then amplify the signal with dual-channel video amplifiers to analyze pulse distortions such as trailing edge overshoot, ringing, and spurious oscillations. The instrument is based on a digital sampling oscilloscope. The digitized pulse information can be processed by a digital signal processor (DSP) to provide pulse characteristics in statistical form and for timing comparison between video and microwave systems. There is an option for digital modulation formats in the microwave range.
Near-carrier phase noise analyzers are specialized signal analyzers designed to detect and process any phase noise of local oscillator (LO) signals. When used in superheterodyne mixing systems, noise near the LO carrier, even if it is 150 dB weaker than the signal, can cause a mismatch between the channel and the transmitted signal. Phase noise analyzers typically operate in a frequency band of up to 40 MHz. To measure phase noise of unknown sources with frequencies up to 18 GHz, a specialphase noise test setupis used, which functions as a superheterodyne downconverter created on the basis of LO, equipped with special filters and having an extremely low level of its own phase noise.
Distortion analyzers first appeared as test instruments for audio recording and playback equipment, designed to measure the total harmonic distortion of an unidentified audio test signal. Using a variable narrowband filter, the fundamental of the carrier can be suppressed, while the remaining harmonics, spurious signals and noise can be measured using a wideband detector. These total signal distortions are then compared as a percentage of the fundamental. Audio analyzers differ from distortion analyzers in that they are sweep spectrum analyzers designed for the audio range of the spectrum, covering a frequency range up to 100 kHz.
Noise figure meters are in the middle category. Although noise figure is actually a parameter that typically characterizes amplifiers and frequency converters, these devices also measure circuit parameters such as gain and attenuation. Noise figure is a critical parameter for amplifiers and frequency converters used in the high-frequency path of receivers, since each amplifier adds its own unwanted noise when amplifying the desired signal — the lower the noise figure, the better the amplifier. A noise figure of 3 dB would mean that the amplifier adds interference equal to the desired signal — this would be a bad circuit element. Modern noise figure meters combine this measurement with the detection of the degree of signal amplification/attenuation so that designers can find the optimal trade-off between gain and noise figure. By using step-down converters, measurements can be made at auxiliary noise generator frequencies up to 50 GHz and higher.
COMBINED NETWORK AND SPECTRUM ANALYZERS
Recognizing that there was significant duplication of instrumentation on test benches equipped with network and spectrum analyzers, manufacturers combined the two devices into a network and spectrum analyzer.These combination instruments are useful for designing and testing active circuits where signal characteristics are also important — for example, for amplifiers and frequency converters. The frequency range covered is from 100 kHz to 1.8 GHz. Some models use FFT techniques to improve resolution and performance in spectrum analysis. Others offer spectrum analysis with time gating for use with devices such as computer disk drives to reduce the impact of interference.
SPECIALISED SIGNAL ANALYZERS
When using standard signal analyzers in specialized test procedures, engineers often encounter situations where a more specific solution would be appropriate. Often, combinations of signal source functions and measurement functions from multiple instruments are used. Several types of such combined instruments are described below as examples.
When designing cellular systems, developing their local oscillator can be a tedious testing process. The local oscillator typically consists of a voltage-controlled oscillator and a phase-locked loop (VCO/PLL) built into the receiver. A typical VCO/PLL requires 9 or 10 parameters to fully characterize its performance under all conditions, depending on the control voltage and signal characteristics. The phase noise studies alone can require many hours of work due to their complexity. Therefore, a specialized analyzer called a VCO/PLL analyzer has been developed.(frequency range from 10 MHz to 3 GHz), which can not only measure all 9 parameters of the VCO signal at once, but is also capable of conducting a new specially modified test that reduces the time required to study phase noise by 10 times.
Another example of a specialized analyzer is a full, inductive, capacitive and active resistance meter.This instrument is configured to perform complete testing of quartz crystals, both in design and production environments. It is based on an impedance meter, but its hardware, software, and fixtures are optimized to determine 9 crystal parameters such as resonant frequency, quality factor, and other essential crystal quality characteristics.
Test “kits” (service monitors) are a generic name given to sets of devices that perform specialized system testing functions. They have been designed for RF and microwave systems ranging from mobile FM transceivers to radar systems. A typical service monitor would be a dual-standard cellular test monitor (e.g. GSM900/DCS1800), which is designed to install and operate cellular transceivers. The service monitor will include a signal generator to produce precision adjustable signals in the format of the system under test. These signals will have nominal modulation and power level for testing the receiving part of the radio. The service monitor also includes spectrum analysis functions for testing the power, modulation characteristics and spectrum of the transmitting part of the radio.