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Axel Bennett
Axel Bennett

Basics Of RF Amplifier Test With The Vector Net... ^HOT^



The basic architecture of a network analyzer involves a signal generator, a test set, one or more receivers and display. In some setups, these units are distinct instruments. Most VNAs have two test ports, permitting measurement of four S-parameters ( S 11 , S 21 , S 12 , S 22 ) \displaystyle (S_11,S_21,S_12,S_22) , but instruments with more than two ports are available commercially.




Basics of RF Amplifier Test with the Vector Net...



The network analyzer needs a test signal, and a signal generator or signal source will provide one. Older network analyzers did not have their own signal generator, but had the ability to control a stand-alone signal generator using, for example, a GPIB connection. Nearly all modern network analyzers have a built-in signal generator. High-performance network analyzers have two built-in sources. Two built-in sources are useful for applications such as mixer test, where one source provides the RF signal, another the LO; or amplifier intermodulation testing, where two tones are required for the test.


For the VNA, the receiver measures both the magnitude and the phase of the signal. It needs a reference channel (R) to determine the phase, so a VNA needs at least two receivers. The usual method down converts the reference and test channels to make the measurements at a lower frequency. The phase may be measured with a quadrature detector. A VNA requires at least two receivers, but some will have three or four receivers to permit simultaneous measurement of different parameters.


The diagram shows the essential parts of a typical 2-port vector network analyzer (VNA). The two ports of the device under test (DUT) are denoted port 1 (P1) and port 2 (P2). The test port connectors provided on the VNA itself are precision types which will normally have to be extended and connected to P1 and P2 using precision cables 1 and 2, PC1 and PC2 respectively and suitable connector adaptors A1 and A2 respectively.


A vector network analyzer achieves highly accurate measurements by correcting for the systematic errors in the instrument, the characteristics of cables, adapters and test fixtures. The process of error correction, although commonly just called calibration, is an entirely different process, and may be performed by an engineer several times in an hour. Sometimes it is called user-calibration, to indicate the difference from periodic calibration by a manufacturer.


A network analyzer has connectors on its front panel, but the measurements are seldom made at the front panel. Usually some test cables will connect from the front panel to the device under test (DUT). The length of those cables will introduce a time delay and corresponding phase shift (affecting VNA measurements); the cables will also introduce some attenuation (affecting SNA and VNA measurements). The same is true for cables and couplers inside the network analyzer. All these factors will change with temperature. Calibration usually involves measuring known standards and using those measurements to compensate for systematic errors, but there are methods which do not require known standards. Only systematic errors can be corrected. Random errors, such as connector repeatability cannot be corrected by the user calibration. However, some portable vector network analyzers, designed for lower accuracy measurement outside using batteries, do attempt some correction for temperature by measuring the internal temperature of the network analyzer.


The three major manufacturers of VNAs, Keysight, Anritsu, and Rohde & Schwarz, all produce models which permit the use of noise figure measurements. The vector error correction permits higher accuracy than is possible with other forms of commercial noise figure meters.


The family of ZNB vector network analyzers (VNA) from Rohde & Schwarz are the perfect instruments for analyzing RF amplifier small signal linear and nonlinear performance. This application note presents information on how to configure and use the R&SZNB vector network analyzer to successfully make accurate and quick measurement of basic RF amplifier parameters.


RF vector network analyzer includes: What is a VNA VNA calibration VNA specifications Scalar analyzer RF network analyzers are vital items of test instrumentation for RF design laboratories as well as many manufacturing and service areas.


RF Network analyzers are used to measure components, devices, circuits, and sub-assemblies. An RF network analyzer will contain both a source and multiple receivers. It will display amplitude and often phase information (frequency or power sweeps) and normally in a ratio format. An RF network analyzer looks for a known signal, i.e. a known frequency, at the output of the device under test, since it is a stimulus response system. With vector-error correction, network analyzers provide much higher measurement accuracy than spectrum analyzers.


Spectrum analyzers can be used for testing networks such as filters. To achieve this they need tracking generator. When used in this way, spectrum analyzers can be used for scalar component testing (magnitude versus frequency, but no phase measurements). With spectrum analyzers, it is easy to get a trace on the display, but interpreting the results can be much more difficult than with a network analyzer.


The key element of the vector network analyzer, VNA, is that it can measure both amplitude and phase. While an amplitude only measurement is much simpler to make, and can be undertaken by less complicated instruments. This may be quite sufficient for many instances. For example when the only consideration is the gain of an amplifier over a certain bandwidth, or the amplitude response of a filter is needed


Only with a knowledge of phase and magnitude from a vector network analyser can circuit models be developed that will enable complete simulation to be undertaken. This will enable matching circuits to be designed based on conjugate matching techniques. Time-domain characterization requires magnitude and phase information to perform the inverse-Fourier transform. Also, phase data is required to perform vector error correction.


The simplified block diagram for the vector network analyzer is provided to give an overall guide to the way the test instrument operates. Any actual VNA will be far more complicated, but contain these essential building blocks.


The VNA has precision connectors on the front panel of the unit itself and then precision cables are used to connect these to the device under test. Precision cables are required because the phase and loss of a standard cable would vary too much with even slight movement, etc.


To test the device, a variable frequency signal is generated within the vector network analyzer and the output is switched to test the DUT in either one direction or the other. In this case the left hand side on the diagram is selected. The signal passes to the splitter where one output is used as the reference signal for the receiver and the other side is passed to a direction coupler and then into the DUT via the external connection on the VNA and the precision cables.


Apart from generating a signal to power the device under test, the signal source also has an output that is connected to the receiver. This enables phase information to be gained from the detected signals. Currently vector network analyzers will make significant use of digital signal processing, and there much of the receiver and detector section will be undertaken in digital format.


Modern RF systems are full of active devices like amplifiers, mixers, and frequencer converters. Testing these type of devices used to require entire racks of equipment. Now, network analyzers are sophisticated enough to handle active device characterization without additional hardware.


Whether you are testing active or passive components, the right mix of speed and performance gives you an edge. In research and development (R&D), Keysight vector network analyzers (VNAs) provide a level of measurement integrity that helps you transform deeper understanding into better designs. On the production line, our cost-effective vector network analyzers provide the throughput and repeatability you need to transform parts into competitive components. Every Keysight vector network analyzer is the ultimate expression of our expertise in linear and nonlinear device characterization. On the bench, in a rack or in the field, we can help you gain deeper confidence.


Vector network analyzers (VNAs) are test instruments that measure electrical network parameters. They are essential for radio frequency (RF) and microwave component analysis of various passive and active devices including filters, antennas, and power amplifiers.


A network analyzer conducts component characterization tests. Network analyzers provide calibrated stimulus signals to the RF network or device under test (DUT) and measure the vector response over the frequency with phase and amplitude information. A VNA captures transmission (transmission coefficient, insertion loss, gain), reflection (reflection coefficient, VSWR, return loss), and impedance measurements, as well as the s-parameters S11, S12, S21, S22.


A vector network analyzer measures a variety of devices and networks with numerous measurements for many different use-cases like spectrum analysis, pulse measurements, power amplifier (PA) characterization, and active device tests. The following guide describes how to set up network analyzer measurements, calibrate your measurement setup, and interpret results: Making Measurements with a Vector Network Analyzer.


Analog Devices offers a variety of solutions for RF signal analysis including phase-locked loops (PLL) for generating LOs and test signals, low-noise amplifiers and gain blocks, mixers, filters, video signal path components and state-of-the-art high speed ADCs to complete the solution. For modular, portable, and low-cost solutions, ADI offers wideband RF detectors as well as fully integrated transceivers.


The circuit shown in Figure 1 precisely converts a 400 MHz to 6 GHz RF input signal to its corresponding digital magnitude and digital phase. The signal chain achieves 0 to 360 of phase measurement with 1 of accuracy at 900 MHz. The circuit uses a high performance quadrature demodulator, a dual differential amplifier, and a dual differential 16-bit, 1 MSPS successive approximation analog-to-digital converter (SAR ADC). 041b061a72


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