Frequency Response Analyzer Introduction

What is a Frequency Response Analyzer?
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A Frequency Response Analyzer (FRA) is a high precision measurement instrument used to analyze components, circuits and systems (known as devices under test, or DUT’s) in the frequency domain. An FRA typically generates a sinusoidal signal and injects it  into a component, circuit or system under test. This signal is measured at the point of injection using one of the input channels on the FRA, usually channel 1. The injection signal travels through the device under test and the same signal is measured simultaneously by the frequency response analyzer at a second reference point – normally the output of the system, using channel 2. The use of sinewaves allows the frequency domain behavior (the frequency response) of a system to be determined.


Frequency Response Analyzer Connection Diagram
Frequency Response Analyzer connection to DUT
The diagram on the left illustrates a basic overview for connecting an FRA to a DUT, the signal generator and reference channel (CH1) are connected to the input of the DUT, CH2 is connected to the output of the DUT.


This connection method enables the frequency domain behavior (also known as the frequency response) of the DUT to be determined. The response of the DUT over a specific frequency range can be determined by performing a “sweep”, this involves stepping the injected frequency across a range of frequencies pre-selected by the user.


Frequency Response Analyzer block diagram
Once the test signals reach the inputs of the frequency response analyzer, they are signal conditioned with N4L proprietary ranging circuitry and then digitized via a high linearity ADC. After digitization, the data is passed to the FPGA/DSP for discrete fourier analysis. The DFT acts as a “notch filter” to extract only the injected signal frequency, all other frequencies are rejected. For example, if a 1kHz signal is injected into the circuit by the FRA generator, the frequency response analyzer utilizes the  DFT process to extract the 1kHz component only from the signal passed to the FPGA.


Without the DFT process, the signal digitized by the frequency response analyzer would also contain noise. The DFT process provides excellent selectivity and very high (120dB) dynamic range.


The output of the DFT from both CH1 and CH2 are compared, with respect to both magnitude and phase shift. The absolute gain (CH2/CH1) is converted into a dB value and both dB gain and phase shift in degrees are displayed.




How can I use a Frequency Analyzer for my development work?
A frequency response analyzer should be considered as important as an oscilloscope to any hardware eopngineer, it is a primary design tool that would play an important role on any hardware engineers test bench. It is important to remember that N4L FRA’s are precision instruments, featuring calibrated inputs and offering measurement accuracies usually only seen within metrology.


An FRA can be used to characterize the gain/phase response of an input filter circuit, determine the AC signal behavior of a transistor, determine whether or not a servo motor control system is stable, enable an engineer to determine the transfer function of a device or subsystem. These are only a few of the many thousands of applications a frequency response analyzer can be applied to.


Example Applications
 
control_loop transistor filter audio opto coax ldo transformer cross_talk emi
Control Loop
Stability
Analysis
Transistor
Performance
Analysis
Filter Design
Audio Amplifier
Design
OptoCoupler
Evaluation
Coax Cable
Frequency
Response
LDO Regulator
Evaluation
Signal Transformer
Performance
Evaluation
Cross Talk
Testing
RFI/EMC Filter
Design
Wide bandwidth Frequency Response Analyzers Combined with Multi Function Measurement
In a world where engineers from many different application areas require ever increasing speed, flexibility and measurement accuracy, the PSM range is a new generation of versatile frequency response analyzers that offer leading performance in every mode without the compromise on accuracy or the additional cost that is commonly associated with such flexible instruments. Newtons4th utilise innovative modern technology and unique circuit design in our instruments to achieve such high accuracy without excessive cost.


 




The PSM range of instrumentation provide not just conventional frequency response measurements but can also be combined with an Impedance Analysis Interface to form a high accuracy impedance analyser, in the case of the PSM3750 this solution is able to provide impedance analysis up to 50MHz


Additional features include an oscilloscope function (PSM3750 + SFRA45) as well as Power Analyzer, Harmonic Analyzer and Vector Voltmeter modes.

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