Frequency Response Analysis
Frequency Response Analyzers (FRA), are also known as a Vector Network Analyzers (VNA) or similarly, Network Analyzers. FRAs perform frequency response analysis to graphically display a systems reaction to a set of test frequencies.
It is important to realize that the FRA is the frequency domain counterpart to the oscilloscope. A key point is that an oscilloscope makes measurements in the time domain, with the horizontal axis as a linear time scale and the vertical axis as the voltage or current scale. This is in contrast to an FRA which makes measurements in the frequency domain, with the horizontal axis as a linear or log frequency scale and two vertical axes representing the magnitude scale and the phase scale.
Generally, designers use an FRA to measure the transfer function of an electrical network. Additionally, the instrument can also be used to measure the input and output impedance of the network.
A point often overlooked is that he term Vector Network Analyzer is used mostly in RF design but both instruments are one and the same when it comes to measuring magnitude and phase.
Below are some example applications and plots.
Power Supply Stability (Bode Plot)
This chart illustrates the open loop gain transfer function of a power supply controller. The magnitude and phase together help determine the stability of the system. Values for the phase margin and gain margin insure the stable operation of the power supply over the specified conditions. The design team will make adjustments during the design phase for optimal performance. Additionally, engineers will use frequency response analysis to verify output impedance, conducted emissions and compliance.
Piezoelectric Transducer Impedance
This chart illustrates the impedance measurement of a piezoelectric transducer. The negative and positive peaks of the red trace show the resonance and anti-resonance frequency points respectively. Depending on the application, a designer can use those data points to optimize system performance. Applications of piezoelectric devices include, ultra-sound, ultrasonic cleaners, actuators, crystals, optics and audio.
Component SPICE Modeling
This chart shows the impedance profile between the gate and source of a MOSFET transistor. Thus, engineers can construct accurate simulation models by interpreting the data. As an example, the negative "straight" slope on the RED trace, along with the -90 degrees of phase shift on the BLUE trace, is an indication of capacitance. You can estimate the ESR of the capacitance at the point of the negative peak of the impedance. Generally speaking, this type of analysis can be performed on almost any type of active or passive device to extract information for more realistic and accurate SPICE models.
Amplifier Transfer Functions
This plot illustrates the magnitude and phase transfer function of an amplifier. The Red trace shows the magnitude and the cutoff frequency of the amplifier as the curve progresses to the right. Measurements like these validate the linearity, bandwidth and phase shift of devices to insure operation within their specified limits. The shape of the magnitude and phase indicate that this is a single pole roll off with the -3dB point at a 45 degree phase shift.
Filter Transfer Function
This chart illustrates the transfer function of a 7-pole, elliptical filter. Filters like these have applications as anti-aliasing filters for ADC or DAC systems. For this reason, it is important to identify the operating parameters of the filter. The most common example is the filters cutoff frequency, the slope of the transition region and the attenuation of the stop band to help determine the sampling rate of the ADC and the number of effective bits.