What is the difference between a Frequency Response Analyzer (FRA) and a Vector Network Analyzer (VNA)?
The FRA is generally designed to measure Bode plots of electronic circuits such as amplifiers or filters. A special application of an FRA is to measure Bode plots of control systems, such as voltage regulators and power converters. FRAs generally offer two high impedance inputs, compatible with oscilloscope probes and an oscillator output port. Some units also offer 50 Ohm input settings in addition to the high impedance inputs. The FRA does not generally include measurement calibration to remove the frequency dependent characteristics of the probes and interconnects to the Device Under Test (DUT).
Both the VNA and the FRA can measure complex magnitude and phase values over frequency.
The VNA, on the other hand, only has two ports that serve as inputs and outputs and the oscillator is not a separated output. The VNA is designed primarily to measure scatter parameters, generally referred to as S-parameters. S-parameters are used to define the response of a system with respect to its “ports” where each port is either an input or an output. The simplest s-matrix has only one port, which would represent a component connected to ground, since ground is not counted as a port. A filter, on the other hand, is generally a 2 port device, with one port being the input and one port being the output. These measurements include reflection, admittance and gain.
The VNA inputs are typically only 50 Ohms and offer measurement calibrations routines for thru calibration and a minimum of Short Open Load (“SOL”) calibration, though some also include a propagation delay calibration. The thru calibration calibrates the gain of the measurement system, including the interconnects, but without the DUT installed. For example, measuring a filter uses two cables to connect the input and output to the VNA. The THRU calibration is performed with the filter removed and the two cables connected together. This calibrates the measurement to 0dB and 0 Degrees without the filter installed.
There are three ways to measure impedance with a VNA. One method is to place the DUT between 2 ports; another method is to place the DUT as a shunt to ground with both ports connected to the DUT. The first method is better for high impedance measurements while the second method is better for low impedance measurements. The single port reflection method uses only one port and is most accurate for impedances that are close to 50 Ohms. The impedance is calibrated using an SOL calibration routine, which calibrates the OPEN circuit (to remove parasitic capacitance), Short, to remove the interconnect impedance and Load, which is used to calibrate precisely to 50 Ohms. Some VNAs also include a propagation delay calibration, which removes the effective inductance related to the propagation of the signal through a short that is physically identical in dimension to the DUT.
The VNA typically offers many more measurement options than an FRA and the VNA also generally offers a much higher maximum frequency capability. The VNA is generally more expensive than the FRA for these reasons.
The OMICRON Lab Bode 100 offers all of the aforementioned features and benefits of BOTH the FRA and the VNA, with the exception that the cost of the Bode 100 is lower than most other FRAs or VNAs.
The configuration screen below shows that the oscillator is a 50 Ohm port, which can be directly connected internally to CH1 (Port 1) as it would be in a traditional VNA. CH1 and CH2 (Port 2) can each be set to either high impedance (oscilloscope probe compatible) or to 50 Ohms as in most VNAs.
The Bode 100 supports both thru gain calibration and SOL impedance calibration. The Bode 100 also offers two calibration algorithms (user & probe calibration) for each of these calibration methods.
The Probe calibration is performed at factory defined frequencies and correction factors for the actual measurement frequencies are calculated by linear interpolation. The User calibration, measures the correction factors for every measured frequency to ensure highest accuracy.
The Bode 100 supports all of the FRA and VNA measurements, including Gain, Reflectance, Admittance and Impedance.
While all of these are necessary for power supply design, many are also especially useful for high speed systems, which require Signal Integrity (SI), Power Integrity (PI) or Power Distribution Network (PDN) measurements.
The Bode 100 includes all of the popular display graphics for both the FRA and the VNA. The FRA screen shows the typical displays of gain, phase, polar, real and imaginary plots while in the S-parameter screen offers typical VNA measurements such as VSWR, Smith charts, etc.
Impedance mode includes all of the typical displays of a full featured Impedance Analyzer, including real, imaginary, capacitance, inductance, resistance and Q in both rectangular and polar formats.
In summary, if you only want to measure the control loop of a power supply or a simple transfer function, then an FRA may meet all of your needs. If you want to measure impedances, PDNs and component parameters or parasitics, a VNA offers many more features that support those more complex measurements.
