When moving from circuit simulation to practical development and test of an electronic design, an oscilloscope will usually be used to investigate the operation of a circuit. However, when the relative magnitude and phase angle between two voltages or the voltage and current at different points in an ac circuit must be quantified reliably, for example in a filter design, amplifier, or attenuator, then a frequency response analyzer (FRA) is the ideal measurement device.
The use of frequency response analyzers has historically been limited to specialist environments since they have been and are generally still assumed to be prohibitively expensive and beyond the budget of most test benches; but that is no longer the case.
For the price of a typical oscilloscope, N4L frequency response analyzers provide wide bandwidth fixed or swept gain and phase measurements in the presence of noise that no oscilloscope can match, representing an invaluable addition to almost any electronic test bench.
Filters are frequency selective devices and given the design objective of significant attenuation outside a desired passband, the difference in signal level between the input and output of a filter may be large.
This presents a challenge for conventional measurement instruments that are unlikely to achieve good accuracy or stability when the signal level is very low or when noise becomes a large proportion of the measured value.
In this case, the meaningful specification and testing of a filter becomes limited by the noise floor or signal to noise rejection ability of a measurement device.
Frequency response analyzers have the particular advantage of a wide signal ranging system, exceptional frequency selectivity, rejecting frequencies that are not of interest and more sophisticated coupling to optimise resolution on the signal of interest.
Amplifier Response and Acoustics
Amplifiers used in electronic and electromechanical applications will often operate over a frequency range that is not ideal for most ac sensing measurement devices that typically operate from 10-20Hz upward. With an ability to measure frequency components well below 1Hz, frequency response analyzers can be used in the design and test of electromechanical systems operating at very low frequency.
Audio amplifier devices, microphones and speakers primarily focus on a frequency range of 20Hz to 20kHz, since this is the range over which human hearing detects sound. This frequency range is within that of many measurement instruments, but two particular factors make audio amplifier design and test more challenging. First, the fact that human hearing has an exceptionally wide dynamic range of 120-130dB, and second, that perceived sound is influenced not only by frequency but also, the relative phase of frequency components. High dynamic range, frequency selectivity and precise phase measurement are ideally suited to frequency response analyzers.
To quantify the ability of a DC power supply or any DC system to reject an unwanted common mode AC input signal, an ideal measurement system will apply both a desired differential DC level and a common mode AC signal. By simultaneously measuring the common mode ac signal at both the input and output of a device, its rejection ratio can be directly measured.
The wide dynamic range and frequency selectively of a frequency response analyzer make them ideally suited to this application.