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 What Are "Transfer Characteristics"? |
 | What Are Transfer Characteristics? |
| "Transfer characteristics" describe the conditions or states under which a device under test (DUT) transfers (transmits, conducts) physical quantities such as electricity, heat, or vibration. They are key parameters for understanding how a DUT behaves, responds, and reacts. |
For an audio amplifier, this is equivalent to the gain. For a temperature sensor, the relationship between heat (input) and the electrical signal (output) is called the transfer characteristic. |
| How Are Transfer Characteristics Measured? |
| Because physical quantities such as heat and vibration are difficult to handle directly, they are converted by transducers or sensors into electrical signals that are easier to process, and then measured. In practice, it is not just the simple output-to-input ratio that matters—response speed and frequency response characteristics are also important parameters. Transfer characteristics that focus on response speed are called "time-domain transfer characteristics"; those that focus on frequency response are called "frequency-domain transfer characteristics." |
 | Time-Domain Transfer Characteristics |
| Waveforms are easy to display graphically, but numerical management is difficult ----> Instrument used: observe waveforms with an oscilloscope |
| Frequency-Domain Transfer Characteristics |
| Instruments are often more complex, but precise measurement suited to numerical management is possible ----> Instruments used: frequency response analyzer (FRA), FFT (fast Fourier transform) analyzer + oscillator |
 What Instruments Are Used to Measure Transfer Characteristics? |
 | Instruments for Measuring Transfer Characteristics |
 ●FFT (fast Fourier transform) analyzer with built-in signal source (FFT servo analyzer) | |
 ●Frequency response analyzer (Frequency Response Analyzer) | |
Both instruments have a built-in signal source for measurement. They digitize and compute (Fourier transform) the signals before and after the circuit under test (input and output), then determine the gain and phase to measure transfer characteristics. Below, we compare and examine the various related instruments. |
| Different Measurement Principles |
| The measurement signals (built-in signal source) used in the FFT (fast Fourier transform) method include pulses, random noise, and swept markers. Each of these contains broad frequency components (a wide spectrum). | |
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| The signal source used for FRA measurement is a sine wave at a fixed frequency. To measure transfer characteristics over a wide frequency range, the frequency is changed slightly for each measurement (frequency sweep), and measurements are repeated. | |
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| Measurement Accuracy |
| The FFT method measures using a fixed range. As a result, small signal components can be buried in noise, increasing measurement error. The FRA method, by contrast, measures only one frequency component at a time, so it can automatically set the optimum range for each measurement. In other words, when measuring weak signals, it increases sensitivity, enabling analysis with a good signal-to-noise ratio (good measurement accuracy). |
| Frequency Resolution Can Be Set Freely |
| In the FFT method, the analysis results are level values at equal intervals on the frequency axis. So if the frequency axis is plotted logarithmically, data becomes sparse at low frequencies and extremely dense at high frequencies. In contrast, the FRA method allows measurement at any arbitrary frequency points, so measurement data can also be equally spaced on a logarithmic axis. |
| Comparison Item |  |  |
| Signal source | Sine wave | Pulse or noise, swept markers, etc. |
| Possibility of saturating the system with the measurement signal | Safe and reliable (low risk of saturation) | Care required during measurement (high risk of saturation) |
| Dynamic range | Very wide Automatic range switching | Narrow Measured on a single range |
| Frequency resolution (distribution of measurement points) | Arbitrary (fine or coarse) / logarithmic distribution with respect to frequency (distribution when frequency is plotted logarithmically) | Determined by sampling / uniform distribution with respect to frequency (if frequency is plotted logarithmically, points are sparser at the low end than at the high end) |
| Injection of the measurement signal | Easy (isolated signal source) | Requires careful consideration |
| Window function setting | Not required | Required (select from several options) |
| Measurement procedure | 1. Enter the frequency range and resolution of the DUT (arbitrary) 2. Set the signal level and number of integrations 
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