PDF1. Measuring Magnitude Response
A typical magnitude response measurement can be performed with a signal generator and a spectrum analyzer, vector signal analyzer, or power meter, depending on the level of amplitude measurement accuracy needed. The signal generator produces tones at a series of frequencies that are ‘swept’ across a bandwidth of interest at the UUT’s input, and the ensuing UUT output power is measured at each of the frequency steps along the way. The UUT output signal power is then divided by the UUT input signal power, with the resulting ratio usually converted to logarithmic units. In order to avoid frequency offset errors due to different timebases present in the generator and analyzer, the devices can usually be locked together with a reference signal. When using a spectrum analyzer or vector signal analyzer, the frequency of the signal at the UUT output can be measured in addition to its magnitude as an alternative. The resulting series of power ratios can then be plotted against frequency to display the system’s overall magnitude response.
The degree of accuracy of the system’s magnitude response will determine the complexity of the measurement system. For high-accuracy characterizations, the measurement system would use power meters as the measurement device, most likely with a calibration grade power splitter used to split the signal into two paths at the input – one path leading to the UUT input and the other path leading to a power meter to measure the actual signal power being fed into the UUT. Another power meter would measure the signal output of the UUT. The signal generator and two power meters would be locked together most likely with a 10 MHz reference signal.
Oftentimes, such high-accuracy measurements are not needed. A signal generator and a spectrum or vector signal analyzer alone can be used in this scenario. The diagram below shows this setup using a NI PXI 565x RF Signal Generator and NI RF Vector Signal Analyzer (VSA).
Figure 1: Simple Magnitude Response Setup
At each frequency increment in the sweep across the bandwidth of interest, the PXI-565x generates a tone of a known, constant power level into the UUT. The NI RF VSA measures the signal power at the output by acquiring an averaged power spectrum centered at the current signal frequency, with software performing an ensuing spectrum peak search measurement to acquire the frequency and power of the peak present in the acquired power spectrum. This power value can then be plotted as a function of the measured frequency values to produce a plot of absolute power measured versus frequency measured. This power vs. frequency plot could also be transformed into a relative power vs. frequency plot simply by subtracting the expected signal generator output power from the measured UUT output power (assuming both are in logarithmic power units such as dBm), resulting in a Y axis measured in units of dB and showing the change from input to output.
2. Resources
Common RF and Microwave Measurements
This is the main page of a series of tutorials focused on common RF measurements involving signal generators and analyzers.
3. Related Products
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