Test & measurement
Cursors on the FFT trace read the nominal filter cutoff frequencies of 400 to 450 MHz, as shown in the info panel to the left.
Figure 4: The digitizer acquired filter output (left grid) and frequency response (right grid). Cursors on the FFT trace read the nominal filter cutoff frequencies of 400 to 450 MHz, as shown in the info panel to the left.
Figure 2: The SBench6 function generator dialog includes the complete description of the waveform, including its sample rate, duration, amplitude range, and equation.
conditional functions, and two constants, which can be combined to create a broad range of waveforms. The swept sinewave has to cover a frequency range greater than the filter’s bandwidth. In this example, the sweep covered a 333 to 625 MHz range in 10 microseconds (µs). The signal described in the function generator is shown in Figure 3.
As shown in the M5i.63xx series, AWGs with 16-bit amplitude resolution can generate complex high-frequency signals like the linear frequency sweep shown with great precision and high signal purity.
MULTI TONE TESTING FOR AMPLIFIER LINEARITY AND EMC MEASUREMENTS Multitone testing is a common way of assessing the linearity of amplifiers. Tests are most commonly done using dual sinusoidal signals that have high signal purity. The two signals are applied to the input of an amplifier, and distortion products due to the non-linearity in the amplifier are measured. The Spectrum 63 series AWGs can generate the dual tone signal in a single channel with very low distortion, as shown in Figure 5.
Figure 6: The results of a two-tone test show the effects of nonlinear operation on the signal. The output spectrum, expanded about the carrier frequencies, shows harmonic and mixing product signals due to device nonlinearity.
is expanded about the carriers to show the classic intermodulation mixing components. Cursors mark the carrier frequencies in the spectrum trace and the beat frequency in the time domain view. The third-order mixing products result from mixing one of the carrier signal’s second harmonics with the other carrier. Other mixing products, such as the second and fifth order products, occur based on similar mixing of harmonics and the carrier signals. The amplitudes and frequencies of these harmonic and mixing components are used to compute several amplifier performance figures of merit. The purity of the signals from the source determines the limits of this type of measurement. Multitone testing is also employed in
electromagnetic compatibility (EMC) measurements. In this type of testing, multiple pairs of signals covering a broad band of frequencies are generated singly. Amplified signal pairs are emitted through an antenna to test a device’s susceptibility to RF interference over a range of frequencies. The use of multitone signals reduces the test time for EMC qualification.
MODULATED SIGNALS
Figure 3: The signal created in the function genera- tor with horizontally expanded views of the begin- ning and end and frequency response showing the range of frequencies output.
The entire sweep appears in the upper left grid. Horizontally expanded views of the beginning (bottom left grid) and the end (left center) show the change in its frequency. The waveform’s Fast Fourier Transform (FFT), one of the calculations available in SBench6, shows the generated signal’s frequency response (right-hand grid). Note that the frequency response is spectrally flat over the sweep range. Cursors mark the start and end frequencies, showing the nominal 333 to 625 MHz range annotated in the Info pane on the left. This signal is transferred to the AWG output as annotated in the Output Channel pane.
The output of the AWG is applied to a properly terminated input of the filter under test. The output of the filter is connected to the M5i.3360-x16 digitizer. A separate instance of SBench6 controls the acquisition, displays the filter output signal, and performs the FFT to show the frequency response, as shown in Figure 4.
Instrumentation Monthly May 2025
Figure 5: A dual-tone test signal consisting of the sum of two sine waves at 1.664 and 1.667 GHz produced by an M5i.6350-x16 AWG.
SBench6 creates the dual-tone waveform by summing two sine waves with frequencies of 1.6667 and 1.6664 GHz, which are created using a formula in its function generator. The resultant waveform shows a beat pattern equal to the frequency difference between the two sinewave components (top and bottom left grids). The FFT of the signal shows a combined frequency peak at 1.66GHz (upper right). Horizontally expanding the FFT shows the two frequency components measured by cursors. This signal is coupled to the AWG output and applied to the amplifier under test. A Spectrum Instrumentation digitizer M5i.3360-x16 is used to measure the amplifier’s output, shown in Figure 6. The figure shows the acquired time signal on the right, which retains the beat frequency of the two carriers. The frequency spectrum of the acquired signal
Radiofrequency communications signals are usually transmitted on modulated carriers. Modulation adds information to the carrier by varying its amplitude, frequency, or phase singly or simultaneously. The linear frequency sweep used for filter testing is a form of frequency modulation. Phase modulation is used in radar applications to improve radar performance. Figure 7 shows the creation of a phase modulated radar pulse. The phase modulation technique used on a radar pulse breaks the pulse into segments, each transmitted with a specific phase shift. There are thirteen segments
Figure 7: Creating a 1 GHz phase-modulated radar pulse with a duration of 21.5µs using the binary data shown as the modulation source and an expanded view showing the phase shift at a binary state transition.
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