Testing Wideband Satcom And Radar

Oct. 7, 2010
The combination of a wide-bandwidth digital oscilloscope and a flexible arbitrary waveform generator allow radar and satcom transmitter testing without custom downconverters.

Wide modulation bandwidths in modern radar and satellite communications (satcom) systems can exceed the intermediate frequency (IF) and analysis bandwidths of even the best commercial-off-theshelf (COTS) spectrum analyzers, vector signal analyzers (VSAs), and Fast Fourier Transform (FFT) analyzers. Custom test solutions can solve the problem, but a more cost-effective solution is based on modern wide-bandwidth digital oscilloscopes and arbitrary waveform generators (AWGs).

The RF and microwave frequencies used by radar and satcom systems also present transmitt er testing challenges since it may not be possible to directly measure a transmitter's outputs to characterize some aspects of performance. One solution is the development of custom frequency downconversion equipment to translate the transmitter's RF/microwave frequencies to an IF that can then be measured with COTS test equipment, such as an oscilloscope. Unfortunately, the nonrecurring- engineering (NRE) time and cost of developing such custom hardware can be significant. And the custom frequency downconversion hardware can introduce its own RF impairments to the test signal and can mask the actual performance of the RF/microwave transmitt er being tested. Local-oscillator (LO) phase noise, RF/IF filter frequency response and group delay, and amplifier gain/phase distortion from the custom frequency downconverter can introduce distortion to the downconverted waveforms and can contribute to the overall error vector magnitude (EVM) being measured. This makes it difficult to determine how much of the EVM being measured is from the actual transmitt er device-under-test (DUT) output, and how much of the EVM is contributed from the custom downconverter hardware.

Fortunately, wide-bandwidth oscilloscopes represent a paradigm shift in how RF engineers can measure the performance of their RF/microwave transmitters by eliminating the need for custom frequency downconversion hardware. Wide-bandwidth oscilloscopes offer bandwidths to 32 GHz with 2 Gpoints of waveform memory, enabling engineers to directly perform RF/microwave measurements on DUTs operating at X-band, Kuband, and Ka-band frequencies (to 32 GHz). Time-domain analysis can be performed to measure transmitt er pulsed RF characteristics such as rise time, fall time, and pulse width. Frequency-domain analysis using vector-signal-analyzer (VSA) software can be used to measure characteristics such as the RF/-microwave spectrum, frequency and phase characteristics (e.g., chirped phase and frequency or frequency-hopping characteristics displayed on an RF spectrogram), EVM, and other characteristics that provide RF/microwave engineers insights into the performance of a radar or satcom transmitter.

In addition to performing analysis on signals with wide modulation bandwidths, creating similar signals for testing a DUT can be a challenge for COTS test equipment. Custom/proprietary radar and satcom test signals may not be supported with COTS equipment, and may require a custom test solution to be designed and builtwhich can be expensive and limited in its flexibility to support a number of different applications. Wide-bandwidth AWGs can be used to generate custom/ proprietary radar signals and wideband modulated signals for radar and satcom applications. Wide-bandwidth AWGs, combined with RF/microwave signal generators with wideband in-phase/ quadrature (I/Q) inputs, provide flexibility in creating custom/proprietary wideband RF/microwave test signals for radar and satcom DUT testing.

What follows are several case-study examples that show how wide-bandwidth oscilloscopes and wide-bandwidth AWGs are being used for wideband radar and satcom applications. For example, an AWG and RF/microwave signal generator with wideband I/Q inputs were combined to create a 2-GHz-wide linear frequency modulated (LFM) chirp radar signal and 1-GHz-wide 16-state quadratureamplitude- modulated (16QAM) signal. The test signals were then analyzed with a wide-bandwidth oscilloscope equipped with VSA sofware. The test setup shown in block-diagram form in Fig. 1 and the actual equipment components in Fig. 2 were used to create the 2-GHz LFM chirp radar signal and wideband 16QAM signal for these case-study examples.

The two test signals were created with MATLAB sofware from The MathWorks running on a personal computer (PC). In each case, the file representing the simulated waveform was downloaded from the PC to the AWG to turn the simulated signal into differential I/Q waveforms. External reconstruction filters were used to filter the differential I/Q waveforms that were fed into the external wideband I/Q inputs on a PSG model E8267D vector signal generator from Agilent Technologies to create the modulated RF/microwave test signal. The model E8267D vector signal generator is available in models operating from 250 kHz to 20.0, 31.8, or 44.0 GHz. The standard modulation bandwidth using external I and Q vector inputs is 160 MHz, which can be extended to 2 GHz with options. The E8267D's output is connected to channel 1 on the widebandwidth oscilloscope for RF/microwave signal analysis.

In the test setup (Fig. 2), an Agilent 81180A wide-bandwidth AWG is shown on the upper lef-hand side, a wide-bandwidth Agilent 90000X oscilloscope with Agilent 89600 VSA sofware is shown on the lower lef-hand side, and an Agilent PSG vector signal analyzer with wideband I/Q inputs is shown on the lower right-hand side. An Agilent MXA signal analyzer is also shown on the upper right-hand side. Tis same test setup was used to create and analyze a 2-GHz-wide LFM chirp radar signal. A custom MATLAB function was used in the oscilloscope to measure and display the envelope of the pulsed RF waveform. Using custom user-defined MATLAB algorithms enable custom/proprietary signal processing to be performed on oscilloscope waveforms for radar and satcom applications. This capability allows preconfigured measurements, such as rise time, fall time, and pulse width, to be made on the RF envelope when performing pulsed radar measurements with a wideband oscilloscope (Fig. 3).

The 2 Gpoints of waveform memory is critical for capturing and analyzing a large number of radar pulses. In addition, segmented memory can be used to further optimize the number of radar pulses captured and analyzed with the available oscilloscope memory (Fig. 4). Segmented memory enables a series of radar pulses to be acquired, effectively capturing the "on" time of the pulse and ignoring the "off" time of the pulse. This optimizes the use of the available oscilloscope memory to analyze a larger number of radar pulses for the available memory.

Employing segmented memory involves setting the horizontal time scale for the RF pulse width characteristics and specifying the number of segments to be captured. The oscilloscope acquisition is then performed and each memory segment (radar pulse) can then be analyzed by scrolling through each segment. In addition to the timedomain pulsed RF measurements shown in Figure 3 and Figure 4, the combination of the widebandwidth oscilloscope running with the VSA software allows frequency-domain measurements to be performed on pulsed signals (Fig. 5). In Fig. 5, the LFM spectrum is shown on the upper left-hand side, the log magnitude is shown on the lower left- hand side, the chirped phase is shown on the upper right-hand side, and the 2-GHzwide chirped frequency is shown on the lower right-hand side.

Using VSA software in the oscilloscope provides the functionality and "look and feel" of a VSA, even though the software is post-processing digitized data from an oscilloscope instead of an RF/microwave signal analyzer. This is a key for RF engineers migrating to oscilloscopes for wide bandwidth measurements on radar and satcom transmitters. Th e VSA so ware provides a familiar user interface for RF/ microwave engineers, enabling traditional RF parameters such as frequency span and resolution bandwidth to be speci ed on the oscilloscope. Then it processes the data from the oscilloscope and displays the results using vector signal analyzer amplitude and phase displays.

The test setup shown in Figure 1 and Figure 2 was also used to create and analyze a wideband 16QAM signal. Figure 6 shows the resulting measurement at 14.5 GHz with a 1 GHz wide 16 QAM modulation bandwidth. An EVM of approximately 1.8 percent was being measured with the VSA software on the oscilloscope (with no DUT in place). Equalization was applied in the VSA to mitigate the effects of amplitude flatness and phase variation on EVM. Note that these measurement results show work-in-progress.

An additional measurement was performed at 19.2 GHz. In this case, the 16QAM modulation bandwidth was reduced to 262.5 MHz because the carrier frequency plus modulation bandwidth were near the upper frequency limit of the signal generator being used (higher frequency models are available, but were not used for this example). Figure 7 shows the results of the measurements performed at 19.2 GHz. An EVM of approximately 1.4 percent was being measured with the VSA sofware on the oscilloscope. Equalization was used in the VSA for this measurement. Note that these measurement results show preliminary work-inprogress. Even though this measurement is being performed at a higher frequency than the measurement shown in Fig. 6, the EVM is lower as a result of the narrower 16QAM modulation bandwidth used.

The use of a wide-bandwidth oscilloscope with VSA sofware enables X-band, Ku-band, and Ka-band radar and satcom transmiter outputs to be directly measured and analyzed (to 32 GHz) without the need for custom and costly frequency downconverter hardware. Tis provides the RF engineer with visibility into the actual transmiter hardware performance, without impairments that may be introduced by external frequency downconversion hardware.

VSA sofware on an oscilloscope provides key measurement capability for the RF/microwave engineer, with a familiar user interface and the widebandwidth benefits of the oscilloscope. A low EVM for a wideband 16QAM signal with 1-GHz modulation bandwidth was measured using the oscilloscope and VSA sofware. Radar characteristics such as chirped phase and frequency were also measured for a 2-GHz LFM chirped signal using the VSA sofware. Using the wide bandwidth AWG and an RF/microwave signal generator with wideband I/Q inputs provides engineers the flexibility of creating custom/ proprietary wide-bandwidth radar and satcom signals for DUT testing in the lab environment without the cost of custom test equipment.

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