Whether for multi-channel analysis, or for wider analysis bandwidth in a measurement, high bandwidth oscilloscopes offer an alternative to traditional spectrum and signal analyzers for both pulsed RF aerospace/defense and I/Q vector modulated communications application measurements. Signals with spectral content beyond 1 or 2 GHz are now being created to support the higher resolution requirements in radar systems and to move the vast amounts of information in new communications systems.
Combining wide bandwidth real-time oscilloscopes with RF analysis software can create a powerful, wideband RF measurement suite for such applications. One application area, Radar/EW, can benefit from this approach through a variety of resultant measurements including pulse amplitude, frequency and phase response across RF pulses, and across multiple channels. Such measurements are explored and found to be useful for the evaluation of pulsed RF systems.
Marrying an Oscilloscope with Vector Signal Analysis Software
The measurement capability of a high bandwidth oscilloscope can be enhanced through the use of external vector signal analysis software. Some of these enhancements, beyond the noise reduction just described through digital down-conversion, include:
- A wide range of vertical scaling options, including linear and log magnitude;
- Key RF measurements including occupied bandwidth (OBW) and power spectral density (PSD);
- Vector demodulation options for communications formats like QAM16;
- Analog demodulation options including AM, FM and PM;
- Set up of segmented memory capture;
- Statistical pulse analysis.
Pulse Amplitude, Frequency, Phase and FFT Measurements
For radar and electronic warfare applications, it is quite valuable to perform a variety of measurements on many pulses, including things like amplitude variation, frequency and phase shift across pulses, and a view of the spectrum of signals. For applications such as aircraft warning receivers, it is also beneficial to have the ability to measure the time difference and the phase difference between pulses associated with the capture of a wave front by multiple antennas on an aircraft. Some of these measurements will now be considered.
In the simplest case, it is possible to measure the basic pulse amplitude, frequency shift, and phase shift across the measured RF pulse. As mentioned previously, the RF pulse train is sampled by the oscilloscope, and then digitally down-converted to reduce noise and allow further signal processing.
For example, in Figure 1, a 15 GHz carrier, 2 GHz-wide linear FM chirped RF pulse signal is shown after vector signal analysis processing, where the 2 GHz-wide spectral content of the signal can be seen in the upper left corner, the real part of the down converted I/Q data seen in the lower left quadrant, the 2 GHz wide linear FM frequency chirp seen across the RF pulse in the upper right quadrant, and the parabolic phase shift seen across the RF pulse in the lower right quadrant. These measurements are taken in the “Vector” measurement mode.
Statistical RF Pulse Analysis
The next level of analysis is possible by shifting into a “Pulse Analysis” mode, where multiple oscilloscope channels are used and RF pulse signals are captured into segments of oscilloscope memory, digitally down-converted into baseband I/Q signals, and then evaluated for single and multiple channel pulse analysis. For single channel measurement, this allows for the comparison of the linear FM frequency chirp to an ideal, best-fit linear FM chirp signal, as well as the phase shift across a pulse to be compared to a best-fit parabolic phase shift profile, and the amplitude of the pulse envelope to be compared to a best fit ideal straight line best fit reference. In Figure 2, such comparisons are being made between measured to reference, and then the “error” between the measured and reference is expanded in vertical scale for a close view.
A Pulse Table also displays RF pulse parameters, including an rms error calculation between the measured frequency or phase across the pulse, compared to a best-fit reference signal. It’s also possible to show statistics for the measurements over all the pulses.
Dual Channel Delta Measurements
It is also possible to make “two channel delta” measurements as shown in Figure 3. Such measurements are becoming increasingly important in applications such as aircraft warning receiver testing, where multiple signals are being captured from multiple antennas and the difference in time delay and frequency difference of arrival between wave fronts need to be measured for angle of arrival calculations.
Notice in this example a 1 nsec time delay being measured between two RF pulses. A 0.2 dB difference in amplitude is measured. And a 16 kHz difference in frequency, on average, is seen.
Three of ten captured pulses that are being placed into oscilloscope memory segments are having additional pulse analysis performed. The parabolic phase shift across pulses (lower left), the linear FM chirp frequency shift across pulses (middle right), and the pulse envelope of pulses (upper left) are superimposed for signals coming into two channels of the oscilloscope. As in the previous example, each scope channel measured signal can have the measured, reference and error signal calculations made. Finally, the FFT spectral content for both scope channel captures of the two pulse trains (center left) is also shown.
Cross-Correlation Between Pulses
In the aircraft warning receiver example mentioned previously, very precise measurements of time delay between RF pulses captured on different antennas on an aircraft can be determined by using a cross-correlation measurement between pulses. In Figure 4, a 50 psec time difference of arrival (TDOA) is being measured between two RF pulses captured on two scope input channels, where pulses have a 10 GHz carrier, 100 MHz wide linear FM chirp modulation, and a 1 μsec width. In this measurement, the channel to channel skew between scope channels, including cable delays at the temperature where measurements will be taken, can first be removed through de-embedding. Then a measurement is made to see the actual time shift between the captured signals. A mean delay of 50 psec was measured with a peak-to-peak variation in delay measured between 47 psec and 53 psec seen with multiple measurements.
The difference in phase between two RF pulses is also of interest in a variety of Radar/EW/warning receiver oriented applications. Through the use of math functions, the measured phase across one pulse can be subtracted from the measured phase across a second pulse, measured on two oscilloscope input channels.
The same two linear FM chirp signals seen in the last example are now measured to view the phase shift between the two pulse trains, as seen by comparing related pulses, as might be seen again from two antennas on an aircraft. The time shift has now been set to zero on an arbitrary waveform generator, but a 25-degree phase shift is being introduced between the two signals. A capture shown in Figure 5, top center trace C, and related blue marker 1, show this 25-degree phase shift in a mean measurement in lower right Trace D, as well as only a 0.8 degree standard deviation, and a 0.7 variance. These are average values over the width of the pulses.
More radar/EW/warning receiver applications are driving toward wider modulation bandwidths to increase range and angle of arrival precision capability in related systems, even extending beyond 1 GHz modulation bandwidths. This increasingly drives designers to use wideband oscilloscopes as RF receivers to evaluate related wideband signals when validating their hardware prototypes.
While scope measurements directly are of interest, it is often advantageous to digitally down-convert captured wideband signals to reduce noise and allow more careful analysis of baseband I/Q signals. Vector signal analysis software can then make useful single- and two-channel measurements, evaluating amplitude, frequency and phase shift across RF pulses captured on a single scope input channel, as well as between RF pulses captured on multiple scope channels. The time shift between multiple, captured RF pulses on two scope channels can also be evaluated by either comparing pulse envelopes, or more precisely through the cross-correlation between two RF chirp pulses. Such techniques are particularly helpful when making angle of arrival calculations for a variety of systems.
This article was written by Brad Frieden, Applications Engineer, Keysight Technologies, Inc. (Santa Rosa, CA). For more information, Click Here .