Modern Radar System Analysis Pdf
Kay Hamling
To understand the best measurement device, we need to understand the signals we are dealing with. First, we assume a sensor (Radar) or an ECM is mostly characterized as a time domain phenomenon, as fundamentally we are dealing with the transmission of a quantity of electromagnetic energy that illuminates a target - we then measure the time it takes for the reflected residual energy to arrive back at the receiver. The signal could be a continuous wave or a sequence of pulses with a specific mission goal. However, pulse rise and fall times, the type of modulation and the behavior of the transmitter amplifier and most importantly the frequency of transmission can create a broad range of responses that need to be considered.
Time domain measurements are traditionally performed with oscilloscopes while spectrum analyzers are best suited for frequency domain measurements. However, our problem is unique; in that we have time domain behaviors we want to observe, but they are exhibited in the frequency domain. While swept spectrum analyzers offer wide frequency and dynamic ranges, their ability to characterize time domain data is limited. Oscilloscopes offer excellent time domain analysis, but lack in dynamic range especially at high frequencies. Advancements however in analog to digital converter technologies and in measurement instrumentation architectures such as FFT based analyzers or Vector Signal Analyzers capturing both the phase and amplitude components of the signal, or its vector) now allow for wide instantaneous bandwidth captures of high dynamic range time domain data in the frequency domain. This is key when dealing with a transient/pulse-based time domain system with transmission frequencies in the giga-hertz ranges such as a radar or ECM.
Real Time Spectrum Analysis drives to the next level of insight. The advantage of real-time measurement capability is the ability to capture transient events in the frequency domain; real time multi-domain triggering, time-selective spectrum analysis, continuous wide bandwidth waveform storage and high-quality persistent displays that provide the higher levels of measurement capability and insight.
CW Radar - Continuous-wave radar is a type of radar system where a known frequency of continuous wave radio energy is transmitted and then received from any reflecting objects. Continuous-wave (CW) radar is excellent for calculating velocity using the Doppler effect by comparing the frequency shift of the received signal with that of the transmitted. Test equipment needs to have the required broadband performance to capture the CW signal with enough fre quency resolution to analyze the Doppler frequency shift.
Pulsed Radar - A pulsed radar emits short and powerful pulses and in the silent period receives the echo signals. It is excellent for determining range by measuring the time difference between the transmitted pulses and the received pulses. The Doppler effect can also be observed to measure velocity, however it is usually calculated over multiple pulses.
Another important factor is to ensure the instrument has enough bandwidth to capture the rise/fall times correctly. For example, an impulse radar may have a very short duration pulse therefore a very broadband oscilloscope may be the best tool to capture the pulse and characterize its parameters such as overshoot and rise and fall times. Very fast transition times or very short duration (sub-nanosecond or shorter) can be accurately seen on a 70 GHz bandwidth oscilloscope such as the DPO70000SX family.
For measuring on/off ratio a wide amplitude is required, Understanding the dynamic range of the instrument is key for this measurement, sometimes referred to the dynamic range of an instrument, ab and can be expressed in decibels, actual bits or effective number of bits (ENOB).
For example, the Tektronix 4, 5 and 6 Series MSO oscilloscope has a 12 Bit analog to digital converter (ADC) and can capture signals with up to 8GHz in bandwidth. The MDO4000C has an optional Spectrum Analyzer built in with an RF frequency range up to 6GHz not only does this allows for cross domain triggering but also provides higher dynamic range for frequency domain measurements The 5 and 6 Series can be used alongside a USB spectrum analyzer such as the RSA300 or RSA500 families when wider frequency coverage and better dynamic range is required, especially for frequency domain measurements.
If the pulse has vector modulation as used in data link or communication systems, this may require specialized demodulation measurements such as error vector magnitude (EVM). Advanced frequency and vector analysis is provided the SignalVu PC that can connect to both Oscilloscopes and Spectrum Analyzers.
Modern Oscilloscope triggering systems are very highly developed and can trigger on both analog and multiple channels of digital data. For example the 5 series oscilloscopes eight input channels can be assigned as trigger inputs for both analog signals such as the envelope of an individual pulse, a defined stream of pulse envelopes or each channel can be assigned to a further eight digital inputs, providing the capability to trigger on multiple parallel data lines or words. Utilizing the external trigger RF devices such as the RSA300 or RSA500 families of spectrum analyzers can be triggered to perform frequency domain measurements based on real-time analog or digital domain events.
The FastAcq display on the oscilloscope can discover baseband pulse time-domain transient errors. Figure 4 shows just one single pulse that has a narrower pulse width than even hundreds of thousands of correct pulses. The blue color on the temperature scale representation of signal persistency represents the least frequent occurrence, while the red areas are the parts of the signal that are the same every time.
The FastAcq capability on the DPO, DSA, and MSO Series provides a time-domain display with a high waveform capture rate. The DPX acquisition technology processor operates directly on the digital samples live from the A/D converter.It discovers rapid variations or one-shot events in the timedomain display.
One of the most highly developed capabilities of the oscilloscope is triggering. Recent advances in oscilloscope trigger have enabled methods of triggering an acquisition or measurement based on the voltages and voltage changes in one or more channels.
The B-trigger is not limited to edge triggering. Instead, the oscilloscope allows the B-trigger to look, after its delay period, for a condition chosen from the same broad list of trigger types used in the A-trigger. A designer can now use the B-trigger to look for a suspected transient, for example, occurring hundreds of nanoseconds after an A-trigger has defined the beginning of an operational cycle. Because the B-trigger offers the full range of triggering choices, the engineer can specify, for instance, the pulse width of the transient they want to find. Over 1,400 possible trigger combinations can be qualified with Pinpoint triggering. Sequences can also include a separate horizontal delay after the A-trigger event to position the acquisition window in time.
The Reset Trigger function makes B-triggering even more efficient. If the B-event fails to occur, the oscilloscope, rather than waiting endlessly, resets the trigger after a specified time or number of cycles. In so doing it rearms the A-trigger to look for a new A-event, sparing the user the need to monitor and manually reset the instrument.
For baseband pulses, the triggers based on edges, levels,pulse width, and transition times are of the most interest. If triggering based on events related to different frequencies is needed, then the RSA Series spectrum analyzer is required.
Traditional measurements of pulses were once made by visual examination of the display on an oscilloscope. This is accomplished by viewing the shape of a baseband pulse. The measurements available using this method were timing and voltage amplitude. These measurements were sufficient, as pulses were generally very simple.
The baseband pulses were used to modulate the power output of the radar transmitter. If it was necessary to measure the RF-modulated pulses from the transmitter, then a simple diode detector was often used to rectify the RF signal and provide a reproduction of its baseband timing and amplitude for the oscilloscope to display. Generally, the oscilloscope did not have sufficient bandwidth to be able to directly display the RF-modulated pulses, and if it did, the pulses were difficult to clearly see, and was even more difficult to reliably generate a trigger.
For these baseband pulse measurements, the measurement technique first used was to visually note the position onscreen of the important portions of the pulse and count the number of on-screen divisions between one part of the pulse and another. This is a totally manual procedure performed by the oscilloscope operator and as such was subject to errors.
Now there are fully automated baseband pulse timing measurements available in modern oscilloscopes. Singlebutton selection of rise time, fall time, pulse width, and others are common. However, most of these measurements do not focus on the measurement envelopes of modulated radar signals.
When used on pulse-modulated carriers, these measurements are of limited utility, because they are presented with the carrier of the signal instead of the detected pulse. This results in pulse width measurements that are made on a single carrier cycle, and rise times of the carrier instead of the modulated pulse. Detectors may be used on the input of the oscilloscope to remove the carrier and overcome this.
When choosing the correct tool, most engineers us an oscilloscopes when performing time domain measurements, but spectrum analyzers are best suited for frequency domain measurements. As discussed earlier, radar and EW is unique, in that time domain behavior is exhibited in both the time and frequency domain. A swept spectrum analyzer offers wide frequency and dynamic ranges, but its ability to characterize time domain data is limited. Oscilloscopes offer excellent time domain analysis and trigger capability, but lack in dynamic range, especially at high frequencies.
c80f0f1006