Applications such as radar and communication systems rely on the control and manipulation of...
From air traffic control operations to weather monitoring to mission-critical military applications, pulsed and pulse compression (CHIRP) radar plays a vital role in target detection and tracking applications.
In this post, we’ll take a closer look at why the shape of a pulse matters and how engineers can measure and characterize pulses to ensure radar systems perform as intended.
Pulse Fidelity & Radar Performance
Radar range and detection capability rely on high pulse fidelity. Distorted pulses can significantly impact the system’s ability to detect targets at the planned-for maximum range, or to distinguish between different target returns, or, in some situations create “ghost” targets.
The shape of a pulse – the way it rises, holds, and falls – can deviate from its intended shape from distortions such as overshoot, ringing, or droop.
What is overshoot?
Overshoot is a signal distortion where the waveform surpasses the desired value immediately after the pulse leading edge.
What is ringing?
Typically occurring after pulse overshoot, ringing introduces unwanted amplitude oscillations until settling at its target value.
What is droop?
Droop occurs when the amplitude reduces between the beginning and end of the pulse.
Limiters are used in radio transmitters to prevent an excess of power from damaging components. Since a pulse overshoot above the threshold can activate the limiter, engineers lower the gain to prevent repeated triggering. While this protects the hardware, the average power is also reduced, which ultimately impacts radar range.
Ringing further alters the pulse shape, and, in the case of pulse compression radars can add time sidelobes, introducing the possibility of “ghost” targets.
Pulse Shape Characterization Test Setup
Below is a typical measurement setup for characterizing the pulse shape of a solid-state power amplifier (SSPA). A key component in radar transmitters, SSPAs amplify pulses before routing the high-power signal to the antenna.
A breakdown of key test setup components includes:
- RF synthesizer: A pulsed microwave signal source (HS9100) generates the input stimulus.
- Driver amplifier: If the power required to drive the SSPA exceeds what the signal source can deliver, a driver amplifier (MPA-series) is used to bridge the gap.
- Directional couplers: Directional couplers (LLC-series) are placed at the input and output of the SSPA to extract a small portion of the signal without interrupting the main signal path, allowing for power measurement and analysis.
- Power sensors: Connected to each coupler, peak power sensors (RTP5000 series) capture the power profile of radar pulses, including artifacts like overshoot, droop, and ringing.
Important Test Instrument Considerations
Choosing the right test instruments for the job ensures your test setup is built for accuracy and success. The Maury Microwave MPA-series are ideal driver amplifier solutions that provide clean, consistent pulses to maintain the quality of the signal going to the DUT.
In terms of peak power sensors, several performance characteristics must be considered to ensure fast, accurate, and reliable test results:
| Characteristic | Definition | Sensor Considerations |
|
Frequency and Dynamic Range |
The range of frequencies and power levels a sensor can measure. |
Sensors must cover expected signal conditions to maintain measurement accuracy. |
|
Rise Time |
The time for the signal’s leading edge to go from 10% to 90% of the pulse magnitude. |
The rise time must be fast enough to capture rapidly rising pulse edges. |
|
Sample Rate |
The rate at which a sensor collects signal data points and processes the measurement. |
The sample rate must be fast enough to support the required time resolution, which indicates the smallest time interval that a sensor can distinguish RF signal characteristics. |
|
Automatic Pulse Measurements |
The automatic extraction of key pulse parameters. |
Automatic measurements should include key distortions such as overshoot and droop. |
|
Pulse Width |
The time interval between a pulse’s leading and trailing edges, measuring how long a single pulse is in its “on” state. |
Sensors need to have the capability to measure the radar’s narrowest pulses; in some cases this can be less than 50 ns. |
The Maury Microwave RTP5000 Series peak power sensors meet all these demands, offering the high performance needed to capture, measure, and analyze subtle, but important, pulse artifacts:
- Frequency range: 50 MHz to 40 GHz
- Sensitivity: -50 dBm sensitivity for pulse measurements
- Rise time: < 3 ns
- Sample rate: 100 MSamples/s (continuous sample rate); 10 GSamples/s (effective sample rate)
- Time resolution: 100 picoseconds
- Minimum pulse width: Ability to make measurements on pulses as narrow as 10 nanoseconds
- Measurement speed: 100,000 per second
- Gap-free acquisition: Capability to collect results from a virtually unlimited number of consecutive pulses or events.
A test setup with the ideal driver amplifier and peak power sensors ensures engineers can confidently capture pulse measurements and maintain system performance at the highest levels.
