Microwave Counters: CW vs. Pulsed Signal Measurements
Filed under Counters
This guide explains the key technical differences between continuous wave (CW) microwave counters and specialized pulse counters. When selecting a frequency counter for microwave applications, engineers must distinguish between steady-state CW signals and transient, time-domain pulsed RF signals (commonly found in radar and electronic warfare systems). While CW counters excel at steady-state sensitivity and high-resolution tracking of stable carriers, pulsed counters incorporate specialized gating, wider bandwidth input paths, and high-speed detection architectures to resolve the carrier frequency within short signal bursts. We explore the mechanical and electrical trade-offs, referencing benchmark instruments to illustrate these capabilities.
- CW Frequency Range
- 10 Hz to 20 GHz or 40 GHz
- Pulse Frequency Range
- 500 MHz to 40 GHz
- Minimum Pulse Width
- 60 ns
- Pulse Repetition Frequency
- 50 Hz to 2 MHz
Continuous Wave (CW) Measurement Basics
CW microwave counters are designed for uninterrupted sinusoidal signals. Standard CW architectures utilize down-conversion techniques to bring microwave frequencies down to a range where digital counting circuits can process them.
Devices like the Keysight Technologies (Agilent HP) 5350B illustrate typical CW counter capabilities. It covers a frequency range of 10 Hz to 20 GHz, using an automatic amplitude discrimination feature to measure the frequency of the highest-amplitude signal present. Its tracking speed reaches 1 GHz/s, and its sensitivity goes down to -40 dBm, which is highly effective for low-level continuous-wave testing. Similarly, the portable Keysight Technologies (Agilent HP) 53150A provides ultra-wideband single-input coverage from 50 MHz to 20 GHz, performing simultaneous frequency and power measurements on CW signals.
The Challenge of Pulsed RF and Radar Signals
Measuring a pulsed RF signal introduces time-domain constraints that defeat standard CW counter architectures. A CW counter expects a continuous signal to gate and count over a set observation period. If the signal is gated on and off—as in radar pulses or electronic warfare systems—a standard CW counter will miscount, display erratic readings, or fail to acquire the signal entirely.
To measure pulsed RF, a counter must acquire the signal within the duration of a single pulse or build up a measurement across multiple coherent pulses. This requires specialized input processing and internal gating circuitry. The counter must handle specific signal parameters such as the pulse width (the duration the RF is on) and the pulse repetition frequency (PRF, or how often the pulses occur).
Pulse Counter Specifications and Architectures
High-performance pulse counters combine both pulse and CW measuring capability. For example, the Keysight Technologies (Agilent HP) 5361B can measure both CW and pulsed signals from 500 MHz to 40 GHz on its microwave channel (Channel 2), while its Channel 1 handles low-frequency signals from 10 Hz to 525 MHz.
When evaluating a pulse counter, engineers must consider several critical specifications:
- Pulse Width Range: The counter must be fast enough to capture short bursts. The Keysight Technologies (Agilent HP) 5361B supports a pulse width range of 60 ns to CW, meaning it can accurately measure a carrier pulsed for only 60 nanoseconds.
- Pulse Repetition Frequency (PRF): The rate of the pulses affects the acquisition timing. The Keysight Technologies (Agilent HP) 5361B accommodates a PRF range from 50 Hz to 2 MHz.
- Signal Integrity and Damage Levels: High-frequency inputs must handle varying power levels. For example, the Channel 2 input on the Keysight Technologies (Agilent HP) 5361B has a maximum input level of +7 dBm for pulsed signals and +5 dBm for CW, with a damage level of +25 dBm peak.
Example instruments
Frequently asked questions
- Can a CW counter like the Keysight 5350B measure pulsed radar signals?
- No. Standard CW counters like the Keysight Technologies (Agilent HP) 5350B or the Keysight Technologies (Agilent HP) 53150A lack the high-speed gating and pulse-detection circuitry required to capture short, transient RF bursts. They require a continuous wave to perform stable down-conversion and counting, and will fail or provide incorrect results on pulsed waveforms.
- What is the minimum pulse width supported by typical microwave pulse counters?
- The minimum pulse width varies by instrument design. For instance, the Keysight Technologies (Agilent HP) 5361B pulse counter has a minimum pulse width capability of 60 ns, allowing it to measure very short radar and communication pulses.
- What is the benefit of automatic amplitude discrimination in a CW counter?
- Automatic amplitude discrimination, found in models like the Keysight Technologies (Agilent HP) 5350B, allows the counter to automatically select and measure the frequency of the highest-amplitude signal present, ignoring lower-power spurious signals or noise.