Hubbry Logo
Continuous waveContinuous waveMain
Open search
Continuous wave
Community hub
Continuous wave
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Contribute something
Continuous wave
Continuous wave
Continuous wave
View on Wikipedia
from Wikipedia
Passband modulation
Analog modulation
Digital modulation
Hierarchical modulation
Spread spectrum
See also

A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration.[1] It may refer to e.g. a laser or particle accelerator having a continuous output, as opposed to a pulsed output.

By extension, the term continuous wave also refers to an early method of radio transmission in which a sinusoidal carrier wave is switched on and off. This is more precisely called interrupted continuous wave (ICW).[2] Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave signals produced by earlier spark gap type transmitters.

Radio

[edit]

Transmissions before CW

[edit]

Very early radio transmitters used a spark gap to produce radio-frequency oscillations in the transmitting antenna. The signals produced by these spark-gap transmitters consisted of strings of brief pulses of sinusoidal radio frequency oscillations which died out rapidly to zero, called damped waves. The disadvantage of damped waves was that their energy was spread over an extremely wide band of frequencies; they had wide bandwidth. As a result, they produced electromagnetic interference (RFI) that spread over the transmissions of stations at other frequencies.

This motivated efforts to produce radio frequency oscillations that decayed more slowly; had less damping. There is a direct relation between the rate of decay (the reciprocal of the time constant) of a damped wave and its bandwidth; the longer the damped waves take to decay toward zero, the narrower the frequency band the radio signal occupies, so the less it interferes with other transmissions. As more transmitters began crowding the radio spectrum, reducing the frequency spacing between transmissions, government regulations began to limit the maximum damping or "decrement" a radio transmitter could have. Manufacturers produced spark transmitters which generated long "ringing" waves with minimal damping.

Transition to CW

[edit]

It was realized that the ideal radio wave for radiotelegraphic communication would be a sine wave with zero damping, a continuous wave. An unbroken continuous sine wave theoretically has no bandwidth; all its energy is concentrated at a single frequency, so it doesn't interfere with transmissions on other frequencies. Continuous waves could not be produced with an electric spark, but were achieved with the vacuum tube electronic oscillator, invented around 1913 by Edwin Armstrong and Alexander Meissner. After World War I, transmitters capable of producing continuous wave, the Alexanderson alternator and vacuum tube oscillators, became widely available.

Damped wave spark transmitters were replaced by continuous wave vacuum tube transmitters around 1920, and damped wave transmissions were finally outlawed in 1934.

Key clicks

[edit]
"Key click" redirects here. For the sound of a Hammond organ, see Hammond organ § Tone generation.

In order to transmit information, the continuous wave must be turned off and on with a telegraph key to produce the different length pulses, "dots" and "dashes", that spell out text messages in Morse code, so a "continuous wave" radiotelegraphy signal consists of pulses of sine waves with a constant amplitude interspersed with gaps of no signal.

In on-off carrier keying, if the carrier wave is turned on or off abruptly, communications theory can show that the bandwidth will be large; if the carrier turns on and off more gradually, the bandwidth will be smaller. The bandwidth of an on-off keyed signal is related to the data transmission rate as: where is the necessary bandwidth in hertz, is the keying rate in signal changes per second (baud rate), and is a constant related to the expected radio propagation conditions; K=1 is difficult for a human ear to decode, K=3 or K=5 is used when fading or multipath propagation is expected.[3]

The spurious noise emitted by a transmitter which abruptly switches a carrier on and off is called key clicks. The noise occurs in the part of the signal bandwidth further above and below the carrier than required for normal, less abrupt switching. The solution to the problem for CW is to make the transition between on and off to be more gradual, making the edges of pulses soft, appearing more rounded, or to use other modulation methods (e.g. phase modulation). Certain types of power amplifiers used in transmission may aggravate the effect of key clicks.

Persistence of radio telegraphy

[edit]
A commercially manufactured paddle for use with electronic keyer to generate Morse code

Early radio transmitters could not be modulated to transmit speech, and so CW radio telegraphy was the only form of communication available. CW still remains a viable form of radio communication many years after voice transmission was perfected, because simple, robust transmitters can be used, and because its signals are the simplest of the forms of modulation able to penetrate interference. The low bandwidth of the code signal, due in part to low information transmission rate, allows very selective filters to be used in the receiver, which block out much of the radio noise that would otherwise reduce the intelligibility of the signal.

Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio transmitter. This mode is still in common use by amateur radio operators due to its narrow bandwidth and high signal-to-noise ratio compared to other modes of communication.

In military communications and amateur radio the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two. Aside from radio signals, Morse code may be sent using direct current in wires, sound, or light, for example. For radio signals, a carrier wave is keyed on and off to represent the dots and dashes of the code elements. The carrier's amplitude and frequency remain constant during each code element. At the receiver, the received signal is mixed with a heterodyne signal from a BFO (beat frequency oscillator) to change the radio frequency impulses to sound. Almost all commercial traffic has now ceased operation using Morse, but it is still used by amateur radio operators. Non-directional beacons (NDB) and VHF omnidirectional radio range (VOR) used in air navigation use Morse to transmit their identifier.

Radar

[edit]

Morse code is all but extinct outside the amateur service, so in non-amateur contexts the term CW usually refers to a continuous-wave radar system, as opposed to one transmitting short pulses. Some monostatic (single antenna) CW radars transmit and receive a single (non-swept) frequency, often using the transmitted signal as the local oscillator for the return; examples include police speed radars and microwave-type motion detectors and automatic door openers. This type of radar is effectively "blinded" by its own transmitted signal to stationary targets; they must move toward or away from the radar quickly enough to create a Doppler shift sufficient to allow the radar to isolate the outbound and return signal frequencies. This kind of CW radar can measure range rate but not range (distance).

Other CW radars linearly or pseudo-randomly "chirp" (frequency modulate) their transmitters rapidly enough to avoid self-interference with returns from objects beyond some minimum distance; this kind of radar can detect and range static targets. This approach is commonly used in radar altimeters, in meteorology and in oceanic and atmospheric research. The landing radar on the Apollo Lunar Module combined both CW radar types.

CW bistatic radars use physically separate transmit and receive antennas to lessen the self-interference problems inherent in monostatic CW radars.

Laser physics

[edit]

In laser physics and engineering, "continuous wave" or "CW" refers to a laser that produces a continuous output beam, sometimes referred to as "free-running," as opposed to a q-switched, gain-switched or modelocked laser, which has a pulsed output beam.

The continuous wave semiconductor laser was invented by Japanese physicist Izuo Hayashi in 1970.[citation needed] It led directly to the light sources in fiber-optic communication, laser printers, barcode readers, and optical disc drives, commercialized by Japanese entrepreneurs,[4] and opened up the field of optical communication, playing an important role in future communication networks.[5] Optical communication in turn provided the hardware basis for internet technology, laying the foundations for the Digital Revolution and Information Age.[6]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Continuous wave

View on Grokipedia
from Grokipedia
A continuous wave (CW) is a waveform characterized by constant amplitude and frequency, resulting in a steady power output over time without interruptions or pulses. This contrasts with pulsed waves, where energy is emitted in discrete bursts, allowing CW signals to maintain a continuous sinusoidal oscillation suitable for various transmission and detection applications. The development of continuous wave technology marked a pivotal advancement in radio communication during the early 20th century. In 1906, Reginald Fessenden achieved the first successful CW transmission using a high-frequency alternator transmitter at his Brant Rock station in Massachusetts, enabling voice and music broadcasts over long distances and surpassing the limitations of earlier spark-gap systems that produced damped waves. This innovation laid the foundation for modern radiotelegraphy and telephony, with widespread adoption by amateurs and professionals by the 1920s, as CW proved more efficient for Morse code signaling and spectrum utilization. CW finds extensive use across electromagnetics, , and acoustics due to its stable output. In radio and systems, CW enables precise Doppler shift measurements for velocity detection, as the continuous transmission allows simultaneous reception of reflected signals to compute frequency changes caused by moving targets. In lasers, CW operation provides uninterrupted light emission for applications like precision , cutting, and alignment in , where constant power ensures smooth material processing without thermal damage from pulses. Additionally, in and , CW techniques facilitate real-time flow monitoring and tissue analysis by injecting steady waves to detect or shifts without range issues common in pulsed methods.

Fundamentals

Definition and Properties

A continuous wave (CW) is an electromagnetic wave characterized by a constant power output over time, typically manifesting as a sinusoidal with unchanging and , in distinction from modulated signals that vary in , , or phase to convey , or pulsed signals that are intermittently transmitted. This steady emission ensures the wave propagates without interruptions, maintaining a consistent carrier signal suitable for applications requiring stable transmission. Key properties of a continuous wave include its fixed carrier , constant , and unbroken phase continuity, which collectively result in a pure, undamped sinusoidal form devoid of transient variations. The of the electric field component in is expressed as uE=12ϵ0E2u_E = \frac{1}{2} \epsilon_0 E^2, where ϵ0\epsilon_0 is the and EE is the instantaneous strength; the total electromagnetic combines this with the contribution, uB=B22μ0u_B = \frac{B^2}{2 \mu_0}, yielding u=ϵ0E2u = \epsilon_0 E^2 for plane waves where the electric and magnetic energies are equal. Mathematically, a continuous wave is represented as y(t)=Acos⁡(2πft+ϕ)y(t) = A \cos(2\pi f t + \phi), where AA denotes the constant , ff the fixed , tt time, and ϕ\phi the initial phase; in the , its power spectral density is a centered at ff, reflecting the monochromatic nature of the signal. The λ\lambda of a continuous wave relates inversely to its via λ=[c](/page/Speedoflight)[f](/page/Frequency)\lambda = \frac{[c](/page/Speed_of_light)}{[f](/page/Frequency)}, with cc being the in (3×1083 \times 10^8 m/s), allowing waves to span vast scales from meters in radio bands to micrometers in . Continuous waves operate across from kilohertz (kHz) in very low-frequency radio communications to terahertz (THz) in advanced imaging and sensing, adapting to diverse and interaction characteristics in each regime.

Generation and Detection

In the radio and regimes, continuous wave (CW) signals are generated using electronic oscillators that produce a stable sinusoidal at a fixed . Early generation methods relied on oscillators, where a high-vacuum tube served as the active device to sustain oscillations and generate continuous waves, particularly at higher frequencies. In modern systems, solid-state devices such as have largely replaced vacuum tubes, forming the basis of oscillators like the Colpitts configuration, which uses a transistor for amplification and feedback to produce clean sine waves suitable for CW transmission. oscillators, employing crystals via the piezoelectric effect, provide exceptional stability by replacing less precise LC tank circuits, achieving Q-factors of 20,000 to 200,000 that minimize drift from environmental factors. Following oscillation, the signal is amplified using power amplifiers to reach desired output levels, ensuring sufficient strength for transmission without distortion. Typical CW transmitters include simple LC oscillators for basic applications, where an inductor-capacitor network sets the resonant frequency, though these suffer from lower stability compared to crystal-based designs. More advanced transmitters incorporate Pierce crystal oscillators, utilizing a (e.g., ) with the in the feedback path to generate signals in the range of 40 kHz to over 100 MHz, ideal for precise communication and sensing. Detection of CW signals primarily involves coherent techniques for phase-sensitive measurement, where the incoming signal is mixed with a (LO) in a mixer to downconvert it while preserving and phase information, often using a or for enhanced sensitivity. detectors, consisting of a for rectification followed by a low-pass RC filter, can recover variations in CW signals, though they are less common for unmodulated carriers and more suited to amplitude-modulated contexts. Superheterodyne receivers tune to the carrier frequency by adjusting the LO to produce a fixed (IF), typically 30–75 MHz, allowing subsequent amplification and of the CW signal. Receivers for CW signals often employ bandpass filters to isolate the narrowband carrier from broadband noise and interference, enabling effective detection even in noisy environments. Key challenges in CW generation and detection include maintaining frequency stability, addressed by quartz crystals that limit deviations to ±20–100 ppm over temperature ranges like -40°C to +85°C, with aging drift as low as ±3 ppm in the first year for high-quality units. Short-term drift rates can be controlled below 1 Hz over seconds in stabilized oscillators, preventing signal misalignment. Signal-to-noise ratio (SNR) improvements are achieved through narrowband filtering, which rejects out-of-band noise while passing the CW signal's limited bandwidth, potentially boosting SNR by several dB in optical or RF systems.

Historical Development

Pre-CW Transmissions

Early radio transmissions relied on spark-gap transmitters, which produced damped electromagnetic waves through the discharge of electrical sparks across a gap. In 1887, demonstrated the existence of radio waves using a spark-gap setup to generate and detect oscillations, confirming James Clerk Maxwell's theoretical predictions. Building on this, developed practical in the 1890s, employing spark transmitters with monopole antennas to send signals over distances, achieving transatlantic communication by 1901. To overcome the limitations of damped waves, inventors explored undamped signal generation using arc transmitters and high-frequency alternators, though these early methods remained inefficient. Arc transmitters, pioneered by around 1902, utilized a sustained between carbon electrodes to produce continuous oscillations, enabling more stable but power-hungry signals suitable for experiments. Meanwhile, Ernst Alexanderson designed high-frequency alternators starting in 1904 at , which mechanically generated sinusoidal waves via rotating armatures, with early models outputting up to 2 kW at 100 kHz. Larger Alexanderson alternators later achieved powers of 10 to 200 kW at low frequencies between 10 and 100 kHz, primarily for long-wave applications. Pre-CW transmissions, particularly from spark-gap devices, exhibited broad spectral characteristics due to their transient, noise-like pulses, occupying wide bandwidths and causing significant interference among stations. These damped waves consisted of rapidly decaying oscillations, making precise tuning difficult as receivers struggled to filter signals without a steady carrier for . Arc and systems offered narrower spectra with undamped outputs but suffered from low , high mechanical complexity, and limited control, restricting their scalability. In the early , spark-gap transmitters saw widespread adoption in maritime and military communications for their simplicity and reliability in distress signaling. By 1904, the Royal Navy equipped ships with wireless sets using spark technology, enabling coordination during maneuvers and battles, such as the where Japanese vessels demonstrated its tactical value. These systems facilitated ship-to-shore and inter-ship messaging but were plagued by mutual interference in congested areas, underscoring the need for spectrum-efficient alternatives. A notable CW milestone occurred in 1906 when used an Alexanderson alternator to transmit the first voice and music broadcasts from Brant Rock, , on December 24, demonstrating the potential of continuous carriers for despite the technology's inefficiencies.

Transition to CW

The transition from damped spark transmissions to continuous wave (CW) radio began with early experiments in the late , driven by the need to overcome the limitations of spark systems, which produced broad, noisy signals prone to interference. In 1906, achieved the first successful CW transmission using a high-frequency , marking a foundational step toward stable, undamped signals suitable for both and voice. This innovation laid the groundwork for subsequent developments, though widespread adoption required further technological and regulatory advancements. Key inventions accelerated the shift. Lee de Forest's 1906 audion tube, a , enabled amplification and laid the basis for vacuum-tube oscillators essential to CW generation. In 1913, patented the regenerative feedback circuit, which provided stable oscillation for reliable CW transmitters by feeding a portion of the output signal back to the input, dramatically improving signal purity and efficiency. These vacuum-tube technologies supplanted earlier mechanical methods like Fessenden's alternators and Poulsen's arcs, offering compact, scalable solutions for commercial and maritime use. The 1912 sinking of the RMS Titanic, where spark transmitters' interference hampered distress signals, underscored the urgency for clearer communication and hastened regulatory reforms. In response, the International Radiotelegraph Convention of 1912 mandated improved radio practices for ships, including continuous monitoring and standardized equipment that favored CW for its reliability in emergencies. Concurrently, the U.S. prohibited inefficient spark transmitters on large vessels over 300 gross tons, requiring CW-capable systems to enhance safety and reduce interference. CW's technical superiority fueled its adoption: unlike spark transmissions, which occupied tens of kilohertz due to damped oscillations spreading energy across multiple frequencies, CW signals maintained a narrow bandwidth under 1 kHz, enabling sharper selectivity, longer range, and minimal interference. This efficiency proved vital for maritime and transoceanic links, with CW circuits spanning thousands of miles by the late . By the , CW had become dominant, outnumbering spark stations two-to-one by 1920 and achieving near-universal use in professional radio. operators embraced CW alongside innovations like superregenerative receivers, invented by Armstrong in 1922, which offered high sensitivity for detecting faint CW signals and proliferated in hobbyist sets throughout the decade.

Persistence of Telegraphy

Despite the advent of (AM) and (FM) technologies in the early , continuous wave (CW) telegraphy persisted due to its inherent simplicity, requiring only basic on-off keying of a carrier signal without complex modulation circuits. This minimalism made CW reliable in adverse conditions, such as during and subsequent conflicts, where it served and for precise, low-bandwidth messaging until the 1990s, when and digital systems began supplanting it. Additionally, CW's low power requirements enabled global reach with modest equipment; for instance, 5 watts of CW power could achieve distances comparable to 100 watts of voice transmission, conserving resources in remote or emergency scenarios. In niche applications, CW telegraphy found enduring roles among amateur radio operators, particularly in Morse code contests organized by bodies like the (ARRL), which host events such as the ARRL DX CW Contest to hone operating skills and test . It also underpinned international distress signaling until the Global Maritime Distress and Safety System (GMDSS) fully phased out on February 1, 1999, replacing manual with automated digital alerts for enhanced maritime safety. Furthermore, CW beacon stations, coordinated through projects like the International Beacon Project (IBP), continue to transmit Morse identifications on HF bands to monitor ionospheric conditions, aiding both amateur and professional radio users in predicting signal paths. The decline of CW telegraphy accelerated post-World War II as commercial broadcasting shifted to voice and data modes for broader accessibility, relegating CW to specialized uses. By 2003, the (ITU) revised its regulations at the World Radiocommunication Conference (WRC-03), eliminating the proficiency requirement for licenses worldwide, which further diminished its mandatory role in spectrum allocations. Echoes of CW's efficiency persist in modern digital modes like , a weak-signal protocol that achieves similar low-power, long-distance contacts by decoding signals down to -24 dB SNR, often outperforming traditional CW in noisy bands while requiring minimal operator intervention. In , CW training remains integral to emergency communications preparedness, with organizations like the ARRL emphasizing Morse for fallback operations in scenarios where voice or data fail, such as during natural disasters.

Radio Applications

CW Telegraphy

CW telegraphy employs on-off keying (OOK) of a to transmit International Morse code, where the carrier is modulated by briefly turning it on for dots and longer for , with defined intervals for spaces. In this system, a dot is represented by a short of one time unit duration, while a is a longer of three time units; the interval between elements (dots or ) within a character is one unit, between characters is three units, and between words is seven units. These timings, standardized for international radiotelegraphy, ensure consistent decoding across operators and equipment. Transmission involves manual keying of the carrier using a straight key or semi-automatic key (often called a "bug"), where the operator depresses the key to generate the on periods for dots and dashes according to the sequence. Straight keys require full manual control for each element, while semi-automatic keys mechanically produce consistent dash lengths and spaces through a vibrating , allowing the operator to focus on timing dots and character separations. At the receiver, decoding traditionally occurs aurally by trained operators who interpret the rhythmic tones at speeds of 20 to 40 (WPM), a proficiency developed through practice and enabling reliable copy under varying conditions. Early mechanical recorders, such as inkwriters or Mills (typewriter-based transcribers), automated initial decoding by printing dots and dashes on paper tape or typing characters, reducing operator fatigue for high-volume traffic. CW offers significant advantages in radio communications, particularly its ability to achieve high data rates within a narrow bandwidth, supporting up to 50 for rapid transmission while occupying as little as 50-100 Hz depending on keying speed. This efficiency stems from the binary-like on-off modulation, which concentrates power in the carrier and minimizes spectral spread, allowing selective filtering to enhance . In high-frequency (HF) bands, CW benefits from superior characteristics, enabling long-distance contacts via ionospheric reflection where wider-band modes like voice may fail due to and ; its narrow footprint permits operation in constrained allocations and excels in weak-signal scenarios, providing a 12-17 dB advantage over single-sideband voice. Essential equipment includes transmitters equipped with key jacks for connecting the manual or semi-automatic key, often integrated with a power to sustain the carrier during on periods. Receivers incorporate a (BFO) to heterodyned the incoming RF signal with a local tone generator, producing an audible beat note typically around 800 Hz for comfortable aural decoding; this is crucial since unmodulated CW appears silent without the BFO.

Key Clicks and Artifacts

Key clicks in continuous wave (CW) radio transmissions refer to broadband transient emissions generated by abrupt on/off keying of the carrier signal, resulting in spectral splatter that extends well beyond the intended narrowband signal. These transients occur when the transmitter is switched without gradual rise and fall times, producing sharp edges in the envelope that excite wide-frequency components due to the Fourier transform properties of sudden changes. In severe cases, such as square-wave keying, the spectrum can spread up to 25 kHz at -60 dB relative to the carrier, causing interference to adjacent channels. The primary causes of key clicks include imperfect keying waveforms lacking sufficient rise/fall times—often less than 5 ms—and non-linearities in transmitter stages like Class-C amplifiers, which amplify the transients into harmonic-rich splatter. This interference manifests as audible "clicks" or "thumps" to receivers on nearby frequencies, degrading copyability of weak signals in crowded bands and potentially violating good operating practices. Effects are exacerbated at higher keying speeds, where faster transitions widen the spectrum further, sometimes reaching several kilohertz of out-of-band emission. To mitigate key clicks, operators employ keying filters or shaped waveforms, such as raised cosine or Gaussian envelopes with 5-10 ms rise/fall times, which confine the bandwidth to under 600 Hz at -60 dB while maintaining readability. Beyond key clicks, other common artifacts in CW transmissions include , a perceptible shift during the key-down period caused by oscillator drift from or loading effects. For instance, in vintage crystal oscillators, keying can induce a downward drift of hundreds of Hertz over the first second due to heating or variations. Unintentional (FM) from mechanical vibrations or inadequate stabilization also generates unwanted sidebands, broadening the signal and mimicking low-level interference. These artifacts are typically measured using spectrum analyzers to visualize the envelope's content and ensure compliance with emission standards. Regulatory bodies like the FCC impose limits on CW emission bandwidth under 47 CFR §97.307 to prevent harmful interference, defining occupied bandwidth as the band where mean power is attenuated at least 26 dB below the total power, with further suppression required for out-of-band emissions. For narrow CW, practical limits and recommendations often target around 250 Hz to fit within allocated sub-bands, though no explicit numerical cap exists for all frequencies; historical complaints about key clicks were prevalent in crowded bands before the , when rudimentary equipment amplified such issues amid post-war band congestion.

Modern Radio Uses

In contemporary radio applications, continuous wave (CW) signals serve as foundational carriers in systems designed for precise time dissemination and ionospheric monitoring. Time-signal stations like WWV, operated by the National Institute of Standards and Technology (NIST), transmit CW markers including second pulses and minute markers at 100% modulation depth to provide global synchronization references. These broadcasts occur across multiple high-frequency (HF) bands, specifically 2.5 MHz (at 2500 W), 5 MHz (10,000 W), 10 MHz (10,000 W), 15 MHz (10,000 W), and 20 MHz (2500 W), enabling receivers worldwide to calibrate clocks and frequency standards with high accuracy. Similarly, ionospheric sounders employ CW modes to assess conditions by detecting levels and frequency shifts through single-frequency transmissions with extended integration times and narrow receiver bandwidths, operating in the 2.8–21.9 MHz range at intervals such as every 10 minutes. This CW-based approach facilitates and pre-scanning of HF channels, particularly in challenging high-latitude environments, yielding real-time data on ionospheric variability essential for reliable long-haul communications. Scientific monitoring leverages CW for specialized radio applications, including in and very low frequency (VLF) transmissions for operations. In , CW signals form the basis for interferometric arrays that combine continuous receptions from multiple telescopes to achieve high-resolution imaging of celestial sources, as the unmodulated carrier nature allows precise across baselines. Propagation beacons operating in CW mode further support scientific ionospheric studies by transmitting Morse-coded identifications, enabling researchers to map HF signal paths and atmospheric effects. For military and strategic purposes, VLF transmitters such as the AN/FRT-31 at NAA Cutler, , utilize CW carriers at 24 kHz (with 2 MW output) to penetrate up to 40 meters, providing submerged with command and control messages via narrowband (FSK) overlaid on the carrier. These systems, including historical installations like those at 15.5 kHz (NSS Annapolis) and 21.4/23.4 kHz (NPM Lualualei), ensure resilient one-way broadcasts critical for naval operations. Digital hybrid modes integrate CW as a carrier for weak-signal propagation testing, enhancing amateur and experimental radio practices. The Weak Signal Propagation Reporter (WSPR) mode, part of the WSJT-X software suite, employs CW-derived carriers shifted by continuous single tones (1400–1600 Hz audio offsets) to probe MF and HF paths, detecting signals as weak as -28 dB signal-to-noise ratio (SNR) for global reporting networks. This enables automated mapping of propagation without manual intervention, often combined with CW beacons for dual-mode transmission across 10 kHz to 230 MHz. Software-defined radios (SDRs) further amplify CW's role in long-distance (DX) communications by generating precise CW signals for DXing, offering advantages like spectrum visualization to identify faint Morse transmissions amid noise, multi-band simultaneous monitoring, and digital filtering that boosts weak-signal recovery by up to 20–30 dB in dynamic range compared to analog rigs. CW's persistence in modern radio stems from its inherent advantages in energy efficiency and jamming resilience, particularly for low-power deployments. In (IoT) sensors, CW modulation—essentially on-off keying (OOK)—minimizes transmitter duty cycles and circuitry complexity, achieving power consumptions below 1 mW during bursts, which extends battery life in remote environmental monitors by factors of 5–10 over complex schemes like (). This efficiency suits ultra-low-power beacons in wireless sensor networks, where CW carriers enable sporadic data pings with minimal overhead. Additionally, CW's narrow bandwidth (typically <100 Hz) provides robustness against jamming, as receivers can employ sharp filters to isolate the signal, maintaining communication links in contested spectra where broader modulations degrade by 10–15 dB under interference. These traits underpin CW's niche in jammed environments, such as tactical networks, where frequency agility and low detectability preserve operational integrity.

Radar Applications

Basic CW Radar

Basic CW radar systems transmit an unmodulated continuous wave signal and simultaneously receive echoes from targets, mixing the received signal with the transmitted signal in a receiver to generate a beat that corresponds to the Doppler shift caused by moving objects. This homodyne receiver architecture simplifies the design by using the transmit signal directly as the local oscillator for downconversion, enabling direct extraction of the Doppler information without intermediate stages. The Doppler shift fdf_d in a basic CW radar is determined by the formula fd=2vfcf_d = \frac{2 v f}{c} where vv is the of the target relative to the radar, ff is the transmitted frequency, and cc is the . This shift produces an audio-frequency beat signal proportional to the , allowing precise speed measurement but introducing range ambiguity since the continuous transmission provides no timing reference for calculation. Without modulation, absolute range cannot be resolved, limiting the system to velocity sensing and motion detection, as stationary targets produce no Doppler shift and cannot be distinguished from clutter. Detection reliability depends on achieving a sufficient (SNR) to ensure reliable target identification above the noise floor. These systems operate at low transmit powers, often in the milliwatt (mW) range, making them suitable for short-range applications where high power is unnecessary and compactness is prioritized. Early examples include experimental German CW developments in , such as the system demonstrated in that detected a in harbor, which laid groundwork for wartime applications. Modern uses encompass intrusion alarms that detect motion across perimeters via Doppler signatures, simple speed guns employed for , where low-power operation suffices for ranges up to several hundred meters, and non-contact vital signs monitoring for detecting and respiration in medical and security applications.

Frequency-Modulated CW Radar

Frequency-modulated continuous wave (FMCW) radar employs a linear frequency sweep, known as a , across a bandwidth BB over a sweep duration TT, enabling range measurement through the resulting beat frequency fbf_b in the mixed received and transmitted signals. The beat frequency is given by fb=2RB[c](/page/Speedoflight)Tf_b = \frac{2 R B}{[c](/page/Speed_of_light) T}, where RR is the target range and cc is the . This modulation allows FMCW systems to resolve target distances without the need for pulsed transmissions, distinguishing it from unmodulated CW radar that primarily measures via Doppler shift. Key advantages of FMCW radar include high range resolution defined by ΔR=c2B\Delta R = \frac{c}{2 B}, which achieves values as fine as 15 cm with a 1 GHz bandwidth, the ability to measure range and velocity simultaneously through phase across multiple chirps, and elimination of precise pulse timing requirements, reducing compared to pulsed radars. These features enable low-power operation with continuous transmission, improving sensitivity via extended integration time while maintaining multi-target discrimination. FMCW radar finds widespread use in automotive applications, such as 77 GHz systems for advanced driver-assistance systems (ADAS) introduced by manufacturers like in the late 1990s and expanded in the 2000s for collision avoidance and . As of 2025, FMCW has become essential for higher levels of vehicle autonomy (Level 3 and above), with 4D imaging variants providing three-dimensional mapping for enhanced in robotaxis and self-driving cars. It also serves in aircraft altimeters for precise height measurement above , leveraging its resistance to environmental interference. In weather sensing, FMCW variants provide high-resolution profiling of and depth from airborne platforms. Implementation typically involves sawtooth or triangular waveforms to generate the , with the received signal processed via (FFT) to extract the beat frequency spectrum for range determination. Triangular modulation facilitates velocity estimation in a single sweep by comparing up- and down-s, while sawtooth requires multiple sweeps for Doppler resolution. Challenges from non-linear s, caused by imperfections, are addressed through algorithms that apply phase corrections or to restore and maintain accuracy.

Optical Applications

CW Lasers

In optics, continuous wave (CW) lasers produce a steady beam of light with constant output power ranging from milliwatts to kilowatts, without the intermittent pulsing characteristic of other laser modes. This steady emission is achieved by continuously pumping the gain medium—typically via electrical discharge for gas lasers, optical excitation for solid-state lasers, or forward electrical bias for semiconductors—to sustain lasing action over extended periods. Unlike pulsed lasers, which deliver energy in short bursts, CW operation ensures a uniform irradiance suitable for applications requiring consistent illumination. Various types of CW lasers exist, categorized by their gain media. Gas lasers, such as the helium-neon (He-Ne) , operate at a wavelength of 632.8 nm in the visible spectrum and are valued for their simplicity and reliability. Solid-state CW lasers, like the neodymium-doped aluminum garnet (Nd:YAG) at 1064 nm in the near-, use crystalline hosts pumped by lamps or s for higher powers. Semiconductor lasers, compact and efficient, emit at diverse wavelengths from to , depending on the material composition, and dominate in consumer and industrial uses due to their low cost and direct electrical pumping. These systems generally maintain output stability with power fluctuations below 1%, often achieved through active feedback mechanisms. The core principle of CW laser operation involves maintaining a steady in the gain medium, where more atoms or molecules occupy the upper than the lower one, enabling net . This inversion is balanced by continuous pumping to counteract and losses, with lasing occurring above a threshold where gain equals cavity losses. The threshold pump power PthP_{th} can be expressed as Pth=hνΔNστ,P_{th} = \frac{h\nu \Delta N}{\sigma \tau}, where hνh\nu is the , ΔN\Delta N represents the required density, σ\sigma is the cross-section, and τ\tau is the lifetime of the upper ; this formula derives from rate equations governing and in . In practice, the gain medium is placed within a resonant cavity to amplify the recursively until equilibrium is reached. A key advantage of CW lasers is their high temporal and spatial coherence, with stabilized He-Ne models achieving coherence lengths on the order of kilometers, far exceeding typical incoherent sources. This property supports precise applications such as optical alignment in and , where a stable beam ensures accurate targeting over long distances, and , where extended coherence enables interference patterns for three-dimensional imaging. Additionally, CW lasers serve as a key external light source for silicon photonics and co-packaged optics (CPO) technology, enabling high-speed optical modules such as 800G and 1.6T for data transmission in AI data centers and server interconnects. These attributes make CW lasers indispensable in fields demanding uninterrupted, high-fidelity light output.

Laser Physics and Uses

In continuous-wave (CW) laser operation, the steady-state behavior is governed by rate equations describing the dynamics of the photon number nn inside the cavity and the population inversion NN between the lasing levels. The rate equation for the photon number is given by dndt=ΓσNnhν−nτc+Rsp,\frac{dn}{dt} = \frac{\Gamma \sigma N n}{h \nu} - \frac{n}{\tau_c} + R_{sp}, where Γ\Gamma is the confinement factor, σ\sigma is the stimulated emission cross-section, hνh\nu is the , τc\tau_c is the cavity photon lifetime, and RspR_{sp} accounts for noise; for the inversion, dNdt=R−γN−ΓσNnhν,\frac{dN}{dt} = R - \gamma N - \frac{\Gamma \sigma N n}{h \nu}, with RR as the pump rate and γ\gamma the inversion decay rate. In steady-state CW conditions, derivatives are zero, yielding n=n0(N−Nth)n = n_0 (N - N_{th}) where NthN_{th} is the threshold inversion, enabling constant output power without oscillations. The fundamental linewidth of a CW laser, limited by quantum phase noise, is described by the Schawlow-Townes formula: Δν=hν(Δνc)24πPout,\Delta \nu = \frac{h \nu (\Delta \nu_c)^2}{4 \pi P_{out}}, where Δνc\Delta \nu_c is the cold-cavity linewidth and PoutP_{out} is the output power; this predicts linewidths below 1 Hz for high-power systems with long cavities. To achieve single-mode operation and mode stability in CW lasers, intracavity etalons filter unwanted longitudinal modes, selecting a single with linewidths narrowed to mHz levels. locking to external references, such as in the iodine-stabilized He-Ne at 633 nm, uses in 127^{127}I2_2 vapor to stabilize the output to hyperfine transitions, achieving fractional stability of 10−1210^{-12} over seconds. CW lasers find essential applications in , where the CO2_2 at 10.6 μ\mum delivers continuous thermal energy for precise soft-tissue , enabling and with minimal in procedures like removal. In optical communications, distributed feedback (DFB) lasers at 1550 nm provide stable CW output for long-haul fiber-optic transmission, leveraging low attenuation in silica fibers for data rates exceeding 100 Gbps over thousands of kilometers. For high-resolution , tunable CW lasers, pumped by argon-ion sources, offer continuous coverage from 400 to 800 nm with sub-MHz linewidths, facilitating Doppler-free studies of atomic and molecular spectra. Post-2000 advancements have produced ultrastable CW lasers critical for optical clocks, where external-cavity lasers locked to Fabry-Pérot cavities achieve Allan deviations below 10−1510^{-15} at 1 s, enabling and lattice clocks with uncertainties rivaling cesium standards. Power scaling in fiber lasers has reached 100 kW CW output by the , using -doped architectures with coherent beam combination to maintain near-diffraction-limited quality for industrial and cutting of thick metals.

References

  1. https://www.sciencedirect.com/topics/engineering/continuous-wave
  2. https://www.comsoc.org/node/28631
  3. https://earlyradiohistory.us/1922smit.htm
  4. https://www.radartutorial.eu/02.basics/Continuous%20Wave%20Radar.en.html
  5. https://www.xometry.com/resources/sheet/continuous-wave-laser/
  6. https://openfnirs.org/2024/01/01/continuous-wave-spectroscopy/
  7. https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/University_Physics_II_-_Thermodynamics_Electricity_and_Magnetism_(OpenStax)/16%3A_Electromagnetic_Waves/16.04%3A_Energy_Carried_by_Electromagnetic_Waves
  8. https://pages.mtu.edu/~scarn/teaching/GE4250/ComplexWave_lecture.pdf
  9. https://science.[nasa](/page/NASA).gov/ems/02_anatomy/
  10. https://www.[sciencedirect](/page/ScienceDirect).com/topics/engineering/continuous-wave
  11. https://pressbooks.online.ucf.edu/phy2053bc/chapter/the-electromagnetic-spectrum/
  12. http://www.tubebooks.org/Books/vto.pdf
  13. https://www.nutsvolts.com/magazine/article/seven-common-ways-to-generate-a-sine-wave
  14. https://www.electronics-tutorials.ws/oscillator/crystal.html
  15. https://www.sciencedirect.com/topics/engineering/coherent-detection
  16. https://www.sciencedirect.com/topics/computer-science/envelope-detector
  17. https://www.radartutorial.eu/09.receivers/rx05.en.html
  18. https://www.allaboutcircuits.com/technical-articles/characterizing-frequency-deviations-of-quartz-crystals-frequency-tolerance-frequency-stability-and-aging/
  19. https://ewh.ieee.org/reg/7/millennium/radio/radio_radioscientist.html
  20. https://ethw.org/Milestones:First_Wireless_Radio_Broadcast_by_Reginald_A._Fessenden%2C_1906
  21. https://ethw.org/Ernst_F._W._Alexanderson
  22. https://www.jmag-international.com/engineers_diary/079/
  23. https://teslaresearch.jimdofree.com/invention-of-radio/spark-gap-transmitter/
  24. https://digital-library.theiet.org/doi/pdf/10.1049/esej%253A19970108
  25. https://rnzncomms.files.wordpress.com/2011/01/history-of-naval-ships-wireless-systems.pdf
  26. https://jproc.ca/rrp/nro_his.html
  27. https://www.megger.com/en-us/et-online/march-2020/have-you-ever-heard-of-the-alexanderson-alternator
  28. https://nationalmaglab.org/magnet-academy/history-of-electricity-magnetism/museum/audion-1906/
  29. https://www.nps.gov/subjects/nationalhistoriclandmarks/edwin-h-armstrong-house.htm
  30. https://www.worldradiohistory.com/BOOKSHELF-ARH/History/The-Continuous-Wave-1900-1932-Aitken-1985.pdf
  31. https://www.nist.gov/blogs/taking-measure/nist-and-titanic-how-sinking-ship-improved-wireless-communications-navigating
  32. https://antiquewireless.org/wp-content/uploads/53-below_535.pdf
  33. https://earlyradiohistory.us/1922sup.htm
  34. https://vu2nsb.com/cw-digital-radio/cw-radiotelegraphy/
  35. https://internationalcwcouncil.org/history-of-morse-code/
  36. https://tigrettod.com/blogs/tigrett-outdoors-blog/what-is-cw-in-ham-radio
  37. https://contests.arrl.org/dxcw/
  38. https://wwwcdn.imo.org/localresources/en/OurWork/Safety/Documents/GMDSSandSAR1999.pdf
  39. https://www.ncdxf.org/beacon/
  40. https://eclecticlight.co/2015/10/20/the-code-lives-on-how-morse-is-still-not-dead/
  41. https://www.k0nr.com/wordpress/2025/03/weak-signal-performance/
  42. https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1677-0-200405-S!!PDF-E.pdf
  43. https://pa3fwm.nl/technotes/tn31-morse-timing.html
  44. https://www.nutsvolts.com/magazine/article/March2017_Morse-Code-Makes-a-Comeback
  45. https://wb3gck.com/2018/11/10/u-s-navy-morse-code-training/
  46. https://www.navy-radio.com/morse.htm
  47. https://collection.powerhouse.com.au/object/213746
  48. https://www.w8ji.com/cw_bandwidth_described.htm
  49. https://hamstudy.org/browse/E2_2022/T8A
  50. https://www.afcea.org/signal-media/high-frequency-communications-features-highs-and-lows
  51. https://www.eham.net/article/32380
  52. https://www.electronics-notes.com/articles/ham_radio/morse_code/what-is-the-morse-code.php
  53. http://www.icarc.org/pdf/CW_friday_1.pdf
  54. https://www.ivarc.org.uk/uploads/1/2/3/8/12380834/keyclicks_version_1.pdf
  55. https://www.w8ji.com/what_causes_clicks.htm
  56. https://www.rfcafe.com/references/qst/why-key-clicks-october-1966-qst.htm
  57. https://www.w8ji.com/keyclicks.htm
  58. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D/part-97/subpart-D/section-97.307
  59. https://www.researchgate.net/publication/266378562_Benefits_of_Coherent_Low-IF_for_Vital_Signs_Monitoring_Using_Doppler_Radar
  60. https://skynet.ee.ic.ac.uk/notes/Radar_6_CW_Radar.pdf
  61. https://www.idc-online.com/technical_references/pdfs/electronic_engineering/Continuous_Wave_Radar.pdf
  62. https://vc.airvectors.net/ttwiz_07.html
  63. https://www.nhtsa.gov/sites/nhtsa.gov/files/812266-downroadradarmodule.pdf
  64. https://corescholar.libraries.wright.edu/etd_all/620/
  65. https://ieeexplore.ieee.org/document/797891/
  66. https://www.armms.org/media/uploads/mike-roberts---armms-nov-2019.pdf
  67. https://barnardmicrosystems.com/media/features_papers/DSTO-TR-1939_PR.pdf
  68. https://www.idtechex.com/en/research-report/automotive-radar-market-2025/1061
  69. https://core.ac.uk/download/pdf/80723506.pdf
  70. https://www.mdpi.com/2079-9292/8/11/1290
  71. https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1198&context=physsp
  72. https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1516
  73. https://ocw.mit.edu/courses/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/18f30fad63a62ef4dd894d3752b55a60_chapter7.pdf
  74. https://www.mi-lasers.com/wp-content/uploads/MG-stablized-HeNe.pdf
  75. https://futurumgroup.com/insights/coherent-q1-fy-2026-shows-strong-datacenter-comms-momentum/
  76. https://newsletter.semianalysis.com/p/co-packaged-optics-cpo-book-scaling
  77. https://sites.unimi.it/aqm/wp-content/uploads/schawlow-townes-1958.pdf
  78. https://pubmed.ncbi.nlm.nih.gov/21452010/
  79. https://www.furukawa.co.jp/review/fr023/fr23_01.pdf
  80. https://tf.nist.gov/general/pdf/882.pdf
  81. https://link.aps.org/doi/10.1103/RevModPhys.87.637
  82. https://www.researchgate.net/publication/268807896_100_kW_CW_Fiber_Laser_for_Industrial_Applications
Add your contribution
Related Hubs
Contribute something
User Avatar
No comments yet.