Specific RF Devices

CW Source

Q: What is the difference between a CW source and a signal generator?

A CW source is the simplest case of a signal generator: it produces a single unmodulated tone at one frequency and one power level. A full signal generator adds modulation capability (AM, FM, phase, pulse, or vector I/Q), sweep, list, and ramp modes, and arbitrary waveform playback. Every signal generator can operate as a CW source by disabling modulation, but a dedicated CW source omits the modulation hardware. This makes it cheaper, often lower in phase noise because there is no wideband modulator in the signal path, and well suited to applications such as local oscillator substitution, blocker injection, and continuous stimulus for amplifier or filter characterization.

Q: How much harmonic and phase noise does a typical CW source have?

A good benchtop microwave CW source delivers harmonics around minus 30 to minus 60 dBc and non-harmonic spurious below minus 60 dBc. Single-sideband phase noise at 10 GHz is typically minus 110 to minus 130 dBc per hertz at a 10 kHz offset for a high-performance synthesized source, degrading roughly 6 dB for every doubling of carrier frequency because multiplication adds 20 log N of phase noise. Output power flatness is usually within plus or minus 1 dB across the band, and amplitude stability is on the order of 0.01 dB over short intervals. Frequency accuracy is set by the internal reference, commonly an oven-controlled crystal at 1 part in 10 to the 8 or better when locked to an external 10 MHz reference.

Q: Why use a CW source instead of a pulsed source for amplifier testing?

A CW source applies continuous power, which represents the worst-case thermal load on a device under test. This is the right stimulus for measuring saturated output power, gain compression, and steady-state efficiency, and for thermal qualification of power amplifiers. A pulsed source duty-cycles the RF on and off, reducing average dissipation so the device stays near its quiescent temperature, which is useful for load-pull and high-power GaN measurements where CW would overheat the part. CW also gives the cleanest single-tone spectrum for spurious, harmonic, and phase noise measurements where pulse modulation sidebands would obscure the result.

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Generating an unmodulated continuous-wave carrier at a single fixed frequency with constant amplitude, a CW source provides the cleanest possible stimulus for RF and microwave measurement. It is the unmodulated subset of a full signal generator: one tone, one power level, no AM, FM, or pulse envelope. Because the output is steady-state, a CW source is the standard stimulus for saturated power and gain-compression testing of amplifiers, for receiver blocker and selectivity tests, and as a substitute local oscillator. Performance is judged by spectral purity, with harmonics held to roughly −30 to −60 dBc and single-sideband phase noise around −110 to −130 dBc/Hz at 10 kHz offset for high-grade synthesized units. Most modern CW sources are derived from a locked frequency synthesizer referenced to a 10 MHz standard, giving frequency accuracy of 1 part in 10⁸ or better.

Category: Specific RF Devices

Typical Harmonics: −30 to −60 dBc

Power Flatness: ±1 dB

Spectral Purity of a Single-Tone Carrier

The defining attribute of a CW source is that it puts essentially all of its energy into one spectral line. In the ideal case the output is a pure sinusoid v(t) = A·cos(2πf₀t), a delta function in the frequency domain. Real sources depart from that ideal in three measurable ways: harmonic distortion (energy at integer multiples 2f₀, 3f₀, and so on), non-harmonic spurious tones from reference feedthrough and synthesizer fractional-N artifacts, and a noise pedestal around the carrier that is quantified as phase noise. A high-quality benchtop unit holds harmonics at −30 dBc when running its output amplifier near compression, improving to −60 dBc or better when backed off, and keeps spurious below −60 dBc across the band.

Frequency stability comes from the reference oscillator rather than the microwave hardware itself. A synthesized CW source phase-locks a tunable oscillator, often a YIG-tuned oscillator or a fractional-N VCO loop, to an internal oven-controlled crystal or to an external 10 MHz house standard. Because frequency multiplication scales phase noise by 20·log(N), a source that synthesizes 40 GHz from a 100 MHz reference inherits about 52 dB of added phase noise relative to the reference, which is why millimeter-wave CW sources are far harder to keep clean than HF or VHF units.

Output power must be both accurate and flat. Internal automatic level control (ALC) loops sense forward power with a diode detector and adjust a PIN-diode or vector attenuator to hold the set level within roughly ±1 dB across a multi-octave span, with finer leveling possible against an external power sensor. For receiver and component testing the absolute level matters because it sets the calibration of insertion loss, gain, and compression measurements.

Power, Level, and Phase-Noise Relations

Ideal CW waveform:
v(t) = A × cos(2πf₀t) → single line at f₀

Power in dBm (50 Ω system):
P_dBm = 10 × log₁₀(P_mW) → 0 dBm = 1 mW; +10 dBm ≈ 0.71 V_rms

Multiplied phase noise:
L_out(f) ≈ L_ref(f) + 20 × log₁₀(N) dBc/Hz

Harmonic distortion (relative):
H_n = 10 × log₁₀(P_nf0 / P_f0) dBc

Where A = peak amplitude, f₀ = carrier frequency, N = multiplication ratio, L = single-sideband phase noise. Example: a 100 MHz reference at −150 dBc/Hz multiplied to 10 GHz (N = 100) yields ≈ −110 dBc/Hz, ignoring loop noise.

CW Source vs. Other RF Stimulus Types

Source Type	Output	Modulation	Typical SSB Phase Noise (10 GHz, 10 kHz)	Best Use
CW source	Single tone, fixed f, fixed power	None	−110 to −130 dBc/Hz	Compression, LO substitution, spectral purity tests
Swept (CW sweep) source	Stepped or ramped frequency	None	−105 to −125 dBc/Hz	Filter and network response, return loss
Pulsed source	Gated CW envelope, low duty cycle	Pulse	Carrier dependent	Radar, high-power GaN load-pull
Vector signal generator	Modulated I/Q waveform	AM/FM/φ/I-Q	−100 to −120 dBc/Hz	5G, Wi-Fi, EVM and BER testing
Free-running oscillator	Single tone, unlocked	None	Worse, drifts with temperature	Low-cost embedded LO, beacons

Common Questions

Frequently Asked Questions

What is the difference between a CW source and a signal generator?

A CW source is the simplest signal generator: one unmodulated tone at a fixed frequency and power. A full signal generator adds AM, FM, phase, pulse, and vector I/Q modulation, plus sweep and list modes. Any signal generator can run as a CW source by turning modulation off, but a dedicated CW source omits that hardware, making it cheaper and often lower in phase noise, ideal for LO substitution and continuous amplifier or filter stimulus.

How much harmonic and phase noise does a typical CW source have?

A good microwave CW source holds harmonics near −30 to −60 dBc and non-harmonic spurious below −60 dBc. SSB phase noise at 10 GHz is roughly −110 to −130 dBc/Hz at 10 kHz offset, degrading about 6 dB per octave of carrier frequency because multiplication adds 20·log(N). Power flatness is within ±1 dB, and frequency accuracy is 1 part in 10⁸ or better when locked to a 10 MHz reference.

Why use a CW source instead of a pulsed source for amplifier testing?

CW applies continuous power, the worst-case thermal load, which is correct for measuring saturated output power, gain compression, steady-state efficiency, and thermal qualification of amplifiers. A pulsed source duty-cycles the RF so the device stays near its quiescent temperature, which suits high-power GaN load-pull where CW would overheat the part. CW also gives the cleanest single-tone spectrum for harmonic, spurious, and phase-noise measurements.

Related Terms

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