CW Sweep
Understanding CW Sweep
Every VNA defaults to its most familiar mode: sweeping frequency across a defined span while holding source power constant. This produces the classic S-parameter frequency response plot. But frequency sweeps cannot answer a critical question about active devices: what happens to gain and phase as you drive the amplifier harder?
A CW Sweep flips the measurement axis. The source locks onto a single frequency, and the VNA instead steps the source power from a low level (deep in the linear region) up through saturation. At each power step, the VNA measures the complex S21 (magnitude and phase). The result is a gain-versus-input-power curve that reveals exactly where the amplifier compresses, how much AM-PM distortion it generates, and at what power level it reaches saturation.
CW Power Sweep vs. CW Time Sweep
There are two distinct CW Sweep modes. In a CW Power Sweep, the VNA increments source power at each data point while holding frequency fixed. This is the workhorse measurement for P1dB, P3dB, and Psat characterization. In a CW Time Sweep, the VNA holds both frequency and power fixed, then records S-parameters continuously over time. CW Time Sweep is used to observe thermal drift, oscillation onset, bias circuit settling, and long-term gain stability after power-up.
ΔG = |S21(Pin)| − |S21ss|
P1dB Definition:
P1dBin = Pin where ΔG = −1 dB
Output P1dB:
P1dBout = P1dBin + |S21(P1dBin)|
AM-PM Coefficient:
KAM-PM = d(∠S21) / d(Pin) [degrees/dB]
Example: A Ka-band PA with small-signal gain of 25 dB and P1dBin = +5 dBm has P1dBout = +5 + 24 = +29 dBm (gain at P1dB is 24 dB, 1 dB below small-signal).
CW Sweep Configuration Comparison
| Parameter | CW Power Sweep | CW Time Sweep | Frequency Sweep |
|---|---|---|---|
| Swept variable | Source power (e.g., −30 to +10 dBm) | Time (e.g., 0 to 60 seconds) | Frequency (e.g., 1 to 40 GHz) |
| Fixed variables | Frequency, bias | Frequency, power, bias | Power, bias |
| Primary output | Gain vs. Pin, Phase vs. Pin | Gain vs. time, Phase vs. time | S-parameters vs. frequency |
| Key measurement | P1dB, Psat, AM-PM | Thermal drift, oscillation | Return loss, insertion loss, group delay |
| Typical points | 201 to 401 power steps | 1001+ time samples | 201 to 1601 frequency points |
| IFBW setting | 1 to 10 kHz (slow, accurate) | 100 Hz to 1 kHz | 1 to 100 kHz |
Frequently Asked Questions
What is the difference between a CW Sweep and a frequency sweep on a VNA?
A frequency sweep steps the source across a range of frequencies while holding power constant. A CW Sweep locks the source to one frequency and sweeps input power from low to high, or records data over time. CW Sweep is how you characterize compression point, AM-PM distortion, and thermal drift, none of which are visible in a standard frequency sweep.
How do you measure P1dB using a CW Sweep?
Set the VNA to CW mode at the frequency of interest. Configure a power sweep from well below compression (say −30 dBm) up through saturation (say +10 dBm). The VNA measures S21 at each power step. At low power, gain is constant (linear region). As input power increases, the amplifier saturates and S21 drops. The input power where S21 has fallen exactly 1 dB below its small-signal value is P1dB. Most modern VNAs calculate this automatically with a compression marker.
Why does AM-PM distortion matter and how does CW Sweep measure it?
AM-PM conversion describes how changes in input amplitude cause unintended phase shifts at the output. As the amplifier compresses, junction capacitances change with signal swing, shifting output phase. A CW power sweep measures both magnitude and phase of S21 at each power step. The slope of phase versus input power (degrees per dB) is the AM-PM coefficient. For 64-QAM or 256-QAM signals, even 2 to 3 degrees of AM-PM distortion can cause EVM failures.