Confronting Measurement Uncertainty in Signal Generation - Part 5: Phase Noise
2019-05-30 | 9 min read
Signal generators produce the signals that you need ranging from simple to complex, and from pure to dirty, to test your design. You use signal generators as a local oscillator (LO), a golden transmitter, or an interference signal to test and characterize devices under test. The fifth, in my series of posts on signal generation and how to obtain the best signal quality deals with phase noise. Previous posts in the series addressed Waveform Discontinuity, Interpolation Shoot, Waveform Sampling Issues, and I/Q Impairments.
The phase noise performance of a signal generator is a key factor in determining test applications, such as radar receiver sensitivity, analog-to-digital conversion (ADC) characterization, and digital modulation. It impacts the signal quality in many aspects and causes measurement uncertainty. In this post, I will explain what phase noise is, why it matters for your measurements, and how to optimize its profile for your test applications.
What Is Phase Noise?
The diagram on the left in Figure 1 shows an ideal sinusoidal waveform in both time and frequency domain. In the frequency domain, the signal is a single spectral line. However, in the real world, there are always small and unwanted amplitude and phase fluctuations present on the signal as shown on the right in Figure 1. In the frequency domain, the signal is no longer a discrete spectral line, but spreads power into different frequencies.
Figure 1. An ideal and a real sinusoidal signal
Phase noise describes the frequency stability of an oscillator and is the noise spectrum around the oscillator’s signal in the frequency domain. Phase noise is usually measured as the single-sideband (SSB) power within one-hertz bandwidth at a specific frequency away from the carrier frequency. Figure 2 shows an SSB phase noise measurement result using a Keysight UXA signal analyzer.
Figure 2. An SSB phase noise measurement
When Phase Noise Matters?
Signal generator phase noise performance can be a limiting factor for specific applications in aerospace and defense, as well as in digital communications. These applications require signal generators with ultra-low phase noise at different frequency offsets. You need to understand what the frequency offset you care most first.
In radar systems, good phase noise is critical for stable local oscillators and coherent oscillators because these signals are at the heart of the radar systems. For example, a radar receiver cannot identify the moving object if the downconverted signal of interest is masked by the poor phase noise as shown in Figure 3. The Doppler shift is proportional to the radar frequency multiplied by the radial velocity, divided by the propagation velocity. The frequency shift might range from hundreds of Hz to hundreds of kHz.
Figure 3. Poor LO phase noise impacts radar receiver sensitivity
Figure 4 shows a simplified QPSK digital receiver block diagram. The phase noise of the LO signal is translated into the output of the mixers. The direct effect of phase noise on the constellation diagram is the radial smearing of the symbols (as shown in green). For a higher order modulation scheme (e.g. 256 QAM), the symbols are closer. The symbols smearing results in a bad receiver sensitivity and higher bit error rate (BER).
Figure 4. A simplified digital receiver block diagram with poor phase noise LO
Orthogonal Frequency-Division Multiplexing (OFDM)
OFDM is a popular modulation scheme for wideband digital communication. It uses many closely spaced orthogonal sub-carrier signals to transmit data in parallel. During frequency conversion with a poor phase noise local oscillator (LO), the sub-carrier with phase noise spreads into other sub-carriers as interference. The phase noise degrades the modulation quality of the OFDM signal. Table 1 illustrates the sub-carrier spacing of modern wireless standards using an OFDM modulation scheme.
Table 1. Sub-carrier spacing of OFDM signals
Optimize Phase Noise Performance
Adjust Reference Oscillator Bandwidth
At frequency offsets below approximately 1 kHz, the internal or external frequency reference determines the frequency stability and phase noise. It is straightforward to have a stable and extremely low phase noise reference oscillator that improves the carrier’s phase noise in the offset frequency range below 1 kHz.
Keysight PSG signal generator allows you to adjust reference oscillator bandwidth (also known as loop bandwidth) in the signal generator in fixed steps for either an internal or external 10 MHz frequency reference as shown in Figure 5. You can optimize the phase noise performance of the signal generator for your applications.
Figure 5. Phase noise measurements with different reference oscillator’ phase-lock loop (PLL) bandwidth adjustments
Adjust Phase-Look Loop Bandwidth
In a signal generator’s synthesizer, the phase-lock loop (PLL) bandwidth also impacts phase noise performance. Keysight PSG signal generators allow you to adjust the loop bandwidth to optimize phase noise you need for the test applications as shown in Figure 6. The light blue curve is optimized for < 150 kHz frequency offset and the yellow curve is for > 150 kHz. Evaluate your application to choose the appropriate phase noise setting for wider offset frequencies.
Figure 6. Optimize pedestal phase noise at synthesizer session
Phase Noise Impairment
Optimizing phase noise performance is not always necessary or desirable. Some applications and tests require a specific amount of phase noise for an accurate signal substitution or tolerance testing.
The Keysight N5182B/N5172B RF signal generator allows users to adjust synthesizer phase noise impairment. Using this feature, you can introduce phase noise into a signal generator by controlling two frequency points (f1 and f2) and amplitude values (Lmid), as shown in Figure 7.
Figure 7. Applying real-time phase noise impairment on a signal generator
Figure 8 shows a continuous wave (CW) signal with phase noise impairment. The start frequency f1 (marker 1) is 1 kHz; the stop frequency f2 (marker 2) is 30 kHz; amplitude values Lmid is -90 dBc/Hz.
Figure 8. A CW signal with phase noise impairment
Internal algorithms produce customized phase noise using the signal generator’s real-time baseband ASIC and processor accelerator. With this feature, you can simulate a more realistic signal which is helpful in evaluating and troubleshooting your devices under test.
Figure 9 illustrates a 64-QAM demodulation analysis at a 30 MHz symbol rate without injecting phase noise impairment. When we apply the phase noise impairment in Figure 8, the root-mean-square (RMS) phase error increases from 1.85° to 2.20°, and the error vector magnitude (EVM) increases from 1.73% to 1.99% as shown in Figure 10.
Figure 9. Demodulate a 64-QAM modulation signal without injecting phase noise impairment
Figure 10. Demodulate a 64-QAM modulation signal with phase noise impairment
The phase noise performance of a signal generator is a key factor in obtaining accurate measurements. Signal generators offer two or more levels of phase noise performance and allow optimization for wide or narrow frequency offsets test applications. On the other hand, you may inject a certain amount of phase noise to simulate a more realistic signal and characterize your device’s performance.
See related posts to learn more about Confronting Measurement Uncertainty in Signal Generation: