Phase noise is rapidly becoming the most critical factor addressed in sophisticated radar and communication systems. This is because it is the key parameter defining target acquisition in radars and spectral integrity in communication systems. There are many papers detailing the mathematical derivation of phase noise but few mention the reasons for its importance. In this Tech Brief, we discuss the importance of phase noise, and what can be done to lessen its effects in microwave systems.
Phase noise is commonly used as a measure of frequency stability within an oscillator. This noise is inherently different than the general background noise of any electrical system, which is defined as kTB, where k is Boltzmann’s constant, B is the bandwidth, and T is the temperature. Instead, phase noise is a secondary effect directly related to the topology and construction of the oscillator. A pictorial representation of phase noise is given below in Figure 1.
FIGURE 1. Pictorial representation of an ideal signal (blue) and a signal with phase noise (red).
In this figure, we have plotted the output power of an oscillator versus frequency. The ideal oscillator is shown in blue, which only outputs power at a single, fixed frequency. The red curve, however, is the output of an oscillator with phase noise, which shows up as power across a spectrum of frequencies very close to the desired output. These skirts, as they are called, are always present and are due to thermal noise within the active devices of the oscillator. The power level of the red skirts is dependent upon the quality of the oscillator and is measured in dBc/Hz at an offset frequency from the desired signal (typically called the carrier).
Phase noise can affect the performance of many different microwave systems. In this Tech Brief, we discuss two in particular: direct down conversion receivers, and radars.
Direct downconversion is a type of receiver in microwave communication systems. One benefit of direct downconversion is the simplicity of the circuit, which is essentially a single mixer driven by a local oscillator (LO) to convert the input RF signal to a baseband (very low frequency). This baseband signal is then directly applied to an analog-to-digital converter for processing. A common term for this architecture is “RF in, bits out”. One problem with direct downconversion, though, is that the input RF signal can be very close in frequency to the LO, which makes the conversion process susceptible to phase noise, especially if the signal strength is low.
In radar systems, the problem is similar in nature. Radar systems operate by transmitting a pulse at one frequency, and then measuring the frequency shift of the returned pulse, as the shift is related to the velocity of the object being imaged through the Doppler effect. Objects moving very slowly will generate a return pulse very close in frequency to the transmitted pulse, and if the cross section of the object is also very small, the power level of this received signal will be also very low. Ultimately this return pulse has to be converted to baseband in order to recover the velocity information, and phase noise can obscure the data.
FIGURE 2. Pictorial representation of an ideal LO signal (blue), an LO signal with phase noise (red), and an RF signal close in frequency (green) we wish to convert to baseband.
A pictorial representation of the dilemma faced by direct conversion receivers and radar systems is shown in Figure 2. In this figure, we can see that if the power level of the RF signal we wish to convert falls below the phase noise spectrum of the LO signal, we will be unable to recover any baseband information, as the signal will be in the noise. Therefore, reducing the phase noise will increase our receiver sensitivity.
FIGURE 3. Pictorial representation of phase noise issues in OFDM systems. Ideal LO signal (blue), LO signal with phase noise (red), RF signal (green).
In Figure 3, we present a second pictorial example of how phase noise can negatively impact a conversion, this time of a multi-carrier orthogonal frequency-division multiplexed (OFDM) signal.
In this figure, we note that if the phase noise of the LO is too high, then the noise will be converted into adjacent channels of the baseband data, thereby ruining the integrity of the information.
Learn more about the remedies for phase noise by reading our blog, "How to Solve RF and Microwave System Phase Noise Problems."
For the full story on phase noise, download out tech brief, "Addressing Phase Noise Challenges in Radar and Communication Systems."