In a perfect world, a crystal oscillator would generate a signal that remains steady, consistent and clear with no deviations, for as long as that signal needed to be transmitted. But of course, we don’t live in a perfect world, do we?
No crystal oscillator produces a 100% perfect signal. Even if the signal is very strong and clear, there will still be tiny, random fluctuations in the waveform. This phenomenon can be represented visually in the frequency domain as sidebands on either side of the carrier. These unwanted fluctuations are referred to as phase noise.
It is important to be aware of phase noise because it is an indicator of how well the devices affected by it will perform. Systems affected by significant phase noise levels will experience a poorer level of performance. Some networks are more susceptible to phase noise than others.
Depending on the application, such as medical technology, GPS in military operations, etc., phase noise can not only be inconvenient, but dangerous. If the noise is bad enough, there is also the risk of interference with adjacent channels, making it a problem for everyone and not just users of the crystal oscillator generating the noise.
So how can phase noise be detected and measured in order take steps to reduce it?
There are numerous methods for measuring phase noise, each with their own pros and cons. The default method in many cases is to use a spectrum analyzer. All those who work with RF technology are familiar with the oscilloscope, which observes and displays signals in the time domain, with the vertical axis representing voltage and the horizontal axis representing the signal over time. A spectrum analyzer follows the same basic principle, but measures and displays a signal in the frequency domain instead, by showing the amplitude of a signal on the vertical axis and the frequency on the horizontal.
When using a spectrum analyzer, the device under test (DUT) will be connected directly to the analyzer’s input. Phase noise is measured by the amount of noise within a specific bandwidth (traditionally 1Hz) relative to the carrier power. Measurements of phase noise are displayed as the number of dBc per Hz at a particular offset.
Spectrum analyzers are complex devices and are considered indispensable tools for testing in RF engineering. A good introductory guide into the specifics of how to use these devices can be found here.
When it comes to a crystal oscillator, however, a spectrum analyzer alone may not be sufficient for measuring phase noise. Some low-noise oscillators produce noise at such a low level that it becomes difficult to accurately detect and measure. One way around this problem is to use a Reference Source/Quadrature Method.
The Reference Source/Quadrature Method
This method involves testing two oscillators set to the same frequencies. One is the DUT (the crystal oscillator in our example) and the second one will be an oscillator with superior performance. This is the “reference source” against which the crystal oscillator under test will be compared. Their signals are combined with a mixer and fed through a low pass filter and a Low Noise Amplifier (LNA). From there, the noise measurements can be taken with a spectrum analyzer, or with a Fast Fourier Transform Analyzer.
If the input signals from both sources are adjusted to be in phase quadrature, the DC voltage output from the mixer will be zero volts. The resulting output from the mixer will provide a direct measurement of the phase difference between the crystal oscillator under test and the reference source, as long as the reference source is performing better than the DUT.
In some situations, a phase locked loop (PLL) with the reference source will have to be used in order to create a feedback system that maintains phase quadrature. Using a PLL, however, can cause problems with calibration of the system since it can remove low-frequency components from the mixer’s output. The phase noise measurements must be corrected mathematically to account for this, or the PLL must be set so that it is below the frequency offset.
A Reference Source/Quadrature approach to measuring phase noise can be very effective, and by following best practices for this method, you can avoid most problems and gain accurate noise measurements for a crystal oscillator.
The Choice of Crystal Oscillators Matter
As the old saying goes, an ounce of prevention is worth a pound of cure. The best way to reduce phase noise as much as possible and improve performance in any application is to pick a well-designed crystal oscillator product in the first place. With over 8 decades of experience in the industry, we specialize in crafting the highest quality low and ultra-low phase noise crystal oscillators on the market. Our products are leading the way into the world of 5G, advanced space-based internet and Positioning, Navigation and Timing (PNT) solutions, and so much more.
The RF industry is on the cusp of some major changes in the next 5 to 10 years. Check out our products to learn how Bliley’s knowledge and superior designs can help you stay ahead of the curve for what’s about to come.