The Most Critical Factor in High-End Radar and Communications Systems:
If you’re anyone involved with the design and implementation of communication systems, you most likely hear that word “phase noise” all the time. Maybe even more times than you’d like. Get used to it for now, it will be used a lot in this post.
There’s a good reason for all this phase noise chat. It’s one of the key factors that determines the overall success or failure of your radar or communications application. It’s even more of an importance in intense environments where strong vibration or g-force is a concern.
So the question is, why is maintaining low phase noise such a concern in these applications and environments? And of course, that question wouldn’t be worth asking unless we also answered how to solve any problems associated with the effects of phase noise. You will know how and why you should decrease phase noise in applications by the end of this article.
The Starting Line: What is Phase Noise?
Phase noise is typically measured as the frequency stability within a crystal oscillator. For those that may be unfamiliar, a crystal oscillator is basically the main component that creates and transmits the radio frequency signal that is used in:
- Radars communication systems
- Microwave systems
- Military applications
- and many more applications
Vibrations from many different sources, even very small vibrations (microvibrations), can cause phase noise and jitter in the crystal oscillator. This inhibits the quality and stability of the signal produced by the oscillator, thus making overall communication of the device more difficult. There are other ways that phase noise can be produced other than just from direct vibrations. Some specific examples are given below.
2 Examples of Phase Noise Problems in Applications
We already discussed that phase noise can cause problems in various applications. Here’s a look at two different microwave systems and how they’re negatively affected by phase noise:
- Direct Down-Conversion Receivers
- Radar Systems
1. Direct Down-Conversion
Direct down-conversion is a fairly basic microwave communication receiver with a simple circuit. It’s basically a single mixer containing a local oscillator (LO) that converts the incoming radio frequency signal down to a very low frequency. This low frequency is then applied right to an analog-to-digital converter for processing.
“Alright, what’s the big idea? What’s the problem here?”
Well, the problem is that many times the RF input frequency can be nearly the same as the frequency of the oscillator. This makes the conversion process easier for some pesky phase noise to slip into the signal. This is even more likely to happen if there’s a weak signal strength.
2. Radar Systems
A similar type of problem occurs in radar systems. Instead of a direct RF input, radar’s pulse a specific frequency and then measure the change of each returning frequency pulse. Each pulse change is related to the velocity of an object in the radar’s view by making use of the doppler effect. Objects moving very slowly will return a pulse that is very close to the original frequency of the pulse sent out.
The returning signal must be converted to a very low frequency to uncover the precise information of the velocity change. Like direct down-conversion, this low frequency conversion causes phase noise to negatively affect the data received.
An Amplifier’s Effect on Phase Noise
For many applications, a crystal oscillator alone cannot provide enough output power to perform at optimal quality. It will sometimes be necessary to call on amplifiers to help the little oscillator emit a stronger signal.
But of course in engineering when there seems to be a solution, there also seems to be more problems. Amplifiers can be great at boosting the signal power of an oscillator, but it can also…
You guessed it! …cause unwanted phase noise to hinder the oscillator’s performance.
Amplifiers, amongst all other electronic devices, add 1/f noise (pink noise) to an input signal. Basically, even if you have an oscillator with low phase noise qualities, amplifiers can completely replace their higher phase noise qualities with the oscillators low phase noise. Of course, that’s no good.
If you're down for some fun, consider learning about the different colors of noise.
How to Eliminate These Phase Noise Problems
The first and most important step to eliminating phase noise is in your designs is to find a high quality crystal oscillator from a trusted supplier. Doing this will help prevent the root cause of the phase noise problem. It’s also a great way to instantly ensure better phase noise performance in your application.
Especially when looking at military, aerospace, or extreme environment applications, high g-force levels and heavy vibrations can cause a lot of unnecessary phase noise in oscillators. Luckily, there are g-force compensated crystal oscillators specifically designed to take on the heavy vibrations and forces of gravity to crush any related phase noise issues. Once again, this is the easiest and most effective way to eliminate phase noise from the root source.
Consider checking out the Poseidon 2 series OCXO. Poseidon 2 is the world's lowest phase noise performance, low-cost OCXO on the market when subjected to dynamic random vibration conditions.
Finally, as far as amplifiers go, the 1/f (pink) noise is caused by random and thermal charge movement in the channel of the device. Amplifiers manufactured with a Gallium Arsenide (GaAs) pHEMT process. The FET devices in the process tend to have high 1/f noise rates due to their high electron mobility.
However, GaAs bipolar devices typically offer a much lower 1/f noise which also means less phase noise. So, use a GaAs HBT process amplifier to significantly reduce the phase noise in your application.
Our brand new, low-cost OCXO (Poseidon 2 series) features the world's lowest phase noise performance when subjected to dynamic random vibration conditions. It's perfect for eliminating phase noise in extreme vibration and g-force environments.
Download a free copy of the datasheet now.