Local oscillators (LO) are crucial components used in the upconversion and downconversion chains of radar and satellite communications systems. In this post, learn the role of the LO and how its performance can significantly affect the way systems operate.
A radar signal typically goes through several upconversion steps before leaving the radar antenna. The purpose of upconversion is to shift the signal to a higher frequency. With an upconverted signal, radar systems can resolve targets in greater detail due to improved resolution, since increasing the frequency shortens the wavelength. This process can also reduce interference with signals from other wireless sources.
During upconversion, the LO’s output and the lower intermediate frequency (IF) radar signal are both applied to a mixer to produce a higher frequency transmit signal. An LO output with high spectral purity and stability ensures the conversion is completed accurately.
After transmission, radio waves bounce off targets in its path. The high-frequency reflections captured by the radar antenna are downconverted to lower IF frequencies that are easier to process. A mixer produces the IF signal based off its two inputs – the radar return and an LO-generated signal.
| LO SPECIFICATION | IMPORTANCE |
| Phase Noise |
A low phase noise output from the system LO enhances target detection. The more phase noise introduced by the LO, the more the receiver’s sensitivity reduces. Excessive phase noise levels can mask close-in, low-power, Doppler-shifted radar returns completely. |
| Frequency Switching Speed |
While operating, certain radar systems can dynamically hop between different frequencies. A quick change in operating frequency can aid in electronic counter countermeasure (ECCM) applications, creating an effective way to evade jamming attempts from hostile parties. LOs must be able to reach required switching speeds for frequency-agile radar. |
In satellite communications systems, LOs are used in user terminals, satellite payloads, and in the uplink and downlink paths of satellite ground stations or gateways. In the uplink from the ground station to the satellite, the LO-generated signal is used to upconvert the transmit signal to a higher frequency. Among other benefits, upconversion can optimize spectrum usage by enabling satellite systems to operate in specifically allocated frequency bands.
The downlink path from the satellite to the ground station utilizes an LO to downconvert the received signal to a lower IF. Similar to radar systems, downconverting to a lower frequency increases the efficiency of subsequent processing steps.
| LO SPECIFICATION | IMPORTANCE |
| Phase Noise |
An LO’s output should introduce the least amount of phase noise possible, since phase noise can distort symbol positions in the constellation diagram. Symbols can rotate to the point where they cross over adjacent decision boundaries, especially in systems using higher order modulation. Excessive phase noise levels increase the error vector magnitude (EVM), symbol errors, and bit errors. |
| Frequency Switching Speed |
Fast switching speed enables greater test throughput, thus increasing efficiency in engineering and reducing the cost of test in manufacturing. |
How can engineers determine if the LO is performing up to necessary standards? One test technique, referred to as LO substitution, involves swapping the system LO with a high-performance RF or microwave signal generator, such as the SGX1000 Series. Engineers can verify that the LO is the root cause of system faults if performance improves considerably when using the highly pure output from the signal source. In cases where the LO is defective or unavailable, engineers can still evaluate upconversion and downconversion chains by using the signal generator as an LO substitute.
For a succinct and informative guide on LO substitution and RF signal generator test solutions, download the Application Brief, “Upconverter and Downconverter Test: Local Oscillator Substitution.” Fill out the form below to learn how to address today’s LO substitution test challenges.