All-optical instantaneous RF signal frequency measurement system based on a linear ACF with a steep slope.

A new microwave photonic structure for measuring the frequency of an RF signal, to the best of our knowledge, is presented. The frequency of an unknown RF signal can be determined by simply measuring the system output optical powers. The proposed frequency measurement system can be designed so that the ratio of the two system output optical powers as a function of the RF signal frequency or the amplitude comparison function (ACF) has a steep linear slope over a wide frequency range. This enables the RF signal frequency to be measured in high resolution and high accuracy. The proposed frequency measurement system has a simple and compact structure, and is free of high-speed photodetectors as well as RF components and instruments. It also has a fast response time compared to many reported photonics-based frequency measurement systems. A proof-of-concept experiment is carried out. Experimental results show a linear ACF with a slope of more than 4.4 dB/GHz over a frequency measurement range of 5-26 GHz and a frequency measurement accuracy of better than ±0.1G H z.

Microwave frequency measurement based on a frequency-to-phase mapping technique.

A new frequency-to-phase mapping technique for measuring a radio-frequency (RF) signal frequency is presented. The concept is based on generating two low-frequency signals where their phase difference is dependent on the input RF signal frequency. Hence, the input RF signal frequency can be determined by using a low-cost low-frequency electronic phase detector to measure the phase difference between the two low-frequency signals. The technique can measure the frequency of an RF signal instantaneously and has a wide frequency measurement range. The proposed frequency-to-phase-mapping-based instantaneous frequency measurement system is experimentally verified over the 5 to 20 GHz frequency measurement range with errors of less than ±0.2 GHz.

Multichannel microwave photonic based direction finding system.

A new microwave photonic topology for RF signal direction finding is presented. It is based on a dual-parallel Mach Zehnder modulator (DPMZM) in series with an optical phase modulator (PM). The direction of an RF signal received by the antennas connected to an RF port of the DPMZM and the PM can be determined from the power ratio of two system output low frequency components, without the need to know the incoming RF signal amplitude in advance. The proposed structure is suitable for implementing a long baseline technique for direction finding and can be extended to have multiple antenna elements in remote locations. In addition to direction finding, the system also has the ability to measure an RF signal Doppler frequency shift to determine an object speed and moving direction when it is used in a radar receiver. Results obtained using the proposed structure demonstrate less than ±2.5° errors over a 3.2° to 81.5° angle of arrival measurement range for different RF signal modulation indexes of 0.02, 0.08 and 0.16. Doppler frequency shift measurement with less than 0.8 Hz errors is also demonstrated.

Simple photonics-based system for Doppler frequency shift and angle of arrival measurement.

A novel photonic approach for simultaneously measuring both the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar system is presented. It has the same structure as a fiber optic link consisting of a laser, an optical modulator and a photodetector. The incoming microwave signal and a reference signal are applied to the optical modulator. Beating of the echo and reference signal sidebands at the photodetector generates a low-frequency electrical signal. The DFS and the AOA can be determined from the frequency and the power of the low-frequency electrical signal measured on an electrical spectrum analyzer. The system has a very simple structure and is low-cost. It has a wide operating frequency range and a robust performance. Experimental results demonstrate a DFS measurement at around 15 GHz with errors of less than ±0.2 Hz, and a 0° to 90° AOA measurement with less than ±1° errors.

Cascaded modulator topology for frequency conversion in antenna remoting applications.

A new cascaded modulator structure that has the ability to realize high conversion efficiency microwave frequency downconversion, while at the same time able to overcome two fundamental limitations in the dual-parallel modulator approach, is presented. It is based on utilizing the polarization-dependent modulation efficiency property in LiNbO3 electro-optic modulators. The new structure allows the modulators for the RF signal and local oscillator (LO) modulation to be placed in different locations suitable for antenna remoting applications, and it has infinite isolation between the LO and RF signal ports. We present experimental results demonstrating that the proposed structure can be used to realize high conversion efficiency frequency downconversion over wide RF and intermediate frequency (IF) signal frequency ranges as the reported dual-parallel-modulator-based microwave photonic frequency downconverter. Very high isolation of more than 70 dB between the LO and RF signal ports is also demonstrated.