Ultra-Wideband Instantaneous Frequency Estimation Daniel Lam, Brandon W. Buckley, Cejo K. Lonappan, Asad M. Madni, and Bahram Jalali D etermining the instantaneous frequency of a sig- nal is required for many applications ranging from radio astronomy to defense applications. Un- fortunately, the scan rate is often too long over a wideband spectrum compared to the time scale of signals of interest. We present an instantaneous frequency measurement receiver, which allows for simultaneous measurement of multiple frequencies and amplitudes across an ultra-wide instanta- neous bandwidth. Powered by the photonic time stretch A/D converter, the high effective sampling throughput of the sys- tem provides high temporal resolution and improvement of frequency and amplitude estimation capability through ad- vanced signal processing. This flexible system has adjustable instantaneous bandwidth and frequency resolution, an ultra- fast sweep time, and reduced hardware complexity compared to other instantaneous frequency measurement systems. Instantaneous Frequency Measurement Receivers The instantaneous frequency measurement (IFM) receiver has been an increasingly important tool for measuring ra- diofrequency (RF) signals over a wide bandwidth. It is used to measure RF frequency, amplitude, pulse width, and time of arrival for a plethora of applications such as radar threat detection, electronic warfare, and signal intelligence [1]. A wideband IFM receiver offers a high probability of inter- cept over wide instantaneous RF bandwidths, large dynamic ranges, good sensitivity and high frequency measurement ac- curacy. Currently, IFM receivers are limited in performance mainly by their ability to measure only single frequen- cies at a time, having limited bandwidths, and slow sweep times across enormous bandwidths. Additional channels would be required to expand the bandwidth which would increase the hardware complexity. The time-stretch instan- taneous frequency measurement receiver (TS-IFM) is able to overcome these challenges and provide a solution capable of ultra-fast sweeping across enormous bandwidths to per- form measurements on transient signals. In today’s spectrally cluttered environments, we need a system that can perform measurements across wider bandwidths and detect frequen- cies of interest quickly and efficiently. Current IFM Methods Traditional IFMs use microwave interferometers and make use of hybrid couplers, power dividers, and delay lines to perform measurements. The basic measurement technology consists of a microwave correlator to measure an unknown signal. A tra- ditional IFM will split the incoming signal into two paths and delay one path by a time t with respect to the other along with a 90-degree phase shift. Subsequently, the ratio of the two paths is taken and an arc tangent operation is performed to deter- mine the input frequency of the received signal. A limitation of using this method is that it can measure only a single frequency at a time, and measuring amplitude re- quires another set of discriminators. While there may be other signals in the band, the IFM receiver measures only the larg- est RF signal in the band [1]. Moreover, the largest signal must also be several dB greater than the others, and two signals can- not be too close in both frequency and amplitude otherwise estimation errors would occur. IFM systems have reduced bandwidths to measure multiple frequencies and also require a series of filters and post-processing with added complexity to measure each frequency. In addition, it is difficult to realize broadband performance because of the bandwidth limitations of the RF components. Most IFMs have an instantaneous band- width of only 1 to 4 GHz. Digital IFMs (DIFM) are popular and provide several major upgrades to analog approaches. DIFMs are capable of hav- ing wider instantaneous bandwidth than analog devices, can measure multiple frequencies, can measure complicated sig- nals, and do not rely on physical delay lines. Digital frequency measurement uses a digital filter bank and several channels to perform the measurement. It also requires a local oscillator to down convert the signal to an intermediate frequency and a high-speed digitizer to sample the signal. An advantage of using DIFMs is that the signal processing backend allows for easier implementation of digital delays, and typically, imple- menting several delays are necessary to perform measurements 26 IEEE Instrumentation & Measurement Magazine April 2015 1094-6969/15/$25.00©2015IEEE