Mohammed El-Shennawy

Fractional-N PLL optimization for highly linear wideband chirp generation for FMCW radars

This work addresses the optimization of Fractional-N Phase Locked Loops (Frac-N PLLs) used to produce frequency chirps for Frequency Modulated Continuous Wave (FMCW) radar applications. In a Frac-N PLL, we have two main clock domains which are the reference and the divided clock domains. Clock domain crossings have to be considered during chirp generation to produce highly linear chirps. Moreover, with wideband chirps, integer divide ratio increments during chirp generation may cause transient frequency glitches which also affect the chirp linearity if not taken care of. In this work we propose techniques to address these issues in Frac-N PLLs. The proposed techniques lead to highly linear wideband chirp generation and thus improve the distance calculation accuracy by a factor of 2 and the distance calculation precision by a factor of 1.5.

Techniques for maximizing input handling and improving linearity of gain interpolating VGAs

This work presents gain interpolating variable gain amplifier (VGA) design aspects. A technique is introduced to improve the VGA input handling capability from a few hundred millivolts to rail-to-rail without affecting any other performance parameters and with low complexity. An additional technique is proven by hand analysis and simulations to reduce the 3rd order intermodulation products (IM3) of the VGA by 4dB. The VGA is designed in an IBM 0.13μm BiCMOS 7LM technology. It has a 56.5dB gain control range and draws 1.76mA from a 3V supply.

Crystal oscillator frequency offset compensation for accurate FMCW radar ranging

Accurate ranging and positioning systems encounter several practical limitations. Crystal oscillator (XO) tolerance is a major challenge that impairs the accuracy of these systems. Two-way ranging with frequency modulated continues wave (FMCW) radars in particular are more influenced by XO tolerance as compared to primary FMCW radars. So this work focuses on the measurement and compensation of the relative frequency offsets between the different XOs in FMCW radar systems in order to improve their ranging accuracy. With a two-way ranging synchronization protocol, a ±1 part per million (ppm) relative XO frequency offset may translate to ranging errors in the range of ±1.2 m. In this work, a three-way ranging synchronization protocol is proposed to compensate for the relative XO offset. The approach was tested in a practical ranging setup. Compared to the conventional two-way ranging synchronization protocol, the precision was enhanced by a factor of around 2.

A technique for robust division ratio switching in Multi Modulus Dividers with modulus extension

This paper presents a detailed study for the logic of Multi Modulus Dividers (MMDs) with modulus extension used in low power high speed frequency synthesizers. With modulus extension, there are two division ranges (the original division range and the extended one) with a boundary between them. In this work, we will show that this architecture needs some care when used in Sigma Delta (ΣΔ) Fractional-N synthesizers, especially when the ΣΔ drives the MMD to switch back and forth this boundary. This switching should be done successfully or else the PLL will completely lose lock.

Fundamental limitations of crystal oscillator tolerances on FMCW radar accuracy

Crystal oscillators (XOs) are widely used in fractional-N phase locked loops (Frac-N PLLs) as reference frequency sources due to their good phase noise performance. However, these XOs have manufacturing tolerances in the range of few tens of parts per million (ppm). When a Frac-N PLL is used in frequency modulated continuous wave (FMCW) radar applications, the XO accuracy affects the frequency and timing accuracies of the radar chirps and so it directly affects the distance measurement accuracy. In this work, the fundamental limits of XO tolerance on distance measurement accuracy of FMCW radars are discussed and modelled. Moreover, an FMCW radar system hardware prototype is developed. Measurement results are in good agreement with the expectations of the simulation model.

A hybrid TDoA/RSSI model for mitigating NLOS errors in FMCW based indoor positioning systems

In this paper, the received signal strength indicator (RSSI) is combined with the time-difference-of-arrival (TDoA) technique as a hybrid approach to mitigate positioning errors due to non-line-of-sight (NLOS) propagation in indoor positioning systems. In order to study the effect of NLOS propagation on frequency modulated continuous wave (FMCW) radars, a simple model based on ray tracing was developed to calculate the positioning error profiles inside an area of interest. The model also calculates the RSSI values inside the area. The model showed a correlation between positioning errors and low RSSI values for some base stations (BSs). Therefore, an algorithm is proposed that uses the RSSI values of each BS as a weighting factor to rule out the NLOS BSs and rely only on the line-of-sight (LOS) BSs. Applying this algorithm on a test scenario with NLOS conditions, an improvement of more than a factor of 3 in the maximum positioning error was achieved. Also the average positioning error was improved by more than a factor of 5.

A coverage efficient FMCW positioning system

Accurate positioning with a limited number of reference stations is challenging. Several commercial systems achieve good accuracy by increasing the system complexity. Simplifying the system and reducing the number of reference stations is highly desirable. This paper presents a coverage efficient highly integrated local positioning system. The system is optimized for minimum positioning error with maximum effective coverage area. It is highly integrated and relies on the traditional frequency modulated continuous wave (FMCW) radar to provide the position information. Starting from the hardware to the final positioning algorithm, the research provides an intensive insight into the system details. The proposed system was practically tested for reliability and accuracy. To mitigate the multipath propagation issue, the system switches between the 2.4 and 5.8 GHz industrial, scientific and medical (ISM) bands. An overall positioning precision of around 10 cm and accuracy of less than or equal to 0.39 m was achieved with a coverage area of around 48 and 24 m2 per reference station, in both outdoor and indoor conditions, respectively.

Fundamental limitations of phase noise on FMCW radar precision

This work presents an approach for estimating the effect of the fractional-N phase locked loop (Frac-N PLL) phase noise profile on frequency modulated continuous wave (FMCW) radar precision. Unlike previous approaches, the proposed modelling method takes the actual shape of the phase noise profile into account leading to insights on the main regions dominating the precision. Estimates from the proposed model are in very good agreement with statistical simulations and measurement results from an FMCW radar test chip fabricated on an IBM7WL BiCMOS 0.18 μm technology. At 5.8 GHz center frequency, a close-in phase noise of −75 dBc/Hz at 1 kHz offset is measured. A root mean squared (RMS) chirp nonlinearity error of 14.6 kHz and a ranging precision of 0.52 cm are achieved which competes with state-of-the-art FMCW secondary radars.

A 55 dB range gain interpolating variable gain amplifier with improved offset cancellation

This work presents the design of a 55 dB dynamic range gain interpolating variable gain amplifier (VGA). In such wide dynamic range amplifiers, small input referred offsets in the range of a few millivolts may cause the outputs to rail especially at the higher gain settings. Therefore an offset cancellation circuit is needed. The VGA design including an improved offset cancellation circuit is fabricated as part of a full automatic gain control (AGC) loop on an IBM 0.18 μm BiCMOS 7LM technology. Measurement results are in good agreement with simulations showing that the AGC could track input power changes in the range from -44 to +11 dBm introduced to the VGAs 100 Ω differential input impedance. The VGA has a measured bandwidth of 18 MHz and input referred noise of 3.5 nV/√Hz. It can directly drive a capacitive load of 23 pF consuming 4 mA from a 3V supply.

Nonlinear modelling of automatic gain control loops considering loop dynamics and stability

This work presents modelling aspects of automatic gain control (AGC) loops based on linear-in-dB variable gain amplifiers (VGAs). In these loops, the VGA control voltage is also an excellent received signal strength indicator (RSSI). The VGA gain is however nonlinearly related to the control voltage. Moreover, VGAs and detectors undergo nonlinear compression under high input amplitudes during settling transients. In this work, these effects are captured by a nonlinear model based on simple and readily available components from the “analogLib” and “functional” libraries in CADENCE design environment making it very easy and fast to build and simulate. The model is capable of verifying the AGC loop stability and capturing the loop dynamics with high accuracy compared to time consuming circuit level simulations. This provides insights into system level parameters such as AGC loop bandwidth, phase margin, settling time as well as estimating the AGC range and RSSI voltage vs. input power. Measurement results from a fabricated AGC prototype are in good agreement with simulation and modelling results thus validating the proposed modelling approach.