Colin Byrne

Radar System Overview

The goal of the broader project is the development of a short-range surveillance radar designed to support air traffic monitoring of delivery drones operating in low-altitude urban environments.

With the growing deployment of autonomous UAV delivery systems, there is increasing demand for compact sensing technologies capable of detecting and tracking small aerial vehicles in cluttered environments.

The radar system integrates antennas, RF front-end hardware, signal processing blocks, and a phase-locked loop responsible for generating a stable microwave reference frequency.

Phase-Locked Loop Architecture

The radar transmitter requires a stable microwave signal around 5.4 GHz. To generate this signal, a phase-locked loop (PLL) architecture is implemented.

The PLL compares a reference oscillator with a feedback signal derived from the VCO output and adjusts the control voltage to maintain phase lock.

Within the project team, different members focus on different subsystems of the PLL including:

• Reference oscillator and frequency multipliers
• Phase detector and mixer
• Loop filter design
• Voltage-controlled oscillator (VCO)

My contribution focuses on the VCO subsystem and its tuning mechanism using a varactor diode.

Varactor-Tuned Colpitts Oscillator

Oscillator Principle

The Colpitts oscillator is a common LC oscillator topology used in RF and microwave circuits due to its simplicity and stability. Its oscillation frequency is determined by the resonant tank circuit.

The oscillation frequency is approximately:

\( f_0 = \frac{1}{2\pi \sqrt{L C_{eq}}} \)

where the equivalent capacitance of the capacitive divider is

\( C_{eq} = \frac{C_1 C_2}{C_1 + C_2} \)

Varactor Tuning

Frequency tuning is achieved using a varactor diode whose capacitance varies with reverse bias voltage.

\( C(V_R) = C_0 (1 + \frac{V_R}{V_j})^{-m} \)

By adjusting the control voltage applied to the varactor, the effective tank capacitance changes, allowing electronic tuning of the oscillator frequency.

ADS Modeling and Simulation

To evaluate the tuning behavior, a varactor model was implemented in Keysight Advanced Design System using an HSPICE subcircuit based on manufacturer datasheet parameters.

The model includes parasitic elements such as series resistance and inductance in order to approximate real device behavior at gigahertz frequencies.

A parameter sweep was performed in ADS to vary the reverse-bias voltage applied to the varactor. This allowed extraction of the capacitance-voltage relationship and verification of the tuning mechanism prior to full oscillator integration.

Simulation Results

The capacitance-voltage characteristic was extracted using small-signal AC analysis.

The capacitance was computed from the imaginary current component flowing through the voltage source:

\( C = \frac{|Im(I)|}{2 \pi f} \)

The simulation results show a monotonic decrease in capacitance with increasing reverse-bias voltage, consistent with junction varactor theory.

Although the absolute values differ slightly from datasheet measurements, the trend matches theoretical expectations and validates the implemented model.

Hardware Implementation Plan

Following simulation verification, the next stage of the project is hardware implementation of the oscillator and PLL subsystem.

• Finalize RF PCB layout using HFSS simulations
• Generate fabrication-ready Gerber files
• Purchase varactor and RF components
• Assemble oscillator on RF PCB
• Perform bench-level testing using spectrum analyzers
• Integrate oscillator into the PLL architecture

This process will transition the project from numerical simulation to experimental verification and full radar system integration.

Appendices

Example HSPICE Varactor Model

* Example Varactor Model

.SUBCKT VARACTOR 1 2

D1 1 2 DMODEL
Rs 1 3 2
Ls 3 2 0.5n

.MODEL DMODEL D
+ IS=1e-14
+ CJO=2.3p
+ VJ=0.7
+ M=0.5

.ENDS VARACTOR