Comparison of a binary quantized image with its dithered version
DSP

Noise is Not Always the Enemy

Noise is usually considered the main enemy in all DSP applications. As David Tse once said: “Noise is the reason for our existence (communication engineers)!” This short article briefly describes why noise sometimes plays a positive role, e.g., in the context of analog-to-digital conversion when the signal is very weak. Introduction Noise is the enemy to be conquered, particularly in communications and radar systems. In a noise-limited regime, we hit a performance brick wall due to the presence of noise. Think of parameter estimation in which the primary criteria is to establish a certain performance against a target signal-to-noise ratio. In this article, we will investigate one of its underappreciated advantages, specifically in a case when the signal of interest

Continue reading
Lime SDR
DSP / Wireless/SDR

Top 5 Software Defined Radios (SDR) for RF Experimentation

In this article, I describe 5 of the most popular SDRs available for RF experimentation today. As a 6th member of this list, I include a surprisingly common and free SDR that can be used for your fun radio projects. Table of Contents Background Where We Came From Top SDRs  5. Universal Software Radio Peripheral (USRP)  4. LimeSDR  3. HackRF One  2. ADALM-Pluto  1. RTL-SDR  0. A Free SDR We start with a little bit of background and where we came from. Background Software Defined Radio (SDR) has revolutionized wireless communication in the same way Microsoft revolutionized the scope of personal computing in its early days. Today the SDRs are used by electronics and DSP engineers, amateur radio hobbyists, physicists,

Continue reading
A figurative example of IQ imbalance depicting potential sources of mismatches
Wireless/SDR

Direct Conversion (Zero-IF) Receiver

The frontend of the transceiver plays a crucial role in determining the ultimate system performance. In a previous article, we described how a superheterodyne architecture helps in enhancing the selectivity and sensitivity of the receiver. Some of the main issues with a superheterydone receiver are the image frequency and a large form factor due to multiple conversion stages. Today we discuss a direct conversion architecture, also known as zero-IF and homodyne. Recall from the concept of frequency domain that a real sinusoid at the Local Oscillator (LO) output has two impulses in its spectrum, one at a positive frequency $+F_{\text{LO}}$ and the other at the negative frequency $-F_{\text{LO}}$. A clue for the root cause of the image frequency problem appears

Continue reading
DDS waveform and spectrum after dithering
DSP / Wireless/SDR

Direct Digital Synthesizer (DDS)

A Direct Digital Synthesizer (DDS) is an integral part of all modern communication systems. It is a technique to produce a desired waveform, usually a sinusoid, through employing digital signal processing algorithms. As an example, in the transmitter (Tx) of a digital communication system, a Local Oscillator (LO) is required to generate a carrier sinusoid that upconverts the modulated signal to its allocated frequency in the spectrum. On the receive (Rx) side, another local oscillator downconverts this high frequency signal to baseband for further processing. Such a process is shown in the Tx and Rx block diagrams of a Quadrature Amplitude Modulation (QAM) system below. In addition to frequency translations, an oscillator with an adjustable frequency is also needed for

Continue reading
A discrete-time FM demodulator block diagram with atan2 and derivative filter
DSP / Wireless/SDR

Frequency Modulation (FM) and Demodulation Using DSP Techniques

Frequency Modulation (FM) is as old as the history of wireless communications itself. The past few decades saw the rise of digital signal processing in all spheres of life that pervaded even the implementation of analog modulation schemes. Today many of the FM systems are built using discrete-time techniques instead of the conventional circuitry as described below. Frequency Modulation In digital communications, data is sent through altering a characteristic of an electromagnetic wave such as amplitude, frequency or phase in discrete steps (e.g., $M$ number of levels). Such systems are known as Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK), respectively. Analog modulation schemes, on the other hand, vary the desired parameter in a continuous

Continue reading