In this keynote address, veteran Software Defined Radio (SDR) developer Hans reflects on his 25 years of experience, detailing the innovative techniques behind his renowned ADT-200A transceiver and outlining future directions for SDR technology. The presentation covers advanced concepts for both receiver and transmitter design. For reception, Hans explains the three key elements to achieving superior audio quality: optimized, non-brickwall audio filters for natural sound, a novel noise blanker algorithm with a moving trigger level, and a unique feed-forward Automatic Gain Control (AGC) system that preemptively handles large signal jumps without delay artifacts. For transmission, he introduces two key innovations: a "Power Enhancer" algorithm that increases average power without the distortion of traditional clippers, and an adaptive "Polar Linearizing" digital predistortion (DPD) system implemented in an FPGA to correct power amplifier non-linearities in real time, achieving significant suppression of unwanted sidebands.
A 25-Year Journey in SDR: Innovations in Receiver and Transmitter Design
0:01:47 The Beginning of SDR: Hans recounts writing his first article on what would become known as Software Defined Radio 25 years ago and later developing the ADT-200A, a direct sampling standalone shortwave transceiver highly praised for its audio quality.
0:04:05 The Secret to Superior Audio: The excellent sound of his transceiver is attributed to three key design principles:
Optimized Audio Filters: Using filters with a rounded, flat response rather than sharp "brick-wall" filters, which can sound metallic.
Advanced Noise Blanker: An algorithm that uses a moving trigger level that adapts to the peak of the received signal.
Innovative AGC: A feed-forward Automatic Gain Control (AGC) solution.
0:09:09 Feed-Forward AGC Explained: Unlike traditional feedback AGCs which react after a strong signal hits, this system uses a preemptive feed-forward algorithm. It analyzes the incoming signal and scales it down before it can cause overload or audio pops, resulting in a very quiet and smooth response to large signal strength changes (e.g., a 100 dB jump).
0:12:18 Optimal SDR Frontend Design: Hans outlines his ideal receiver frontend, consisting of a step attenuator, a very narrow preselector, a specific low-noise/high-intercept preamplifier (Minicircuits PGA-103+), and a sharp anti-aliasing filter to prevent out-of-band noise from being folded into the baseband.
0:19:33 Advanced Preselector Implementation: To achieve the narrow filtering required from 1.8 to 30 MHz, his design requires approximately 150 different filter configurations, which are switched using PIN diodes and fine-tuned with varactor diodes.
0:21:33 Transmitter Power Enhancer: This is a DSP algorithm that increases the average transmitted power without clipping. It analyzes the audio signal between zero-crossings and scales down any half-wave that exceeds the maximum level, preserving the waveform shape and creating minimal distortion compared to traditional clippers.
0:24:44 Polar Linearization (Digital Predistortion): Hans details a new, real-time digital predistortion (DPD) system. It uses a high-speed feedback loop within an FPGA to measure the power amplifier's amplitude and phase distortions and applies an inverse correction to the transmitted signal. This adaptive system can achieve intermodulation suppression greater than 60 dB.
0:30:40 Modern Implementation Tools: The complex FPGA logic for features like the predistortion system was developed using high-level tools like the Xilinx System Generator, which allows for simulation and implementation without writing extensive low-level code.
0:37:40 User Testimony on Power Enhancer: An audience member, who owns an ADT-200A, confirms that activating the Power Enhancer is perceived by the receiving station as if a 100-watt amplifier has been switched on, validating its effectiveness.