Chapter 9 / 9.1 / 9.1.5

9.1.5 Modern Digital And SDR Transmitters

The fundamental principles of frequency translation and power amplification remain unchanged, but their implementation has shifted almost entirely into the digital domain. Modern transmitters no longer rely on analogue oscillators and modulators for carrier generation. Instead, modulation, filtering, frequency synthesis, and gain control are performed digitally, allowing precise control, reconfiguration, and monitoring in real time. This evolution has led to the software-defined radio (SDR)—a transmitter whose operating parameters can be modified entirely by software while the underlying hardware remains constant [3].

9.1.5.1 Direct-Conversion Architectures

A direct-conversion, or homodyne, transmitter generates the required RF signal directly from digital in-phase (I) and quadrature (Q) baseband components without an IF stage. The digital baseband is synthesized within a field-programmable gate array (FPGA), digital signal processor (DSP), or system-on-chip (SoC). These digital I/Q signals are applied to high-speed digital-to-analogue converters (DACs) that feed an I/Q modulator or a complex-mixing stage producing the RF output.

Direct conversion eliminates the multiple up-conversion stages of the classical heterodyne transmitter, simplifying the signal path and minimizing group-delay distortion. Because there are no analogue mixers or IF filters, frequency agility is limited only by the numerical resolution of the synthesizer. Careful design is nevertheless required to suppress LO leakage, DC offsets, and I/Q imbalance—imperfections that can produce unwanted spectral components near the carrier. Modern calibration techniques use digital feedback loops that continuously measure and correct these impairments during operation.

9.1.5.2 Digital Up-Conversion (DUC) And Baseband Processing

In digital transmitters, modulation and spectral shaping occur entirely in software prior to conversion to analogue form. The digital up-converter (DUC) performs interpolation, carrier mixing, and filtering to translate the modulated baseband waveform to the desired RF output frequency. Adjustable digital filters implement the precise spectral mask required by the relevant standard—for example, ETSI EN 300 421 for satellite links or 3GPP TS 38.104 for 5G NR [4].

Digital up-conversion provides several advantages over analogue methods:

• Programmable modulation formats. The same hardware can support QPSK, 8PSK, 16/64/256-QAM, OFDM, and amplitude/phase-shift keying schemes.
• Adaptive filtering. Passband shape, bandwidth, and roll-off can be varied dynamically to meet regulatory and link-budget constraints.
• Digital predistortion (DPD). Non-linearities in the RF power amplifier are compensated by applying an inverse transfer function to the digital baseband signal before conversion, achieving adjacent-channel leakage ratios (ACLR) better than –60 dBc.
• Integrated monitoring. Power, spectrum, and error-vector magnitude (EVM) are measured in real time for automatic calibration.

9.1.5.3 Frequency-Agile, Software-Controlled Transmitters

The flexibility of SDR architecture allows a single hardware platform to operate over multiple frequency bands and modulation schemes by reconfiguring software parameters. Carrier generation is accomplished using direct digital synthesis (DDS) or fractional-N phase-locked loops (PLLs) disciplined by temperature- or oven-controlled crystal oscillators (TCXO/OCXO) or by satellite-based frequency references such as GPSDOs. These systems routinely achieve frequency stability better than ±0.01 ppm and phase-noise levels below –120 dBc Hz^–1 at 10 kHz offset.

Modern transmitters also integrate networked control interfaces that allow frequency plans, power levels, and waveform parameters to be updated remotely. Embedded microcontrollers or FPGAs manage automatic power calibration, linearization, and spectrum monitoring, ensuring compliance with ITU and ETSI emission masks.

These advances have transformed the transmitter from a fixed-function circuit into a reconfigurable, frequency-agile system. In contemporary satellite, cellular, and defense applications, a single SDR platform can generate multiple carriers, adapt modulation to channel conditions, and operate seamlessly across several frequency allocations—all while upholding the same fundamental objectives of efficiency, linearity, and spectral purity that have governed transmitter design since the earliest days of radio.