Preamble

Circuit Design and Signal Processing Technology for Medium-Wave Broadcast Transmitters
(Bomi Medium-Wave Relay Station, Tibet Autonomous Region Radio and Television Bureau, Lhasa, Tibet 850000)

Abstract

[Objective] This study investigates key technical issues in medium-wave broadcast transmitter circuit design, optimizes signal processing schemes, and enhances system performance and spectrum utilization efficiency. [Methods] Based on digital signal processing technology, the transmitter circuit is optimized, a distributed monitoring scheme is adopted to measure transmitted signal quality, and system performance indicators are verified through high-power dummy load testing systems. [Results] The improved medium-wave transmitter demonstrates significant performance improvements in spectrum control, modulation depth, and spurious emission suppression. The double-tuned structure of the output tank circuit effectively enhances spectral characteristics, while the distributed monitoring system ensures equipment operational stability. [Conclusion] The optimized digital circuit design significantly improves overall transmitter performance, offers substantial engineering application value, and provides a practical solution for technological innovation in medium-wave broadcast transmission equipment.

Keywords: medium-wave transmitter; digital modulation; signal processing; spectrum optimization; circuit design
CLC Number: G222
Document Code: A
Article ID: 1671-0134(2025)04-145-04
DOI: 10.19483/j.cnki.11-4653/n.2025.04.030
Citation Format: Suolang Zhuoma. Circuit Design and Signal Processing Technology for Medium-Wave Broadcast Transmitters[J]. China Media Technology, 2025, 32(4): 145-148.

Medium-wave broadcast transmitters constitute the core transmission equipment of radio stations, with their circuit design and signal processing technology directly impacting broadcast quality and transmission efficiency. As broadcast technology advances, medium-wave transmission equipment faces increasingly stringent technical requirements regarding modulation depth, spectral characteristics, and spurious emission indicators. Traditional analog modulation methods exhibit inherent limitations in spectrum utilization and signal quality, making them inadequate for meeting the high-quality transmission demands of modern broadcast systems. Adopting digital technology to transform medium-wave transmitters holds significant engineering value and contributes positively to enhancing overall broadcast system performance.

1.1 Broadcast Signal Modulation Principle

The modulation principle of medium-wave broadcast transmitters is based on amplitude-modulated carrier technology, where the carrier signal undergoes digital modulation to produce amplitude-modulated waveforms. The specific implementation process proceeds as follows: the audio signal first passes through an analog input board for processing, where a Bessel filter removes out-of-band spurious components and converts balanced audio signals to unbalanced audio signals. The input balanced audio signals are processed at a standard sampling rate through adaptive sampling rate conversion methods. The analog-to-digital conversion board completes the transformation from analog to digital signals, converting composite audio signals into digital audio signals. The modulation encoding board processes the digital audio signals according to power synthesis rules, converting them into switching control signals for the power amplifier modules through encoding distribution. The application of digital modulation technology significantly enhances modulation depth stability, effectively reduces spurious emissions, and improves spectrum utilization efficiency \cite{1}.

1.2 Digital System Architecture Design

The digital system architecture primarily comprises an RF amplification system, audio processing and encoding system, monitoring and display system, control and protection system, and power supply system. The RF system employs direct digital drive technology, including power amplifier modules, RF audio distribution boards, module control boards, audio processing boards, as well as output matching networks and antenna switching components. The audio system completes audio signal processing based on an FPGA+DSP platform, implementing functions such as audio sampling, conversion, and power control. Power control includes startup power level control, fine power adjustment, stepwise power ramping, and power feedback control. The system utilizes a wide-frequency design to achieve rapid switching between preset frequencies, with all technical indicators meeting national Grade A standards. The control and protection system adopts a modular design, primarily consisting of main control boards, digital I/O boards, and analog boards, which collect operational data in real-time through a distributed monitoring system.

1.3 Performance Indicator System Construction

The performance indicator system establishes standardized specifications across multiple dimensions, including transmitter output power, carrier frequency stability, spurious emissions, and modulation depth. The transmitter employs automatic adjustment technology for standing wave ratio monitoring circuits, enabling automatic parameter switching according to configured frequencies to ensure safe and stable operation under various working conditions \cite{2}. The power amplifier module utilizes differential balanced input methods to effectively reduce RF output influence on the input. The output tank circuit incorporates a bandpass filter to remove spurious signals outside the transmitter frequency, ensuring output signal purity. The direct digital drive power amplifier module employs programmable logic devices and surface-mount chips, enhancing system integration. The system design incorporates multiple protective measures including temperature monitoring, airflow monitoring, arc detection, and standing wave ratio sampling, forming a comprehensive performance monitoring system.

2.1 Modulator Parameter Configuration

Modulator parameter configuration primarily includes analog input gain settings, power adjustment, and dither signal level control \cite{3}. Key parameter settings in the modulator include triangular wave frequency adjustment, maintaining the frequency within the 70-74 kHz range to ensure flat binary amplifier steps. The analog input board features multiple adjustment potentiometers: audio gain adjustment for achieving 100% modulation depth, maximum carrier power adjustment for controlling the transmitter's output power ceiling, and triangular wave frequency and level adjustment for optimizing modulation quality. The modulator also incorporates audio signal preprocessing circuits that filter and adjust the amplitude of input audio signals, removing out-of-band spurious components \cite{4}. Signal preprocessing employs Bessel filters, which exhibit favorable group delay characteristics. For power control, digitally controlled electronic attenuator technology is utilized, enabling continuously adjustable control ranges. In amplitude modulation transmitters, the waveform equation for single-tone modulation is:

$$e(t)=E_c[1+m\cdot\cos(\omega_m\cdot t)\cos(\omega_c\cdot t)]$$

where $t$ represents the time variable, $E_c$ denotes carrier amplitude, $m$ indicates modulation depth, $\omega_m$ represents the modulation angular frequency, $\omega_c$ denotes the carrier angular frequency, $\cdot$ signifies multiplication operations, and parentheses indicate operational precedence.

2.2 Sampling and Quantization Circuit Design

The sampling and quantization circuit achieves efficient sampling and data conversion of audio signals through high-precision analog-to-digital converters. The design employs sampling pulse width adjustment technology to ensure sampling accuracy by regulating pulse width \cite{5}. For DC bias adjustment across different power levels, adaptive algorithms are implemented to achieve optimal amplitude modulation. The circuit structure utilizes differential input methods to effectively suppress common-mode interference and improve signal sampling accuracy. The settling and hold times of the sample-and-hold circuit are optimized through precise control \cite{6}. The quantization circuit employs a pipelined architecture to enhance data conversion speed. The analog-to-digital conversion board features two critical adjustment potentiometers: one for regulating sampling pulse width to ensure sampling precision, and another for adjusting DC bias to achieve optimal amplitude modulation at different power levels. The circuit also incorporates overvoltage protection and anti-interference circuits to improve system reliability. According to the Nyquist sampling theorem, the sampling frequency $f_s$ must satisfy:

$$f_s \geq 2f_{\text{max}}$$

where $f_s$ represents the sampling frequency and $f_{\text{max}}$ denotes the signal's maximum frequency. Quantization noise power is given by:

$$P_q = \frac{\Delta^2}{12}$$

where $\Delta$ represents the quantization interval.

2.3 Timing Synchronization Module Development

The timing synchronization module addresses timing coordination among various functional units within the system \cite{7}. The module employs Phase-Locked Loop (PLL) technology to achieve clock synchronization, generating the required system clock through PLL circuits. Key parameters for timing synchronization include lock time, phase noise, and jitter characteristics. The timing synchronization module ensures synchronized operation of all power amplifier stages through a master-slave clock distribution architecture. During signal transmission, differential signal transmission methods are utilized to reduce clock jitter and enhance system stability. The timing synchronization circuit includes clock distribution buffers to minimize clock skew and jitter. The circuit design adopts digital phase-locked loop technology, which offers superior temperature stability and interference immunity compared to analog PLLs \cite{8}. System clock distribution employs a tree structure to minimize clock offset maximally. For power amplifier module control, high-speed serial synchronous transmission technology is implemented, significantly reducing wiring connections and improving system reliability. The typical transfer function of a digital phase-locked loop is:

$$H(s) = \frac{2\zeta\omega_n \cdot s + \omega_n^2}{s^2 + 2\zeta\omega_n \cdot s + \omega_n^2}$$

where $s$ represents the complex variable of Laplace transform, $\zeta$ denotes the damping coefficient, and $\omega_n$ indicates the natural angular frequency, with $\cdot$ signifying multiplication operations.

3. Research on Transmitter Signal Processing Methods

3.1 Spectrum Control Technology

Spectrum control technology primarily optimizes the transmitted signal spectrum through digital pre-distortion and bandpass filtering \cite{9}. The filtering system employs an LC bandpass filter structure to remove spurious signals outside the transmitter frequency, ensuring output signal purity. During spectrum control, the quality factor $Q$ of the output tank circuit directly affects spectral purity, with $Q$ calculated by:

$$Q = \frac{f_0}{f_2 - f_1}$$

where $f_0$ represents the center frequency (0 is a digit, not a subscript, indicating fundamental frequency power), $f_2$ and $f_1$ (with 2 and 1 as subscripts) denote upper and lower cutoff frequencies, and $f_2 - f_1$ indicates the 3dB bandwidth.

The spectrum control system dynamically adjusts pre-distortion parameters through real-time monitoring of output signal spectral characteristics \cite{10}. Digital pre-distortion technology employs polynomial models to describe amplifier nonlinearities, enhancing spectral purity through inverse nonlinear compensation. The output tank circuit utilizes a double-tuned loop structure, employing vacuum variable capacitors for resonance point adjustment, with inductors featuring multi-tap designs for coarse tuning and capacitors for fine tuning. The pre-distortion processing unit in the spectrum control circuit is implemented via FPGA, storing nonlinear correction coefficients through lookup tables. The system establishes spectrum monitoring points for real-time spectrum data acquisition and analysis. To improve spectral purity, 3dB attenuation circuits with isolation are added between RF power amplifier stages to prevent mutual interference \cite{11}.

3.2 Modulation Depth Stabilization Technology

Modulation depth stabilization technology employs closed-loop feedback control, as illustrated in Figure 3 [FIGURE:3], to achieve precise modulation control \cite{12}. The modulation depth detection circuit utilizes envelope detection principles, measuring maximum and minimum values of the modulated wave to calculate modulation depth $m$:

$$m = \frac{E_{\text{max}} - E_{\text{min}}}{E_{\text{max}} + E_{\text{min}}}$$

where $E_{\text{max}}$ represents the envelope maximum and $E_{\text{min}}$ denotes the envelope minimum.

During modulation, dynamic range compression of audio signals is performed through the digital audio processing board to prevent overmodulation. The audio processing circuit incorporates precision limiters and automatic gain control circuits to maintain modulation depth within reasonable ranges \cite{13}. The system utilizes digital audio processors for audio signal amplitude control, implementing dynamic range compression and expansion through software algorithms. The modulation depth control system employs dual protection mechanisms, with analog limiting circuits at the audio input and digital limiting algorithms in the digital processing unit. Audio modulation achieves dither optimization through the addition of 72kHz triangular waves, improving modulation quality. The audio processing circuit also includes frequency response compensation units that separately process different frequency bands through multiple filter groups to optimize audio frequency response characteristics \cite{14}.

3.3 Spurious Emission Suppression Technology

Spurious emission suppression technology is achieved through multiple filtering and shielding measures. The output tank circuit incorporates a bandpass filter bank to remove spurious signals outside the transmitter frequency. The filter's passband attenuation characteristics employ Chebyshev approximation, providing steeper stopband performance. The system's overall spurious emission suppression ratio is determined by:

$$S = 20\log_{10}\left(\frac{P_0}{P_s}\right)$$

where $P_0$ represents the power at the transmission frequency (0 is a digit, not a subscript, indicating fundamental frequency power), $P_s$ denotes spurious signal power (with $s$ as a subscript indicating spurious signal), and $\log_{10}$ signifies the base-10 logarithm.

Spurious emission suppression employs a multi-stage LC filter structure, with the RF power amplifier utilizing multi-point grounding technology to reduce ground loop interference. The circuit layout design adopts partitioned shielding and filtering techniques to minimize electromagnetic interference. The output tank circuit's tuning circuit uses vacuum capacitors to avoid nonlinear distortion caused by conventional capacitors under high-power RF conditions \cite{15}. The system incorporates standing wave ratio monitoring circuits that automatically perform power control when load matching anomalies are detected, preventing excessive spurious emissions. Isolation circuits are added between the RF excitation circuit and power amplifier stages to reduce harmonic interference from signal reflections.

4.1 Test Platform and Methods

The system test platform comprises three components: a power test system, spectrum analysis system, and audio test system \cite{16}. The power test system employs high-power dummy loads to replace antennas for testing, with the dummy load consisting of 80 parallel-connected 12kΩ non-inductive wire-wound resistors, providing an output impedance of 150Ω and a maximum power capacity of 400kW. The spectrum analysis system measures transmitted signal quality through real-time spectrum analyzers, enabling monitoring of spurious emissions, spectral purity, and other indicators. The audio test system includes audio signal generators and audio analyzers for testing modulation characteristics. Test methods strictly adhere to national radio and television equipment technical standards, conducting comprehensive tests on the transmitter at high, medium, and low power levels. During power testing, thermal condition tests verify system stability, with individual resistors rated for maximum 5kW power, exceeding the 2.5kW carrier power requirement and satisfying power design specifications. Complete system operation testing validates long-term stability, ensuring equipment reliability across various operating conditions.

4.2 Indicator Measurement and Evaluation

Indicator measurement employs a distributed monitoring scheme, with test points established at critical nodes throughout the transmitter. Power detection is implemented at two locations: the power amplifier output and antenna input, with power data collected through directional couplers. Temperature monitoring covers critical components including thyristors, freewheeling diodes, and power filter inductors, with temperature switches generating monitoring signals when temperatures exceed preset thresholds. Each cabinet is equipped with airflow detection boards to monitor internal airflow conditions; when airflow anomalies occur in any cabinet, the transmitter reduces power or shuts down. Both power amplifier cabinets and network cabinets are fitted with arc detection boards to monitor for arcing events. Standing wave ratio detection includes automatic switching control circuits, network voltage and current tuning, and amplitude adjustment circuits, which activate automatic switching control signals based on frequency signals from the modulation controller. System efficiency testing is calculated through the ratio of input to output power. Measurements for output power stability, spectral purity, modulation depth, and other indicators all employ standard test methods to ensure measurement accuracy and reliability, as illustrated in Figure 4 [FIGURE:4].

Through improved circuit design and systematic testing of the medium-wave broadcast transmitter, the significant effectiveness of digital signal processing technology in enhancing transmitter performance has been confirmed. The improved transmission system has achieved design expectations for key indicators including modulation depth, spectral purity, and spurious emission. Experimental data demonstrates that this technical solution possesses substantial engineering practical value and can be widely applied to technical renovations of medium-wave broadcast transmission equipment. Positive progress has been achieved in reducing spurious emissions and improving spectrum utilization efficiency. The system exhibits excellent stability across different operating environments, with operational reliability fully validated.

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Author Biography: Suolang Zhuoma (1991—), female, from Nagqu, Tibet, holds a bachelor's degree and works as an assistant engineer. Her research focuses on medium-wave broadcast transmitters.

(Responsible Editor: Li Jing)