Modulation method of medium-short wave signal and transmitter thereof

Abstract

The application discloses a modulation method of a medium-short wave signal and a transmitter thereof. The modulation method is based on PSM and replaces M small steps with one continuously adjustable pulse width modulation voltage. The main components of the corresponding transmitter include a power supply system, a control system, a signal processing and distribution system, a superposition system, a filter, and a switching amplifier. The application combines the advantages of PDM and SM, improves the overall efficiency and overall performance index of the transmitter, and realizes multiple power amplifier redundancy. All power amplifiers work in turns, the power amplifier with fault is intelligently skipped and alarmed, and other power amplifiers work normally, so that the output of the transmitter is not affected. The transmitter has a harmonic distortion of less than or equal to 0.8%, and an overall efficiency of greater than or equal to 89%. The transmitter has good economic and social benefits, and is worth popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of medium and shortwave transmitter technology, specifically a modulation method for medium and shortwave signals and a transmitter thereof. Background Technology

[0002] PSM is a fusion of PDM and SM (step modulation) technologies. The source signal is a sine wave, which is modulated into a step waveform. Currently, PSM is the mainstream modulation method for medium and shortwave transmitters in China, and it has higher overall efficiency compared to the previous PDM method.

[0003] A PSM modulator mainly consists of a power supply system, a control system, a superposition system, filters, and switching amplifiers. The power supply system converts the input AC to DC. The superposition system is primarily used to superimpose the output signals of the switching amplifiers. The switching amplifiers can be rapidly switched on and off, controlled by the control system. The working principle of this type of transmitter is that 48 final-stage power amplifiers use a magnetic ring transformer with copper rods (equivalent to one turn in series) as the secondary windings for synthesis. 42 of these are large-step power amplifiers with the same number of primary turns, while the other 6 are binary power amplifiers that vary their primary turns or operating voltage to produce output voltages of 1 / 2U, 1 / 4U, 1 / 8U, 1 / 16U, 1 / 32U, and 1 / 64U, respectively, where U is the large-step voltage. The synthesized output yields the overall output power. However, the fixed step size of the binary power amplifiers results in insufficient precision, leading to higher distortion, increased number of power amplifier modules, increased cost, and larger space requirements.

[0004] PSM also includes various modulation methods, such as N large step modulation or N large step + M small step modulation. The N large step modulation method uses N steps of equal height to approximate the curve. This modulation method will produce a huge truncation error, i.e., distortion, at positions where the fluctuation is less than an integer number of step heights. The final modulated waveform is a stepped waveform with the same height, and its error is one U0. While meeting the requirements, it brings huge distortion, so it is necessary to improve the accuracy. To improve accuracy, a factory developed a method using N large steps and M small steps, with the height of each small step being U1. To minimize the average error, U1 should be taken as U0 / (M+1). Initially, the large steps are used to approximate the position to a point less than one large step, and then the small steps are used for further approximation, up to a maximum of M small steps. M+1 small steps constitute one large step. However, this method still introduces some distortion, although the error becomes 1 / (M+1), or one U1, compared to the large step. While this improves accuracy, the increase is limited, and it also significantly increases costs. Since each transmitter idealizes its power combining and supply, and treats them as linear and constant, the resulting error will become increasingly larger. Therefore, a better method is needed to improve accuracy and reduce error. Summary of the Invention

[0005] To address the problem that increasing accuracy in PSM modulation is limited and incurs a significant cost increase, this invention provides a modulation method for medium and shortwave signals and its transmitter.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A modulation method for medium and short wave signals employs a combination of N large steps and one continuously adjustable pulse width modulation voltage. When the N large steps approach the height of less than one large step, the continuously adjustable pulse width modulation voltage is used to supplement them.

[0008] The continuously adjustable pulse width modulation voltage is generated by a pulse width modulator.

[0009] The output power of the N large-step power amplifier modules is a fixed 2.5kW, while the continuously adjustable power amplifier modules can output power from 0 to 2.5kW.

[0010] A medium-shortwave signal transmitter, wherein the power amplifier module consists of N power amplifier modules and one power module with a continuously adjustable pulse width modulation voltage; the N power amplifier modules are powered by a constant voltage power supply, and the N power amplifier modules are controlled by a signal processing and distribution system respectively.

[0011] The power module with continuously adjustable pulse width modulation voltage is powered by a PDM power supply, which is controlled by a signal processing and distribution system.

[0012] The N power amplifier modules are the same power amplifier module.

[0013] The constant voltage power supply can be supplied individually, or multiple power supplies can be connected in parallel, or a high-power power supply can be supplied together.

[0014] The signal processing and distribution system is implemented using an FPGA.

[0015] The beneficial effects obtained by this invention are:

[0016] This invention overcomes the limitations of traditional medium and shortwave broadcast transmitters that struggle to achieve high power and efficiency using PDM modulation, as well as the distortion issues associated with PSM modulation. All power amplifier modules used in the transmitter are identical and interchangeable, thus reducing the need for spare parts and significantly lowering maintenance costs.

[0017] By adopting this invention, high efficiency and good performance indicators can be obtained. At the same time, it can be used in a wide frequency bandwidth range, adapting to the requirements of 1+1 and N+1 systems of various medium and shortwave broadcasting stations, and also meeting the requirements of various medium wave stations that need to change frequencies at any time. It has good economic and social benefits and is worth promoting and applying.

[0018] The transmitter manufactured using this invention exhibits harmonic distortion ≤0.8% and overall efficiency ≥89%. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the transmitter circuit of the present invention.

[0020] Figure 2 This is a schematic diagram of the modulation principle of the present invention.

[0021] Figure 3 This is a block diagram of the PSM structure of a certain factory.

[0022] Figure 4 This is a block diagram of a PDM transmitter from a certain factory.

[0023] Figure 5 This is a schematic diagram illustrating the effects of N steps, N+M steps, and the N+1 steps of this invention.

[0024] Figure 6 This is a block diagram of a PDM transmitter from a certain factory.

[0025] Figure 7 This is a schematic diagram of sinusoidal voltage pulse step modulation (PSM).

[0026] Figure 8 This is a structural block diagram of a PSM transmitter from a certain factory. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings.

[0028] Medium and shortwave transmitters mainly consist of power supply, audio, radio frequency (RF), cooling, and detection systems, with the RF and audio systems being the core components. The main part of the audio system is the pulse width modulator, which performs the modulation and processing of audio signals.

[0029] To improve accuracy while reducing cost, this invention provides a new modulation method based on the N large step + M small step PSM modulation scheme. The modulation principle is as follows: Figure 2 As shown, unlike the traditional N+M steps, by... Figure 4The small steps are replaced with a nearly continuous curve. When N large steps approach the threshold and are insufficient to reach the height of a single step, an adjustable power amplifier is used to obtain the accurate voltage at that moment. In other words, a single continuously adjustable pulse width modulation voltage replaces the M small steps in N+M step modulation, hence the name N+1 step modulation. This method combines the advantages of low distortion in PDM (the final modulated curve is completely consistent with PDM) and high efficiency in SM (only one small power amplifier operates at full power) to improve the overall efficiency and performance of the transmitter.

[0030] Based on the above method, the improved circuit schematic is as follows: Figure 1 As shown in the diagram. In this embodiment, N power amplifiers are 2.5kW power amplifier modules, where N is 4. The entire unit has a total of 5 RF power amplifier modules. Four power amplifier modules with an output power of 2.5kW form the main power combining ladder, and the total output power after combining is 4×2.5kW. A continuously adjustable power amplifier works simultaneously with the other four power amplifiers. The continuously adjustable power amplifier is powered by a variable voltage using pulse width modulation (PDM), allowing it to output power that is continuously adjustable from 0 to 2.5kW. This can accurately compensate for the effects of the ladder and various nonlinear devices. In other words, this scheme is equivalent to the N+M ladder modulation method, where the continuously adjustable power amplifier replaces M steps, except that there are enough M steps here to be equivalent to continuously adjustable. Each power amplifier module is a broadband power amplifier with the same function and performance. The only difference is that the first four power amplifiers are supplied with a fixed voltage, while the last continuously adjustable power amplifier is supplied with a pulse width modulation voltage. All power amplifiers receive RF excitation, supply voltage, and control voltage from the exciter stage.

[0031] The output of each RF module is sent to the primary coil of a series combiner transformer. The combined output power is then sent to the RF output network. The four final-stage power amplifiers use a magnetic ring transformer with copper rods (equivalent to one turn in series) on the secondary side for combining. Four of the large-step power amplifiers have the same number of primary turns and the same supply voltage. Another power amplifier has the same number of primary turns as the N large-step power amplifiers. By changing its operating voltage, it creates output voltages of 1 / 1024 (the number of steps depends on the selected PDM adjustment accuracy; theoretically, it can be considered continuously adjustable) steps, thus achieving continuously adjustable power output. This large-step and continuously adjustable step form a high-efficiency, low-distortion, stable, and reliable medium- and short-wave transmitter. The combined output impedance of the entire transmitter is approximately 4Ω. To achieve a transmitter output impedance of 50Ω, a matching output network is added after the combiner to match the combiner's 4Ω output to a standard 50Ω output. To facilitate transmitter maintenance without shutting down the system, each module is equipped with a corresponding relay. When the RF power amplifier module is removed from the transmitter, the relay forces a contact to close on the primary coil of the series-combined transformer. When the transmitter is turned off, all relay contacts are closed, thus isolating the RF power amplifier modules from the output network and enhancing their resistance to lightning strikes. Since the four power amplifier modules at the front of the transmitter operate at full power, with only the last module used for fine-tuning and slightly lower efficiency, the overall efficiency of the transmitter is relatively high.

[0032] The main components of the system include a power supply system, a control system, a signal processing and distribution system, a superposition system, filters, and switching amplifiers. The signal processing and distribution system primarily processes the input audio signal, including sampling and quantization, and generates control voltages to control the operation of N+1 power amplifiers. It also generates a PDM control signal to control the PDM power supply to the power amplifier module that acts as a variable-step amplifier. This system also generates a carrier signal (RF) with the same operating frequency as the transmitter, which is set according to the transmitter's operational requirements. The constant voltage power supply can be a single, one-to-one connection, multiple power supplies connected in parallel, or a single high-power power supply. The RF signal, control voltage, and power supply are distributed to each power amplifier module, controlling their operating status. After output from the power amplifier modules, the combined output is sent through a serial combiner, then through a matching output network before being transmitted.

[0033] The power amplifier module is hot-swappable. When the module is removed, the primary coil of the cascade synthesizer at that module location automatically closes, eliminating the need for any additional key module for isolation. The RF power amplifier module consists of four wideband RF amplifiers with a carrier output level of 2500 watts and a peak power level of up to 13.25 kilowatts. Various control signals are directly assigned and sent to the power amplifier module for signal processing. The processed signals are then directly sent to the high-efficiency Class D RF amplifier. All circuitry is integrated onto a single circuit board, eliminating the need for interconnections between stages. This streamlined power amplifier chain facilitates troubleshooting and maintenance. The Class D power amplifier contains four switching devices arranged in an H-bridge configuration to generate an unfiltered, modulated RF signal. The switching devices are controlled as follows: a pair of devices on one diagonal is simultaneously activated, while a pair on the other diagonal remains deactivated. This generates two opposing power surges across the load, which is located at the center of the H-bridge. The pins of the power amplifier tubes are connected using a stable and reliable crimping method. When replacing the power amplifier, no soldering machine is needed; it can be removed directly with screws, avoiding damage caused by soldering during maintenance and replacement.

[0034] The synthesized RF output is filtered through an RF network consisting of two T-networks and a third harmonic notch filter that acts as a shunt. The system monitors the RF output using RF current probes, RF voltage probes, and RF sampling probes. The samples obtained from these probes are used for control and monitoring functions.

[0035] The coupled output port feeds back the RF signal to the modulation controller. The modulation controller demodulates the signal and compares it with the input audio signal to derive a predistortion control signal. The modulation controller then adjusts the predistortion of the control signal. The introduction of predistortion control can compensate for the effects of power supply voltage variations following load power changes (i.e., carrier drop), as well as the effects of nonlinearity in synthesizers and other components, and also increases the bandwidth used by the transmitter.

[0036] The performance parameters of the transmitter obtained from the test are shown in Table 1.

[0037] Table 1. Operating parameters of N+1 step modulation

[0038]

[0039] Figure 6The diagram shows the principle block diagram of a PDM transmitter from a certain factory. The power amplifier of this transmitter achieves continuously adjustable power output by changing its operating voltage to generate output voltages in steps of 1 / 1024 (the number of steps depends on the selected PDM adjustment accuracy; theoretically, it can be considered continuously adjustable). This results in a low-distortion, stable, and reliable medium- and short-wave transmitter. However, because the power amplifier operates at less than full power output most of the time, the overall efficiency is low. The operating performance range of this transmitter is listed in Table 2.

[0040] Table 2 PDM Performance Indicators

[0041]

[0042] PSM is a fusion of two major technical modes: PDM and SM (step modulation). Its working diagram is shown below. Figure 7 The source signal is a sine wave, which is modulated into a stepped waveform. Currently, the mainstream modulation method for medium and shortwave transmitters in China is PSM, which has higher overall efficiency than the previous PDM method.

[0043] A PSM modulator mainly consists of a power supply system, a control system, a superposition system, filters, and a switching amplifier. For example... Figure 8 The diagram shows the PSM (Power Supply Management) structure of a certain factory. The power supply system aims to convert the input AC power into DC power. The superposition system is mainly used for superimposing the output signals of the switching amplifier. The switching amplifier can be quickly turned on and off, and is controlled by the control system. Its operating parameters are listed in Table 3.

[0044] Table 3. Operating parameters of N+M step modulation

[0045]

[0046] The working principle of this type of transmitter is that the 48-channel final stage power amplifier uses a magnetic ring transformer method with copper rods (equivalent to one turn in series) as the secondary stage for power synthesis. 42 of these are large-step power amplifiers with the same number of primary turns, while the other 6 are binary power amplifiers that vary their primary turns or operating voltage to produce output voltages of 1 / 2U, 1 / 4U, 1 / 8U, 1 / 16U, 1 / 32U, and 1 / 64U respectively, where U is the large-step voltage. The synthesized output yields the overall output power. Because the binary power amplifier's steps are fixed, the precision is insufficient, resulting in higher distortion, increased number of power amplifier modules, increased cost, and larger space requirements. When the number of modules is insufficient, the precision decreases. Therefore, low-power transmitters do not use this modulation method. As can be seen from the table, the harmonic distortion of low-power transmitters is significantly greater than that of high-power transmitters. However, because each power amplifier module operates at full power, the overall efficiency is relatively high.

[0047] PSM also includes various modulation methods, such as N large step modulation or N large step + M small step modulation. N large step modulation uses N steps of equal height to approximate the curve, such as... Figure 3 The step modulation, also known as coarse amplitude modulation, is used at positions where the fluctuation is less than an integer number of step heights, such as... Figure 3 Point A) produces a huge truncation error, i.e., distortion, resulting in a stepped waveform of uniform height with an error of U0. While meeting the requirements, this introduces significant distortion, necessitating improved accuracy. To enhance accuracy, a certain factory developed... Figure 8 The diagram shows N large steps plus M small steps, with the height of the small steps being U1. To minimize the average error, U1 should be taken as U0 / (M+1). In this case, for... Figure 4 Point A is first approximated by a large step to a position less than a large step, and then approximated by small steps, up to M small steps. M+1 small steps constitute a large step. However, some distortion will still occur, but the error becomes 1 / (M+1), or one U1, compared to the large step, which greatly improves the accuracy. However, the increase in accuracy is limited, and increasing accuracy will also bring huge cost increases. Moreover, each transmitter will idealize the transmitter's power synthesis and power supply, and will process it according to linearization and constantness. Therefore, the final error will become larger and larger.

[0048] Comparing Tables 1, 2, and 3, it can be seen that the overall efficiency of the medium-wave transmitter designed in this scheme is similar to that of the N+M step-modulation PSM transmitter, and significantly better than that of the PDM transmitter. Moreover, compared to the PDM transmitter, the N+1 step-modulation transmitter can achieve multiple power amplifier redundancies, with all power amplifiers operating in turn while others continue to function normally, without affecting the overall output. Therefore, the stability of the N+1 step-modulation PSM is also sufficiently high, meeting a wider range of practical needs.

Claims

1. A modulation method for medium and shortwave signals, characterized in that, The method combines N large steps with one continuously adjustable pulse width modulation voltage. When the N large steps approach the height of less than one large step, the continuously adjustable pulse width modulation voltage is used to supplement it.

2. The modulation method for medium and shortwave signals according to claim 1, characterized in that, The continuously adjustable pulse width modulation voltage is generated by a pulse width modulator.

3. The modulation method for medium and shortwave signals according to claim 1 or 2, characterized in that, The output power of the N large-step power amplifier modules is a fixed 2.5kW, while the continuously adjustable power amplifier modules can output power from 0 to 2.5kW.

4. A medium- and short-wave signal transmitter, manufactured using the method described in claim 1, characterized in that: The power amplifier module consists of N power amplifier modules and one power module with a continuously adjustable pulse width modulation voltage; the N power amplifier modules are powered by a constant voltage power supply, and the N power amplifier modules are controlled by the signal processing and distribution system respectively.

5. A medium-shortwave signal transmitter according to claim 4, characterized in that: The power module with continuously adjustable pulse width modulation voltage is powered by a PDM power supply, which is controlled by a signal processing and distribution system.

6. A medium-shortwave signal transmitter according to claim 4, characterized in that: The N power amplifier modules are the same power amplifier module.

7. A medium-shortwave signal transmitter according to claim 5, characterized in that: The constant voltage power supply can be supplied individually, or multiple power supplies can be connected in parallel, or a high-power power supply can be supplied together.

8. A medium-shortwave signal transmitter according to claim 4, characterized in that: The signal processing and distribution system is implemented using an FPGA.

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