Analysis of digital tuning filter principles and various solutions

0 Introduction

The digital or analog signal information to be transmitted is generally a low-frequency signal, which must be modulated by the carrier to a specific radio frequency band before it can be transmitted through the antenna. With the development of communication technology, fixed carrier frequency technology has gradually exposed problems in confidentiality, anti-interference, and frequency band utilization in military communications. In order to solve these problems, frequency hopping (Frequency Hopping Spread Spectrum, FH-SS) communication technology has gradually developed. Digital tuning filters are a type of digital tuning control band filter with a certain power capacity developed after the emergence of computer control technology in frequency hopping systems.

1. Current Status of Digital Tuning Filter Technology

The information transmitted by the traditional fixed carrier frequency signal transmitter can be in the form of analog or digital signals. After the signal is modulated, a modulated wave signal with a fixed subcarrier frequency is obtained, which is then mixed with the main carrier frequency signal output by the frequency synthesizer to make the carrier frequency of the modulated wave signal output meet the requirements of the RF passband, and then fed to the antenna for transmission. The receiver selects the required carrier frequency signal through a bandpass filter, and obtains the information transmitted by the transmitter after amplification, frequency synthesizer, and demodulation.

The frequency synthesizer of the frequency hopping system controls the frequency of the output carrier signal through the frequency hopping instruction. The frequency hopping instruction generator can continuously issue instructions to control the frequency synthesizer to continuously change the frequency of its output carrier. Therefore, the carrier frequency of the modulated wave output by the mixer will also continuously jump with the instructions, so the frequency hopping signal is sent out through the filter and antenna. This is the frequency hopping communication technology, as shown in Figure 1.

Frequency hopping communication is one of the most commonly used spread spectrum methods. Its working principle refers to a communication method in which the carrier frequency of the transmission signal of the sender and receiver changes discretely according to a predetermined rule. In other words, the carrier frequency used in the communication jumps randomly under the control of a pseudo-random change code. In terms of the implementation of communication technology, "frequency hopping" is a communication method that uses a code sequence for multi-frequency frequency shift keying, and is also a communication method for code-controlled carrier frequency hopping. From the time domain, the frequency hopping signal is a multi-frequency frequency shift keying signal; from the frequency domain, the spectrum of the frequency hopping signal is a random jump at unequal intervals over a very wide frequency band. Among them: the frequency hopping controller is the core component, including functions such as frequency hopping pattern generation, synchronization, and adaptive control; the frequency synthesizer synthesizes the required frequency under the control of the frequency hopping controller; the data terminal includes error control for the data.

Compared with fixed-frequency communication, frequency-hopping communication is a communication method in which both the transmitter and receiver change the frequency synchronously, making it difficult for the adversary to track the frequency-hopping pattern and thus unable to intercept the communication content, thus having a strong anti-decryption capability. At the same time, frequency-hopping communication also has good anti-interference capabilities. Even if some frequencies are interfered with, normal communication can still be carried out on other frequencies that are not interfered with. In the civilian field, since the frequency-hopping communication system is an instantaneous narrowband system, it is easily compatible with other narrowband communication systems. In other words, frequency-hopping radio stations can communicate with conventional narrowband radio stations, which is conducive to equipment updates; frequency hopping also uses the advantages of frequency division and code division multiple access to improve spectrum utilization and solve problems such as spectrum resources and channel capacity congestion.

The world's first frequency hopping radio came out in the late 1970s, and its technology has been continuously developing and improving since the 1980s. Industry insiders point out that frequency hopping communication is an effective means of confidentiality and countering radio interference, calling it the "killer weapon" of radio communication. The use of frequency hopping technology in wireless communications is one of the most important means of confidentiality and anti-interference in military communications. Compared with frequency hopping technology, cracking frequency hopping codes, fast tracking of carrier frequencies, and broadband high-power interference are three different levels of countermeasure technologies.

Cracking the frequency hopping code is actually cracking the fast frequency hopping password and communication password, and the difficulty is needless to elaborate; fast tracking of carrier frequency is the main countermeasure of the current frequency hopping technology. The current technology of the United States has reached a frequency sweep of 1,000 times/s, that is, it cannot be captured if the frequency hopping time is less than 1 ms, while the current frequency hopping technology can reach a frequency hopping time of less than 10μs, and the current processing speed is also 2 orders of magnitude behind the frequency hopping technology; broadband high-power interference is a common interference solution, but it is affected by the transmitter power. The power needs to reach dozens of times the transmission power of the frequency hopping radio station to achieve the interference effect. Countering the frequency hopping radio station often requires a large investment and small results. Therefore, the three main types of electronic countermeasures have not yet threatened the developing frequency hopping technology. A frequency hopping filter is a bandpass filter inserted into the channel of the transmitting and receiving system. Changing the center frequency of the bandpass filter according to the frequency hopping pattern (the law of carrier frequency change in frequency hopping communication) can significantly improve the anti-interference performance of the system and effectively improve the signal-to-noise ratio of the receiver, thereby reducing the system's requirements for frequency hopping transceivers, making frequency hopping communication equipment more efficient and reliable, while also meeting confidentiality requirements. Frequency hopping filtering technology is one of the key technologies of frequency hopping communication. The following will explain and compare several principle schemes of frequency hopping filters.

2 Principle and solution of frequency hopping filtering technology

The frequency hopping filter needs to achieve rapid change of the center frequency of the filter. The core is to achieve this by continuously and rapidly changing or using switches to change all or part of the parameters of the bandpass filter. In practical applications, there are four main implementation schemes for frequency hopping filters: unit combination filter group, filter using variable parameter device, digital filter, and digital tuning filter. The following is an analysis and comparison of the technical principles of various filters.

2.1 Unit combination filter bank scheme

The scheme of the unit combination filter group is shown in Figure 2. Its circuit principle is relatively easy to understand. Each bandpass filter corresponds to a center frequency. The "control code" of digital tuning controls the "switch array" of the filter group input and output through the "controller", switching different internal filter sub-units to achieve the purpose of digital tuning frequency selection filtering; the controller can also be manually adjusted to achieve the purpose of tuning the frequency selection filter sub-unit.

The advantages of the unit combination filter bank design structure are as follows:

Since its internal filter unit is independent, filters of various principles can be made according to requirements. The topological structures of filter 1 and filter 2 can be completely different, and the design and application are more flexible.

Since the switching unit only performs digital control of the switch array, the frequency hopping speed of the digital filter is very fast, which can reach the order of microseconds; the filter unit is mainly composed of LC, which is not affected by the static DC operating point of the semiconductor device, so the power capacity is large. At the same time, LC can select devices with good temperature characteristics, so the temperature characteristics such as the center frequency temperature drift of the filter are good. This design structure is simple in principle and easy to implement. It has been improved and developed in the civilian field. The high-frequency head tuner of early communication machines and televisions is a successful example.

The disadvantages of the unit combination filter group are also obvious:

The number of filter units required depends on the frequency hopping points required by the system, and each filter unit is debugged independently. Multiple groups of filter units have a large number of components, large volume, and high debugging difficulty. Generally, when the system requires more than 8 groups, the large volume of multiple groups of filters will be troublesome.

When multiple groups of filters are assembled in a space very close to each other, they will inevitably cause mutual interference in the radio frequency or higher frequency bands. That is to say, the filter is not only affected by the centralized parameters and distributed parameters of the connected unit, but also by the distributed parameters of the adjacent units. Often, when one filter unit is adjusted, the adjacent unit is not normal. This places high demands on the design and debugging difficulty of the filter, and the space occupied by the unit combination filter group, making it difficult to apply it in modern frequency hopping communications.

Therefore, this design structure is widely used in the design of frequency hopping filters with a small number of units, but it cannot be implemented in a frequency hopping communication system with more than 250 frequency hopping points.

2.2 Variable parameter device filter solution

The scheme of variable parameter device filter is shown in Figure 3. The key of the scheme is that there is a device with controllable parameters inside the filter. Similarly, the "control code" of digital tuning controls this device through the "controller", for example, by controlling the parameters of the voltage-controlled element and the variable capacitance diode, the overall parameters of the filter are changed, thereby changing the center frequency of the filter, achieving the purpose of digital tuning frequency selection filtering.

The advantage of the variable parameter device filter solution is that the circuit volume can be made very small and the circuit debugging is relatively convenient. Therefore, in conjunction with phase-locked and digital storage technology, it has been improved and developed in the civilian field. The electric tuning tuner of color TV is a very successful example.

The disadvantages of the variable parameter device filter solution are: the linearity of the "variable parameter" device is generally not good, and the controllable range is small, which can be improved by the segmented tuning method; the accuracy of active control of such devices is poor, although it can be compensated by phase-locking and other technologies, but the phase-locking frequency and speed are low. Limited by the phase-locking frequency and speed, variable parameter device filters are difficult to use in the field of high-frequency frequency hopping; generally, the temperature characteristics of such devices are poor, and the electrical performance of the filter cannot be extended to the military temperature range; the power capacity of the controlled device is small, and it cannot pass the power signal, which is suitable for signal processing, but not suitable for use in transmitters and receivers. Therefore, variable parameter device filters are difficult to use in military frequency hopping communications.

2.3 Digital filter solution

The principle block diagram of the digital filter is shown in Figure 4. Its solution is based on digital signal processing. The input analog signal is first converted into a digital signal through an A/D converter, and then transformed through the microprocessor's FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) and other transformations to perform filter function algorithm to process the data, and finally the processed signal is output through D/A conversion.

Advantages of digital filter solution: With the help of microprocessor, very complex algorithms can be used. By designing appropriate programs, the functions that can be completed can be far more than filtering requirements, and even various signal analysis and recognition tasks can be realized. The software processing method is flexible and the processing accuracy is high. This is the advantage of this solution and also the development trend of all signal processing fields in the future.

However, the disadvantages of this solution are: based on signal processing, the power capacity is small and it cannot be transmitted through power signals; it relies on the microprocessor and the speed of A/D and D/A conversion, and the processing speed is currently slow and can only be suitable for processing signals below a few hundred kHz; the signal can only be transmitted in one direction.

In addition, there is another branch of digital filters - programmable filters. MAXIM has produced a single-chip programmable filter MAX264, which integrates the resistors and capacitors required for the filter, does not require external devices, and its center frequency, Q value and working mode can be controlled by pin programming. MAX264 can work in bandpass, lowpass, highpass, bandtrap or fullpass mode, and its passband cutoff frequency can reach 140 kHz; it can complete simple digital tuning functions, but its controllable points are very few, the maximum operating frequency is low, and the temperature characteristics are also poor, which is far from meeting the requirements of frequency hopping radio stations currently working in the RF field, and the frequency gap is 3 to 4 orders of magnitude. Some people also use "programmable logic devices (FPGA)" or "application-specific integrated circuits (ASIC)" to make related products, and the so-called "high frequency" can only reach 1 MHz, which is also several orders of magnitude different from the requirements for military communications. Single-chip programmable filters are the direction of development, and the gap of several orders of magnitude requires at least ten years of technological development to keep up.

2.4 Digitally tuned filter solution

The principle block diagram of the digital tuning filter is shown in Figure 5. The control code controls the digital logic through the input interface circuit. The digital logic contains logic circuits such as shift registers and programmable oscillators. The data stored in the memory is read and converted into a control sequence to control the switch drive array, drive the combination of closing and opening of the capacitor array in the filter, and then control the filter parameters. That is, the filter frequency selection is realized by controlling the combination of a small number of components with different weights in the filter, so as to achieve the effect of controlling the amplitude-frequency characteristics of the filter.

Advantages of digital tuning filters: Frequency is the result of controlling the combination of parameters of different weighted components in the filter. Each specified frequency may have dozens, hundreds, or even thousands of combinations to choose from. We can select the best combination of filter amplitude-frequency characteristic indicators, so the control characteristics of the digital tuning filter frequency are very good.

Since the frequency is generated by combination, it overcomes the problem of nonlinear parameters of single components in other solutions, and can make the selection points evenly distributed. For example, 250 selection points are evenly distributed in 10-30 MHz, and each step is 0.08 MHz. Since the frequency is generated by combination, it can be combined with a small number of components with the least different weights, so a set of hardware can combine multiple frequencies, overcoming the disadvantage that multiple sets of filter groups are required for multiple center frequencies.

This scheme controls the capacitor element array in the LC filter, retaining the advantage of the large power capacity of the LC filter. The signal can be transmitted in both directions, which is also an advantage. The control of this digital tuning filter is controlled by the switch control array of the frequency hopping pattern, and the speed can be very fast, meeting the requirements of military frequency hopping communication.

The disadvantages of this scheme are: the circuit is complex, it is a combination of digital circuit and analog circuit; the volume is medium; control devices are added, and the difference loss is added; it is difficult to make a high-order filter due to the complexity of the structure; it needs the support of the internal software of the circuit, and has high requirements for the hardware and software of the peripheral tuning and testing system. Through the above analysis and comparison of the principle scheme of the frequency hopping filter, the digital tuning filter is suitable for the requirements of military frequency hopping communication.

3 Key technologies of digitally tuned filters

The digital tuned filter circuit involves a variety of component hardware technologies such as digital logic circuits, high-voltage driver circuits, microwave PIN arrays, capacitor arrays, and magnetic powder core transformers. The design and production of the filter unit group is the key hardware technology of the digital tuned frequency hopping filter.

In principle, the circuit uses software to obtain a combination of device parameters with multiple weights, and then generate the filter module function. Therefore, pure hardware does not have the function of frequency hopping and frequency selection, and it also requires internal software support in the module circuit. During the manufacturing process, the filter subunit group of the digitally tuned frequency hopping filter will produce the uniqueness of the circuit hardware, that is, the electrical performance characteristics of each circuit are different from each other, so the internal adaptation software data of each circuit is also different, and the installed copied software will not be able to meet the module performance indicators. Therefore, in addition to the hardware requirements, a complex software debugging system is also required to debug and test each circuit, and give each circuit's internal software its own unique performance software. The debugging and testing software system of the digitally tuned frequency hopping filter is the key technology of the digitally tuned frequency hopping filter software.

4 Conclusion

Digital tuning frequency hopping filter is one of the key components of military frequency hopping communication. Digital tuning frequency hopping filter is a kind of programmable filter with multi-point frequency hopping, high frequency accuracy, certain power passing, and suitable for radio frequency segment. It has irreplaceable advantages such as unit combination filter, variable parameter device filter, digital filter, etc. The biggest feature of digital tuning frequency hopping filter is that each module has unique and distinctive software inside. The best software and hardware work together to achieve the best performance of the module.