Design of binary amplitude shift keying (ASK) digital frequency band transmission system

0 Introduction

In modern digital communication systems, the application of frequency band transmission systems is the most prominent. The original digital baseband signal is transformed into a frequency band signal suitable for transmission on the frequency band through spectrum shifting. The system that transmits this signal is called a frequency band transmission system. In the frequency band transmission system, different frequency band modulation methods are formed according to the control of different parameters of the carrier by the digital signal. The carrier amplitude of the amplitude shift keying method (ASK) changes with the modulation signal. Its simplest form is that the modulation signal in the digital form of the carrier is turned on and off under control, which can also be called on-off keying (OOK). In this design, a sine wave is selected as the carrier, and a binary baseband signal is used to modulate the amplitude of the carrier signal. The carrier is turned on or off under the control of the digital signal 1 or 0. When the signal is 1, the carrier is connected, and there is a carrier on the transmission channel; when the signal is 0, the carrier is turned off, and there is no carrier on the transmission channel. The bandwidth of the modulated signal is twice the width of the binary baseband signal. This modulation is called binary amplitude shift keying (2ASK).

1 2ASK signal algorithm

1.1 Time Domain

Where an=1 or 0, g(t) is the pulse shape, Ts is the symbol interval, and the carrier c(t)=COSωct. When s(t) is a rectangular pulse, 2ASK modulation is called on-off-key control OOK (on-off-key Control). The OOK signal uses the on-off (presence or absence) of the carrier to represent the baseband "1" code or "0", as shown in Figure 1.

1.2 Frequency domain

Assume that the spectrum of S(t) is S(ω), and the spectrum of S2AKS(t) is:

This shows that the spectrum of the 2ASK signal is to move the center of the digital baseband spectrum to the carrier frequency, and the bandwidth is twice the baseband bandwidth; and from we can see that the baseband signal is composed of several basic pulses, so the bandwidth of the baseband signal is completely determined by the basic pulse bandwidth. The bandwidth of the 2ASK signal depends on the bandwidth of the baseband basic pulse, which is twice the basic pulse bandwidth. Assume a rectangular pulse:

The power spectrum of a single basic pulse from equation (7) is shown in FIG2 , where the code rate Rs=1/Ts.

As can be seen from Figure 2,

Each of its zero points satisfies: sin(ωTs／2)=0==>ωTs／2=πi, i≠0==>ω=2πiRs, i≠O, the first side lobe peak is about 14 dB attenuated than the main peak. Therefore, the first zero point is usually taken as its effective bandwidth, and the first zero point bandwidth (effective bandwidth) converted into frequency units is: 2πRs／2π=Rs. Therefore, the effective bandwidth of the 2ASK signal is 2Rs, and the effective bandwidth is fc±Rs.

2 2ASK modulation circuit

The purpose of modulation is to move the signal spectrum to an area suitable for channel transmission and to realize channel frequency division multiplexing. Although the spectrum components of the baseband signal have been significantly attenuated outside the first zero point bandwidth, it may still interfere with another signal under frequency division multiplexing, so a corresponding circuit is necessary to suppress harmonics. The 2ASK modulation circuit generally includes a high-frequency oscillation circuit, a modulation switch circuit, and a filter circuit. In this design, the oscillation circuit generates a 107MHz sinusoidal signal as a carrier, which is connected to a bandpass transmission filter (surface acoustic wave filter 107±2MHz) with a bandwidth greater than 2Rs after the modulation switch circuit, and then connected to a 7th-order Butterworth low-pass filter to limit the harmonic frequency band of the modulated signal entering the channel.

2.1 Quartz crystal LC oscillator circuit design

The sine wave generating circuit can generate a sine wave output. It is formed by adding positive feedback to the amplifier circuit. It is the core circuit of various waveform generators and signal sources. The sine wave generating circuit is also called a sine wave oscillation circuit or a sine wave oscillator. It is usually composed of an amplifier circuit, a feedback network, a frequency selection network, an amplitude stabilization circuit, etc.

The frequency stability of the oscillator is an extremely important technical indicator. Whether the frequency of this modulator is stable depends on the frequency stability of the main oscillator (excitation source) in the system. If the frequency is unstable, it will affect the reliability of communication and cause large errors. Therefore, this design uses a highly stable quartz crystal.

Quartz crystal has the characteristics of a resonant circuit. If the angular frequency ω of the applied voltage is equal to the inherent resonant angular frequency ωq of the quartz mechanical vibration (which is determined by the geometric dimensions and cut type of the quartz crystal), the quartz crystal will resonate. That is, when the amplitude of the applied voltage remains unchanged, the elastic deformation is greatly enhanced, and thus the current reaches a maximum. At the same time, it has excellent single-frequency properties, that is, each crystal can only provide a stable oscillation frequency and cannot be directly used in a band oscillator. It is only suitable for a single-frequency oscillation generator.

The quartz crystal must be inductive to form an LC parallel resonant circuit and generate oscillation. Since the Q value of the quartz crystal is very high, reaching more than several thousand, the circuit shown can achieve very high oscillation frequency stability. The specific circuit design is shown in Figure 4.

2.2 Modulation switch circuit design

Figure 6 is the modulation circuit in this design. The modulation switch uses one of the NAND gates in the TTL logic device 74F00. The high-frequency oscillation signal is generated by the circuit shown in Figure 4. After being amplified by the two-stage high-frequency power tube 3355, it enters the input end (pin4) of the NAND gate after being isolated by the capacitor C206. At the same time, pin4 is raised to the potential:

As shown in Figure 7, this potential stabilizes the NAND gate in the linear region.

Before the modulation signal (data pulse) enters the gate circuit, a 47l capacitor is added to the ground, and a 1μH inductor is connected in series to filter out some high-frequency components of the pulse, making it easier to modulate. But in general, the modulation signal edge is too steep, the harmonic component is too heavy, and the pulse width of the modulation signal is also very narrow (the maximum frequency of the modulation signal in this design is 50kHz), which is not conducive to the transmission of the modulation signal in the network.

When the data pulse is input from pin 5, when the data pulse is "0", no matter what the carrier signal is, the output is always high level "1"; when the data pulse is "1", the output is the carrier signal. The output terminal is connected to a DC blocking capacitor C207 (2200pF). When the output is high level, it is equivalent to DC and will be blocked by the DC blocking capacitor. In this way, the input sound meter is shown in the waveform marked in Figure 6.

The modulated signal in this design contains rich harmonic spectrum, so it is necessary to filter its harmonics to improve the stability of the system, thus applying it to the surface acoustic wave bandpass filter.

2.3 Surface Acoustic Wave Filter

The surface acoustic wave filter (SAW filter) is composed of a metal film evaporated on a substrate with piezoelectric effect, and then a pair of interdigital electrodes are formed at both ends through photolithography. When a signal voltage is applied to the transmitting transducer, an electric field is formed between the input interdigital electrodes to cause the piezoelectric material to vibrate mechanically (i.e., ultrasonic waves) and propagate to the left and right sides in the form of ultrasonic waves. The energy on the edge is absorbed by the sound absorbing material.

This filter is small in size and light in weight. The center frequency can be made very high and the relative bandwidth is relatively wide. Ideally, it has a rectangular frequency selection characteristic. However, the actual frequency response cannot be rectangular. When the sound meter is working, there are still some false signals that affect its characteristics. The most important of these is the third-order transit signal. It is a part of the sound wave emitted by the receiving transducer and sent to the receiving transducer through the transmitting transducer. Its time delay is three times that of the main signal, which interferes with the main signal and causes the signal in the passband to fluctuate. Its actual frequency response is shown in Figure 8, so it is necessary to cascade a low-pass filter afterwards.

2.4 Design of Butterworth LPF

Separating signals and suppressing interference are the most extensive and basic applications of filters, which allow the required frequency signals to be transmitted smoothly and interfere with the unwanted frequency signals. The modulation signal frequency in this design is 107MHz and the bandwidth is ±2MHz, so the cutoff frequency of the low-pass filter should be designed at 110MHz.

The attenuation rate of the Butterworth filter is slower than other types of filters, but it is very flat, with no amplitude variation, low requirements in all aspects, simple design, and no obvious disadvantages in performance. The Q value requirements for the components that make up the filter are very low, so the inductor can be made with enameled wire; and the selectivity of this filter in this design is not very high, mainly for filtering out harmonic components such as the second harmonic (214MHz) and the third harmonic (321MHz) to avoid interference with other signals. Considering the system requirements comprehensively, this design uses a 7th-order Butterworth LPF.

When designing a Butterworth LPF, the filter is based on the normalized Butterworth LPF design data, and its cutoff frequency and characteristic impedance are converted into corresponding values ​​of the filter to be designed, as shown in the flow chart 10.

According to the theoretical calculation and the debugging results on the network analyzer, the specific filter parameters are shown in Figure 11. The capacitor uses a ceramic capacitor with a package of 0603, and the inductor is manually wound with enameled wire, where φ is 2mm and D is 0.13mm. The signal waveform obtained after the modulated waveform passes through the filter is shown in Figure 12, with a center frequency of about 107MHz and a bandwidth of 2MHz.