300M internal clock frequency
• Frequency shift keying (FSK), binary phase shift keying (BPSK), phase shift keying (PSK), pulse frequency modulation (CHIRP), amplitude modulation (AM) operation
· Quadrature dual 12-bit D/A converter
Ultrafast comparator, 3 picoseconds effective jitter deviation
External dynamic characteristics:
80 dB Spurious-Free Dynamic Range (SFDR) @ 100 MHz (±1 MHz) AOUT
4x to 20x programmable reference clock multiplier
Two 48-bit programmable frequency registers
Two 14-bit programmable phase compensation registers
12-bit amplitude modulation and programmable on-off shaping keying
Single-pin FSK and BPSK data input interfaces
· PSK function can be realized by I/O interface
· Linear and non-linear FM CHIRP function with pin controllable pause function
· With transitional FSK function
· Less than 25 ps RMS jitter deviation in clock generator mode
Automatic bidirectional frequency scanning
Can perform sin(x)/x correction on the signal
Simple control interface:
Configurable for 10 MHz serial interface, 2-wire or 3-wire SPI-compatible interface or 100 MHz 8-bit parallel programmable interface
·3.3V single power supply
· With multiple low-power features
Single or differential input clock
Small 80-pin LQFP package
Signal generation circuit formed by AD9854The AD9854 DDS chip is used here. It can generate signals up to 150 MHz according to the Nyquist sampling law when driven by a 300 MHz clock. In order to obtain a better signal frequency, only signals up to 100 MHz are generally obtained. To get a signal higher than 100 MHz, use its higher harmonics. Based on the AD9854 signal generation circuit as shown:
Since the AD9854 supports FSK modulation, the MSK signal we actually need is a special kind of orthogonal FSK signal. Its particularity is that the difference between the two sets of frequencies is the minimum frequency that satisfies the orthogonality of the two sets of frequencies. difference. It is worth noting that the phase of the MSK signal is continuous. In the modulation process, we need to add phase constants to ensure the phase continuity of the generated MSK signal. This will increase the complexity of the MSK modulation system. One of the advantages of the AD9854 chip is that the output phase of the chip is continuous. Therefore, using the AD9854 to generate MSK signals will greatly reduce system complexity.
1, FPGA configurationIn practical applications, the AD9854 is required to generate the MSK signal that meets the requirements. The FPGA must be used to configure the AD9854. FPGA and AD9854 hardware connection is shown as in Fig. 1, the pin definition in the picture is shown in Table 1.
After the hardware circuit is set up, you can use the FPGA to configure the registers in the AD9854 to implement the corresponding functions of the AD9854 so that the AD9854 generates the MSK signal.
For the MSK signal, the two sets of frequencies are orthogonal to each other, and the frequency difference Δf=f2-f1=1/2Tb, that is, the modulation index h is:
From equation (5), we can see that in order to guarantee the continuity of the phase, the phase constant added in this bit interval is not only related to the input of this bit interval, but also related to the input and phase constant in the previous bit interval, which will This greatly increases the difficulty of generating MSK signals in non-AD9854 methods. However, in the first section mentioned that the AD9854 chip itself can guarantee the continuity of the output signal phase, so in this system design, there is no need to consider the MSK signal phase continuity problem.
The parameters of the MSK signal generated by the AD9854 in this system are shown in Table 2.
According to equation (3), the two sets of frequencies for the MSK signal are:
The AD9854 has a high degree of operability. It has 39 configurable registers and can be configured with parameters such as frequency, phase, amplitude, and clock according to user requirements. All the AD9854 need to configure the registers shown in Table 3.
According to the MSK signal generation method proposed in this paper, the MSK signal is generated, and the signal generated by the oscilloscope observation is shown in FIG. 2 , and the frequency spectrum of the signal generated by the spectrum analyzer is shown in FIG. 3 .
The frequency difference between the MSK signals in the system is only 100 Hz. Therefore, the time domain waveform of the MSK is similar to the sine wave. It can be seen from the waveforms in the oscilloscope that the MSK signal generated by this method has constant envelope, continuous phase, and stable jitter.
The frequency spectrum of the MSK signal observed by the spectrum analyzer can obtain the output center frequency of 300 KHz, the output level of -14 dBm, and the spurious-free dynamic range of 60 dB, which satisfies the specifications.
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