Direct digital synthesis (DDS) is a key technology to realize digitization, which is widely used in the field of telecommunication and electronic instruments. DDS is usually implemented by setting logic circuit in CPLD or FPGA, but the output of DDS is limited by the rate of D / a converter and the bandwidth gain and response time of operational amplifier in I / V conversion after D / a conversion, so the signal amplitude of CPLD and FPGA is unstable in high frequency band. Therefore, this paper introduces a signal generator design based on DDS device AD9851.
AD9851 circuit module and control logic
As AD9851 works at a high frequency, it is easy to introduce large noise, so we need to pay attention to the connection between power supply and ground wire to reduce the noise. In order to avoid high frequency interference, AD9851 and its periphery are realized by PCB. The circuit is shown in Figure 2.
There are two ways to write frequency control word and phase control word, parallel and serial, which can be realized by PFGA internal state machine. The FM modulation of the system is divided into two levels of maximum frequency offset: 5KHz and 10kHz, and the PSK modulation signal is realized by changing the phase control word. The control word and theoretical value are calculated as follows: the output data of DDS wavetable in low frequency band is 14 bits (214 = 16384). When the output value of DDS wavetable is greater than 16384 / 2 = 8192, the phase of PSK control word is changed by 180 degrees. Because the AD9851 phase control word is the upper 5 bits, if 180 ° is changed, the phase control word 8’h90 will be changed. The highest output of AD9851 is 150MHz, which corresponds to the frequency control word 32’hfffff (hexadecimal), so 1Hz corresponds to 28.633,1 (decimal).
The output range of sinusoidal signal generator is 100 Hz ~ 10 MHz, the frequency stability is better than 10-4, the amplitude is stable and adjustable, at the same time, it can output all kinds of commonly used modulation signals, and has the advantages of instant frequency conversion, flexible control, stable amplitude, small volume, low cost, etc., which makes the system can be used as a signal source in various portable fields. Because of the advantages of small product and low cost, the system can be used as a signal source in various portable fields.