QPSK的载波同步采用costas环实现，用Verilog实现其中15位IQ信号进入硬判决鉴相器计算误差信号，再由环路滤波器输出dds的频率控制字

，实现载波同步。

以下是Verilog代码实现：

module costas(
    input clk,
    input [14:0] i,
    input [14:0] q,
    output reg [31:0] freq_ctrl_word
);

reg [14:0] i_delay;
reg [14:0] q_delay;
reg [14:0] i_delay_2;
reg [14:0] q_delay_2;
reg [31:0] error_signal;
reg [31:0] error_signal_delay;
reg [17:0] dds_phase_accumulator;
reg [31:0] dds_increment;
reg [31:0] dds_frequency;
reg [31:0] dds_frequency_delay;

parameter Kp = 100;
parameter Ki = 1;
parameter Kd = 1;

assign dds_frequency = freq_ctrl_word >> 1;

always @(posedge clk) begin
    i_delay <= i;
    q_delay <= q;
    i_delay_2 <= i_delay;
    q_delay_2 <= q_delay;
end

assign error_signal = i_delay * q_delay_2 - q_delay * i_delay_2;
assign error_signal_delay = error_signal;

always @(posedge clk) begin
    dds_increment <= error_signal_delay * Kp + error_signal * Ki + (error_signal_delay - error_signal) * Kd;
    dds_phase_accumulator <= dds_phase_accumulator + dds_increment;
    dds_frequency_delay <= dds_frequency;
end

assign freq_ctrl_word = {32{1'b0}} + ((dds_phase_accumulator >> 18) & 32'h3fffff) + dds_frequency_delay;

endmodule

其中，i和q分别表示15位的I/Q信号，freq_ctrl_word为32位的频率控制字。Kp、Ki、Kd为PID控制器的系数。DDS相位累加器和频率控制字的计算采用了类似于FPGA中的DDS模块的方法。

注意，在实际应用中，需要根据具体情况调整PID系数和DDS的输出频率范围和分辨率，以达到更好的性能和适应不同的信号。