FSK解码原理及实现方法

2512785 2006-04-18 | rar | 50 | 次下载 | 2积分

资料介绍

FSK解码原理及实现方法

1.解码数学原理
条件:
??? FSK的频率为：1200/2200-->1/0;
??? wc = 1700,即(1200+2200)/2,设delta = +500/-500;
??? T是采样周期
??? 则:
??????? 1200 可表示为cos((wc-delta)*t);
??????? 2200 可表示为cos((wc+delta)*t);
设第n次采样值为cos((wc+/-delta)*(t-T)),第n+1采样值为cos((wc+/-delta)*t).
有：
Value(n)*Value(n+1) =
cos((wc+/-delta)*t)*cos((wc+/-delta)*(t-T))?
= [cos((wc+/-delta)*t+(wc+/-delta)*(t-T))+cos((wc+/-delta)*t-(wc+/-delta)*(t-
T))]/2
= [cos(2*(wc+/-delta)*t-(wc+/-delta)*T) + cos((wc+/-delta)*T)]/2????
??????? (1)
??????? (H)??????????????? (L)
将(1)式通过一个低通滤波器,则(1)式的(H)项即2位频率被滤掉，只剩下(L)项:
(1)--->Lowpass filter--->cos((wc+/-delta)*T)
再看:
??? cos((wc+/-delta)*T) = cos(wc*T+/-delta*T)??? (2)
??? IF:??? wc*T = PI/2???
??? 则 cos(wc*T+/-delta*T) = cos(PI/2+/-delta*T)
?????????????????????? = -/+sin(delta*T)??? (3)
(3)式则是FSK的值，
2.滤波器.
??? 对于来电显示，下面这段程序可以达到解码的要求
??? 定义:
??? #define??? FSKBUF 4
??? byte??? g_cADCResult;//A/D的采样值
??? int??? currentx,currenty,lastx,last_sample;
??? int??? g_iFSKBuf[FSKBUF];
??? int??? g_iFSKAvg;
??? int??? g_iFSKBuf1[FSKBUF];
??? int??? g_iFSKAvg1;
??? int??? g_iFSKBuf2[FSKBUF];
??? int??? g_iFSKAvg2;
??? byte??? g_cFSKBufPoint;
??? //在滤波之前将变量初化为0
??? 程序实现:(每次采样要做以下工作，注意采样频率和CID的波特率不是倍数关系
??? currentx = g_cADCResult;
??? currenty = last_sample;
??? last_sample = currentx;
??? //last sample in currenty,now sample in currenx;
??? currenty? *= currentx;//cos(t)*cos(t-T) = -/+sin(delta*T);
??? //------avg--lowpass filter;
??? g_iFSKAvg -= g_iFSKBuf[g_cFSKBufPoint];
??? g_iFSKBuf[g_cFSKBufPoint] = currenty;
??? g_iFSKAvg += currenty;
??? currenty = g_iFSKAvg;
??? //---------end filter;
??? g_iFSKAvg1 -= g_iFSKBuf1[g_cFSKBufPoint];
??? g_iFSKBuf1[g_cFSKBufPoint] = currenty;
??? g_iFSKAvg1 += currenty;
??? currenty = g_iFSKAvg1;
??? //second filter over
??? g_iFSKAvg2 -= g_iFSKBuf2[g_cFSKBufPoint];
??? g_iFSKBuf2[g_cFSKBufPoint] = currenty;
??? g_iFSKAvg2 += currenty;
??? currenty = g_iFSKAvg2;
??? //third filter over
??? g_cFSKBufPoint++;
??? g_cFSKBufPoint %= FSKBUF;
??? if(currenty>0)
??? {
??????? //接收到bit 1
??? }
??? else
??? {
??????? //接收到bit 0
??? }