晶體(ti)元件的(de)負(fu)(fu)(fu)載(zai)(zai)電(dian)(dian)容(rong)是指在電(dian)(dian)路(lu)中跨接晶體(ti)兩(liang)端的(de)總的(de)外界有效(xiao)電(dian)(dian)容(rong)。是指晶振(zhen)要正常震蕩所需要的(de)電(dian)(dian)容(rong)。一般(ban)外接電(dian)(dian)容(rong),是為了使晶振(zhen)兩(liang)端的(de)等效(xiao)電(dian)(dian)容(rong)等于或接近(jin)負(fu)(fu)(fu)載(zai)(zai)電(dian)(dian)容(rong)。要求(qiu)高的(de)場合還要考慮ic輸(shu)入端的(de)對地電(dian)(dian)容(rong)。應用時一般(ban)在給出負(fu)(fu)(fu)載(zai)(zai)電(dian)(dian)容(rong)值附近(jin)調整可以得(de)到精確頻(pin)(pin)率。此電(dian)(dian)容(rong)的(de)大(da)小主要影響負(fu)(fu)(fu)載(zai)(zai)諧(xie)振(zhen)頻(pin)(pin)率和等效(xiao)負(fu)(fu)(fu)載(zai)(zai)諧(xie)振(zhen)電(dian)(dian)阻。
晶(jing)振的(de)(de)負載(zai)電容(rong)=[(Cd*Cg)/(Cd+Cg)]+Cic+△C式中(zhong)Cd,Cg為(wei)分別接(jie)在晶(jing)振的(de)(de)兩(liang)個(ge)腳(jiao)上(shang)和(he)對地(di)的(de)(de)電容(rong),Cic(集成電路內(nei)部電容(rong))+△C(PCB上(shang)電容(rong)).就是(shi)說負載(zai)電容(rong)15pf的(de)(de)話(hua),兩(liang)邊(bian)個(ge)接(jie)27pf的(de)(de)差不多(duo)了,一般a為(wei)6.5~13.5pF
各(ge)種邏輯(ji)芯(xin)片(pian)的(de)(de)晶(jing)(jing)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)可(ke)以(yi)(yi)等(deng)(deng)效(xiao)為(wei)(wei)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)三點(dian)(dian)式振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩器(qi)(qi)(qi)(qi)(qi)。晶(jing)(jing)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)的(de)(de)內部(bu)通(tong)常(chang)是(shi)一(yi)(yi)個(ge)(ge)(ge)反(fan)相(xiang)器(qi)(qi)(qi)(qi)(qi), 或者是(shi)奇數(shu)(shu)個(ge)(ge)(ge)反(fan)相(xiang)器(qi)(qi)(qi)(qi)(qi)串聯。在(zai)晶(jing)(jing)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)輸(shu)出(chu)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao) XO 和(he)晶(jing)(jing)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)輸(shu)入(ru)(ru)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao) XI 之間(jian)用一(yi)(yi)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu)連(lian)接(jie), 對于 CMOS 芯(xin)片(pian)通(tong)常(chang)是(shi)數(shu)(shu) M 到數(shu)(shu)十 M 歐(ou)之間(jian)。很(hen)多芯(xin)片(pian)的(de)(de)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)內部(bu)已(yi)經(jing)包含了這(zhe)(zhe)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu), 引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)外部(bu)就(jiu)不(bu)(bu)用接(jie)了。這(zhe)(zhe)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)阻(zu)是(shi)為(wei)(wei)了使反(fan)相(xiang)器(qi)(qi)(qi)(qi)(qi)在(zai)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩初始時(shi)(shi)處與線性(xing)狀態(tai), 反(fan)相(xiang)器(qi)(qi)(qi)(qi)(qi)就(jiu)如(ru)同一(yi)(yi)個(ge)(ge)(ge)有很(hen)大(da)(da)(da)增(zeng)益的(de)(de)放(fang)大(da)(da)(da)器(qi)(qi)(qi)(qi)(qi), 以(yi)(yi)便于起振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)。石(shi)(shi)英晶(jing)(jing)體也連(lian)接(jie)在(zai)晶(jing)(jing)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)的(de)(de)輸(shu)入(ru)(ru)和(he)輸(shu)出(chu)之間(jian), 等(deng)(deng)效(xiao)為(wei)(wei)一(yi)(yi)個(ge)(ge)(ge)并(bing)聯諧(xie)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)回(hui)路(lu)(lu), 振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩頻(pin)率(lv)應該(gai)是(shi)石(shi)(shi)英晶(jing)(jing)體的(de)(de)并(bing)聯諧(xie)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)頻(pin)率(lv)。晶(jing)(jing)體旁邊的(de)(de)兩(liang)(liang)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)接(jie)地(di), 實際上就(jiu)是(shi)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)三點(dian)(dian)式電(dian)(dian)(dian)(dian)路(lu)(lu)的(de)(de)分(fen)(fen)壓(ya)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong), 接(jie)地(di)點(dian)(dian)就(jiu)是(shi)分(fen)(fen)壓(ya)點(dian)(dian)。以(yi)(yi)接(jie)地(di)點(dian)(dian)即分(fen)(fen)壓(ya)點(dian)(dian)為(wei)(wei)參考點(dian)(dian), 振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩引(yin)(yin)(yin)腳(jiao)(jiao)(jiao)(jiao)的(de)(de)輸(shu)入(ru)(ru)和(he)輸(shu)出(chu)是(shi)反(fan)相(xiang)的(de)(de), 但(dan)(dan)從并(bing)聯諧(xie)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)回(hui)路(lu)(lu)即石(shi)(shi)英晶(jing)(jing)體兩(liang)(liang)端來看, 形成一(yi)(yi)個(ge)(ge)(ge)正反(fan)饋以(yi)(yi)保證電(dian)(dian)(dian)(dian)路(lu)(lu)持續振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩。在(zai)芯(xin)片(pian)設(she)計時(shi)(shi), 這(zhe)(zhe)兩(liang)(liang)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)就(jiu)已(yi)經(jing)形成了, 一(yi)(yi)般是(shi)兩(liang)(liang)個(ge)(ge)(ge)的(de)(de)容(rong)(rong)(rong)(rong)量(liang)相(xiang)等(deng)(deng), 容(rong)(rong)(rong)(rong)量(liang)大(da)(da)(da)小(xiao)依工藝和(he)版圖而(er)不(bu)(bu)同, 但(dan)(dan)終歸是(shi)比較小(xiao), 不(bu)(bu)一(yi)(yi)定適(shi)合(he)很(hen)寬的(de)(de)頻(pin)率(lv)范圍。外接(jie)時(shi)(shi)大(da)(da)(da)約(yue)是(shi)數(shu)(shu) PF 到數(shu)(shu)十 PF, 依頻(pin)率(lv)和(he)石(shi)(shi)英晶(jing)(jing)體的(de)(de)特性(xing)而(er)定。需要注意的(de)(de)是(shi):這(zhe)(zhe)兩(liang)(liang)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)串聯的(de)(de)值是(shi)并(bing)聯在(zai)諧(xie)振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)回(hui)路(lu)(lu)上的(de)(de), 會影(ying)響振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩頻(pin)率(lv)。當兩(liang)(liang)個(ge)(ge)(ge)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)量(liang)相(xiang)等(deng)(deng)時(shi)(shi), 反(fan)饋系數(shu)(shu)是(shi) 0.5, 一(yi)(yi)般是(shi)可(ke)以(yi)(yi)滿足振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩條件的(de)(de), 但(dan)(dan)如(ru)果不(bu)(bu)易起振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)或振(zhen)(zhen)(zhen)(zhen)(zhen)(zhen)蕩不(bu)(bu)穩定可(ke)以(yi)(yi)減小(xiao)輸(shu)入(ru)(ru)端對地(di)電(dian)(dian)(dian)(dian)容(rong)(rong)(rong)(rong)量(liang), 而(er)增(zeng)加輸(shu)出(chu)端的(de)(de)值以(yi)(yi)提高反(fan)饋量(liang)。
設計考慮事項:
1、使晶(jing)振(zhen)、外部(bu)電容器(qi)(如果(guo)有(you))與 IC之(zhi)間的(de)信號線盡可能保持短。當非常低(di)的(de)電流通過IC晶(jing)振(zhen)振(zhen)蕩器(qi)時,如果(guo)線路太長,會使它對 EMC、ESD 與串擾產(chan)生非常敏感的(de)影(ying)響。而且長線路還會給振(zhen)蕩器(qi)增(zeng)加寄生電容。
2、盡可能(neng)將其它時鐘線(xian)(xian)路(lu)與(yu)頻繁切換的信號線(xian)(xian)路(lu)布置在遠離晶振連接的位置。
3、當心(xin)晶振(zhen)和地的走線
4、將晶(jing)振外(wai)殼接地(di)
如果實(shi)際的負載電容(rong)配置不當(dang),第(di)一會引(yin)起線路參考頻(pin)率的誤(wu)差。另外如在發射接(jie)收(shou)電路上(shang)會使晶振(zhen)的振(zhen)蕩幅度(du)下降(jiang)(不在峰點),影響混(hun)頻(pin)信號(hao)的信號(hao)強度(du)與信噪。
當波形(xing)出(chu)現削(xue)峰,畸變時,可增(zeng)加(jia)負載電(dian)阻調(diao)整(zheng)(幾(ji)十K到(dao)幾(ji)百K).要穩(wen)定波形(xing)是并聯一個1M左右的反饋(kui)電(dian)阻。