Insait long ol ilektronik seket, ol 'crystal oscillator' em i wanpela bilong ol samting we planti manmeri i save yusim long kisim klok. Long mekim gutpela wok bilong 'crystal oscillator', ol 'external load capacitors' insait long 'circuit' i mas gat gutpela disain bihainim 'load capacitance' (CL) bilong en. Sapos ol i no bungim gut ol 'capacitor' dispela inap long kamapim hevi long 'oscillation', 'frequency deviation' i go antap, na tu sistem i no inap stap gut.
Dispela atikol bai givim wanpela gutpela wok painimaut long mining bilong 'crystal oscillator load capacitance', rot bilong skelim ol 'external capacitors', na ol samting yu ken tingim long wokim 'circuit design'.
Ⅰ. Wanem em i 'load capacitance' (CL) bilong wanpela 'crystal oscillator'?
Ol man i wokim ol kristal i save makim wanpela 'nominal load capacitance value' (CL) insait long datasit, olsem 18pF, 20pF, 12pF, na ol narapela. Dispela i makim 'capacitive load' we ol i save skelim kristal aninit long en long winim 'nominal oscillation frequency' bilong en.
Long narapela tok, 'crystal oscillator' inap long kamapim stretpela 'frequency' tasol taim 'load capacitance' bilong en i wankain olsem CL.
Ⅱ. Olsem Wanem Ol Ekstenel Lod Kapasitor i save Afektim Lod Kapasitens?
Wanpela kain 'crystal oscillator circuit' i save kisim dispela kain:
C1 na C2 em tupela ekstenel kapasitor we i konektim long graun (planti taim ol SMD kapasitor);
Tupela 'terminal' bilong XTAL i konektim long tupela 'pin' bilong 'crystal oscillator';
Long praktis, ol i makim ikwil lod kapasitans CL bilong kristal long C1 na C2 long seris, wantaim parasitik kapasitans Cp we PCB na paket i kamapim.
Ⅲ. Fomula bilong Kalkulesen: Hau long painimaut 'External Capacitor Value' bihainim CL?
Fomula bilong 'equivalent load capacitance' (CL) em i olsem:
We:
C1, C2: Ol ekstenel kapasitor i konektim long graun;
Cp: Parasitic capacitance we i kamap long PCB traces, packaging, na ol narapela samting (planti taim 2pF i go inap 5pF, ol i ken skelim long rot bilong simulesen o ekspiriens);
CL: Nominal lod kapasitans we i stap insait long datasit bilong kristal osileta.
【Eksampel】
Tingim olsem 'crystal specification' em CL=18pF, na PCB 'parasitic capacitance' Cp ≈ 5pF. Long skelim ol stretpela veliu bilong C1 na C2:
Pastaim, senisim ol veliu i go insait long fomula:
Sapos yumi ting C1=C2=C, fomula i kamap isi long:
Risal bilong dispela em:
✅ Olsem na, ol ekstenel kapasitor we ol i rekomendim em: C1=C2 =27pF (Standad Value).
Ⅳ. Bilong wanem ol ekstenel kapasitor i no wankain i save kamapim hevi?
Frekwensi deviesen: Sapos ol ekstenel kapasitor i liklik tumas o bikpela tumas, kristal bai deviet long nominal frikwensi bilong en, na dispela bai kamapim ol asua long sistem klok.
Stat-ap hevi o no stap gut: Sapos yu putim planti samting tumas dispela i save daunim 'gain', we inap long mekim 'crystal' i no inap long stat o kisim 'frequency jitter'.
Pawa i yusim moa: Sapos yu no putim gut pawa, dispela i save mekim na pawa i yusim bikpela pawa insait long 'crystal oscillator circuit'.
EMI i go antap: Ol 'oscillations' we i no stap gut i ken kamapim ol giaman 'frequencies', na dispela i ken bagarapim 'system electromagnetic compatibility' (EMC).
V. Ol rekomendesen bilong disain na ol wok bilong enjiniaring
Lukluk gut long 'crystal oscillator datasheet'
Painimaut CL veliu.
Yu mas kliagut long 'circuit topology' we ol i rekomendim.
Sekim sapos ol i rekomendim wanpela mak bilong ol 'external capacitor' veliu.
2. Tingim trupela bod envairomen
Skelim Cp yusim PCB simulesen o ekspiriens.
Long ol hai-spid o stretpela klok (olsem, ol komyunikesen sistem, MCU main frikwensi > 100MHz), stretpela kalkulesen o tes em i gutpela.
3. Yusim ol simetrik kapasitor
Em i gutpela long yusim C1=C2 long sentaim frikwensi na skelim nois.
Ol i ken yusim tu 'asymmetrical design' long ol spesel wok bilong bungim ol samting.
4. Yusim wanpela SMD kapasitor arei long stretim ol hevi
Traim wantaim ol SMD kapasitor (olsem, 10pF, 15pF, 22pF, 27pF, na ol narapela) long painim stretpela fit;
Sekim autput stebiliti na frikwensi ekyuresi yusim wanpela 'spectrum analyzer' o 'oscilloscope'.
6,Planti taimFAQ
|
Ol askim |
Ol bekim |
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Sapos CL=12pF, wanem mak bilong ol ekstenel kapasitor i mas stap? |
Sapos Cp=5pF, orait C1=C2≈14pF |
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Bilong wanem sampela MCU i no nidim ol 'external capacitors'? |
Sampela 'crystal oscillator circuits' i gat ol 'load capacitors' (olsem, STM32), olsem na ol i no nidim ol 'external capacitors'. |
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Ol 'external capacitors' we i no wankain inap long bagarapim 'crystal oscillator'? |
Planti taim, ol i no inap kukim o bagarapim, tasol ol i ken kamapim hevi long stat o ol rong long 'frequency'. |
Konklusen
Disain bilong ol 'external load capacitors' bilong wanpela 'crystal oscillator' em i wanpela liklik samting tasol em i bikpela samting we i save afektim stret 'stability' na 'accuracy' bilong 'clock system'. Sapos yu kliagut long kalkulesen rilesensip namel long CL na C1/C2, na stretim ol paramita bihainim trupela PCB parasitik kapasitans, em i ki bilong kisim gutpela 'oscillation' na stretpela frikwensi.
Insait long ol hai-spid dijitel sistem, ol ikwipmen bilong toktok, o ol sistem we i gat bikpela wok long klok, wok bilong makim na stretim gut 'load capacitance' bilong 'crystal oscillator' em i wanpela wok bilong enjiniaring we ol i mas tingim gut.
HCIi gat wanpela profesenel leboretori na wanpela tim bilong ol aplikesen enjinia wantaim moa long 20 yia ekspiriens, na ol i save givim fri profesenel mating na tes sevis long ol kastoma. Sapos yu gat sampela nid long dispela samting, plis yu ken toktok wantaim ol 'sales' o 'technical staff' bilong mipela.