マルツ パーツまめ知識 |
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デジタルテスタ |
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周波数測定機能の活用 第3回目 |
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◎プローブアダプタを作る |
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★振幅レベル減衰の改善 |
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オシロスコープ用プローブは「10:1」で用いた場合、抵抗は9MΩです。 |
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元々、オシロスコープ側が1MΩの前提となっているために実際のプローブ出力では信号 |
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レベルが1/10になります。 |
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デジタルテスタを周波数測定ファンクションで用いた場合の入力インピーダンス(抵抗)は |
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不明なのですが、このままでは用いるデジタルテスタによっては感度不足となる場合が |
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あります。 |
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そこで、図31のように減衰した分を増幅し、本来の振幅レベルとなるような「アダプタ」を |
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製作しましたので紹介します。 |
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★回路 |
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図32のように一旦、1MΩの抵抗で受けた後にFETによるバッファアンプを経てから、オペ |
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アンプにより10倍の増幅をします。 |
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実際にはFET部とオペアンプ部の接続部で若干のロスが発生し、おおよそ10倍近い増幅度に |
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なります。 |
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R6,R7は出力インピーダンスを決める抵抗で、出力保護を兼ねています。 |
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対象とする周波数は100KHz〜30MHz程度です。 |
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電源は市販のスイッチングACアダプタを前提とし、DC12Vを供給します。 |
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なお、電源電圧を低くしていますので、扱える信号レベル(プローブ入力できるレベル)の |
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最大値は10V程度です。 |
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電源は抵抗分割(R8,R9)により簡易的に「単電源入力→両電源変換」を行い、これにより |
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オペアンプを両電源動作としています。 |
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★部品表 |
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表6 |
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部品番号 |
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品名 |
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型番 |
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メーカー |
数量 |
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C1,C2 |
セラコン 0.01μF |
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CCDC50V103*10 |
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2 |
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C3,C4 |
ケミコン100μF |
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25YK100 |
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Ruby-con |
2 |
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C5,C6 |
セキセラ 0.1μF |
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CT4-0805B104K*10 |
Linkman |
2 |
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IC1 |
オペアンプ |
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LT1227CN8#PBF |
LT |
1 |
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J1,J2 |
BNCコネクタ(角座、メス) |
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BNCR |
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トーコネ |
2 |
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J3 |
DCジャック |
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MJ14ROHS |
マル信 |
1 |
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Q1 |
FET |
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2SK192AGR |
東芝 |
1 |
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R1 |
カーボン抵抗
1MΩ |
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1 |
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R2 |
カーボン抵抗
1K |
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1 |
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R3 |
カーボン抵抗
1K |
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1 |
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R4 |
カーボン抵抗
680Ω |
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1 |
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R5 |
カーボン抵抗
75Ω |
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1 |
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R6,R7 |
カーボン抵抗
100Ω |
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2 |
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R8,R9 |
カーボン抵抗
1K |
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2 |
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XIC1 |
ICソケット 8P(丸ピン) |
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21218NE |
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Linkman |
1 |
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ユニバーサル基板 |
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LUPCB-7247-NS |
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Linkman |
1 |
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ケース |
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MB1 |
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TAKACHI |
1 |
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スイッチングACアダプタ12V/2A |
STD-12020U |
Linkman |
1 |
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ビス、アースラグ類 |
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1式 |
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オシロスコーププローブ |
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LA05044 |
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1 |
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両端BNCコネクタ同軸ケーブル |
LA06001 |
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1 |
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セラコンおよびセキセラの型番、数量に注意 |
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◎製作 |
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★基板 |
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基板は72×47サイズの「片面ユニバーサル基板で、これをケースに収納できるサイズに |
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カットします。 |
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GNDは図33のように「導電性テープ」を用いて、これを「ベタGND」にします。 |
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R1,R2,R4のみ基板はんだ面実装です。 |
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★ケース実装 |
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ケース実装の要領を図34に、様子を写真4に示します。 |
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ケースへの基板固定は「金属スペーサ等」を用いる方法が良いのですが、BNCコネクタへの直付 |
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およびメッキ線を利用しています。 |
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◎特性を確認する |
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★プローブアダプタを含んだ感度 |
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表7 |
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周波数(KHz) |
81D |
86D |
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プローブアダプタを含んだ感度実測結果を |
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100 |
32 |
13 |
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表7、グラフ5に、また、測定方法を図35に |
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1000 |
32 |
14 |
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示します。 |
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5000 |
34 |
15 |
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10000 |
53 |
16 |
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用いたテスタはLinkmanのLDM-86Dおよび |
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15000 |
64 |
17 |
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LDM-81Dの2機種です。 |
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20000 |
76 |
20 |
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25000 |
87 |
23 |
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LDM-86Dとの組み合わせでは100KHz〜 |
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30000 |
87 |
26 |
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30MHzの範囲で、約30mVrms以上の電圧 |
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レベルであれば測定可能です。 |
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★プローブ補正有無の影響 |
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オシロスコープの「プローブ補正」の原理を図36に示します。 |
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a ) ではC1の影響で「ある高い周波数」からはポイントV1では電圧値が下がり、b ) のように |
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R2にC2が並列接続された形では電圧値が上昇します。 |
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今度はc ) のようにC1,C2を組み合わせてC2の値を調整すればV1において周波数によらず |
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電圧値が一定になります。 |
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オシロスコープ用のプローブはこれを応用してC2を調整することにより周波数特性が一定と |
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なるように操作し、これを「プローブ補正」と言います。 |
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実際の補正手順はオシロスコープに内蔵されている矩形波を観測し、d ) のようにきれいな |
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矩形波となるようにトリマー(C2)調整を行います。 |
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今回製作のプローブアダプタは周波数測定用の補助装置ですから、厳密にはそれほど |
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周波数特性の平坦性は要求されないと思います。 |
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ただし、特性は把握しておく必要がありますので周波数特性の実測結果をグラフ6、グラフ7 |
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表8、表9に、測定回路を図37に示します。 |
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(グラフ6) |
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プローブ補正しない場合。 |
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赤が「過補正」。10MHz以上で特性が上昇。 |
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黒が「補正不足」。20MHz以上で特性が下降。 |
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(グラフ7) |
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プローブ補正した場合。 |
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40MHzまで特性は、ほぼフラット。 |
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結論としては30MHzまでの周波数カウンタ用途であればプローブ補正は必要ないと思います。 |
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参考として、プローブ補正要領を図38に示します。 |
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オシロスコープの「プローブ補正信号」を利用し、通常のプローブ補正と同じ要領でQ1の |
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ソース波形を観測し、トリマ調整します。 |
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表8 |
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周波数 |
過補正 |
補正不足 |
|
表9 |
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0.1 |
0.3 |
0.3 |
|
周波数 |
補正 |
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1.0 |
0.0 |
0.0 |
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0.1 |
0.3 |
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5.0 |
0.0 |
0.0 |
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1.0 |
0.0 |
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10.0 |
0.3 |
0.0 |
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5.0 |
0.0 |
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15.0 |
0.7 |
0.0 |
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10.0 |
0.3 |
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20.0 |
1.1 |
-0.2
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15.0 |
0.4 |
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25.0 |
1.5 |
-0.6
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20.0 |
0.5 |
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30.0 |
1.9 |
-1.2
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25.0 |
0.5 |
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35.0 |
2.2 |
-2.0 |
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30.0 |
0.3 |
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40.0 |
2.5 |
-3.0 |
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35.0 |
0.0 |
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45.0 |
2.7 |
-4.1
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40.0 |
-0.6
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50.0 |
2.8 |
-5.5
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45.0 |
-1.4
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55.0 |
3.0 |
-7.0 |
|
50.0 |
-2.5
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60.0 |
3.0 |
-7.7
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55.0 |
-3.6
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65.0 |
3.0 |
-8.7
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60.0 |
-5.0 |
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70.00 |
2.8 |
-10.1
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65.0 |
-6.2
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75.00 |
1.1 |
-11.8
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70.00 |
-7.6
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80 |
-1.2
|
-13.5
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75.00 |
-8.5
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85.00 |
-4.5
|
-16.0
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|
80 |
-10.3
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90.00 |
-9.5
|
-19.5
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85.00 |
-12.5
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95.00 |
-15.5
|
-22.0
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|
90.00 |
-16.5
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100.00 |
-17.0
|
-25.0
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95.00 |
-19.0
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100.00 |
-22.0
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◎使用風景 |
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★マイコンのクロック |
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写真5はマイコンのクロック回路の発振チェックに用いている例です。 |
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プローブアダプタを用いることにより感度不足を補うことが出来ます。 |
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ただし、クロック回路の測定ポイントによってはプローブの「容量成分」による影響があります |
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ので常にこのことを意識しておく必要があり、写真5では本来の発振周波数から若干低く測定 |
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されています。 |
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★アナログ発振回路 |
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写真6はアナログ発振回路での応用例です。 |
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コルピッツ発振回路ですが、測定ポイントは「バッファーアンプ」を通った後にします。 |
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これにより発振回路への影響が少なくなり、「発振周波数調整」として用いることが出来ます。 |
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