{"id":5380,"date":"2023-06-05T16:19:39","date_gmt":"2023-06-05T08:19:39","guid":{"rendered":"http:\/\/www.dcl-controls.com\/?p=5380"},"modified":"2025-03-31T09:18:37","modified_gmt":"2025-03-31T09:18:37","slug":"why-voltage-signal-oscillate","status":"publish","type":"post","link":"https:\/\/dclcontrols.com\/ja_jp\/%e3%82%b5%e3%83%9d%e3%83%bc%e3%83%88\/%e3%82%bf%e3%83%bb%e3%82%a2%e3%82%ad%e3%83%a5%e3%83%86%e3%83%bc%e3%82%bf%e3%83%bc\/why-voltage-signal-oscillate","title":{"rendered":"\u96fb\u5727\u51fa\u529b\u4fe1\u53f7\u306f\u306a\u305c\u7279\u5b9a\u306e\u6761\u4ef6\u3067\u767a\u632f\u3059\u308b\u306e\u304b\uff1f"},"content":{"rendered":"
\n\t\t\t\t\t\t
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\u00a0<\/h3>

1. Voltage Signal Transmission Path<\/strong><\/h2>
\"Fig.
Fig. 1 Transmission path of voltage source signals<\/figcaption><\/figure>

As shown in Figure 1<\/strong>, a voltage signal transmission system consists of three key components:<\/p>

  1. Negative feedback voltage output circuit<\/strong><\/p><\/li>

  2. Signal transmission cable<\/strong><\/p>

    • Includes parasitic capacitance (C1)<\/strong> \u305d\u3057\u3066 parasitic inductance (L1)<\/strong>, which depend on cable length<\/strong> \u305d\u3057\u3066 wiring layout<\/strong>.<\/p><\/li><\/ul><\/li>

    • Input signal acquisition circuit<\/strong><\/p>

      • May include a low-pass filter (L2, C2)<\/strong>.<\/p><\/li><\/ul><\/li><\/ol>

        2. Why Does Oscillation Occur?<\/strong><\/h2>

        In a negative feedback voltage signal system, the input signal is calculated as:<\/p>

        \u2223Xi\u2032\u2223=\u2223Xi\u2223\u2212\u2223Xf\u2223|X_i’| = |X_i| – |X_f|<\/span><\/p>

        However, if the feedback signal (|Xf|) is phase-shifted by 180\u00b0<\/strong>, the equation changes to:<\/p>

        \u2223Xi\u2032\u2223=\u2223Xi\u2223+\u2223Xf\u2223|X_i’| = |X_i| + |X_f|<\/span><\/p>

        This means that even if no input signal<\/strong> is applied (|Xi| = 0<\/strong>), the feedback signal sustains the output<\/strong>\u7d50\u679c\u3068\u3057\u3066 self-sustaining oscillation<\/strong>.<\/p>

        For oscillation to occur, two conditions must be met:<\/p>

        1. Loop gain is greater than 1:<\/strong> |AF| > 1<\/strong><\/p><\/li>

        2. Phase shift satisfies:<\/strong> \u03c6A + \u03c6F = (2n+1)\u03c0<\/strong><\/p><\/li><\/ol>

          \u306b\u3064\u3044\u3066 phase shift<\/strong> in the loop comes from multiple sources:
          \u2714 Parasitic capacitance (C1) and inductance (L1) in the transmission cable<\/strong>
          \u2714 Low-pass filter components (L2, C2) in the input circuit<\/strong>
          \u2714 Filter capacitors in the output circuit<\/strong><\/p>

          \u3082\u3057 total phase shift reaches 180\u00b0<\/strong>, self-oscillation can occur.<\/p>

          3. How to Prevent Oscillation?<\/strong><\/h2>

          To prevent oscillation, one of the two conditions above must not be met<\/strong> within the operational frequency range.<\/p>

          \"Fig.
          Fig. 2 Condition of negative feedback loop without oscillation<\/figcaption><\/figure>

          As shown in Figure 2<\/strong>, two critical frequencies are considered:<\/p>

          • fc:<\/strong> The frequency where loop gain |AF| drops to 0 dB<\/strong>.<\/p><\/li>

          • fo:<\/strong> The frequency where loop phase shift exceeds -180\u00b0<\/strong>.<\/p><\/li><\/ul>

            To maintain stability, the gain should be below 0 dB<\/strong> when the phase shift reaches -180\u00b0.<\/p>

            4. Why Current Output Signals Are More Stable<\/strong><\/h2>
            \"Fig.
            Fig. 3 Current source feedback loop<\/figcaption><\/figure>
            \"Fig.
            Fig. 4 Voltage source feedback loop<\/figcaption><\/figure>

            Current signal output circuits<\/strong> are less prone to oscillation because:
            \u2714 Their feedback path is confined to internal components<\/strong> (as shown in Figure 3<\/strong>).
            \u2714 External load variations have little effect on phase shift or gain<\/strong>.<\/p>

            In contrast, voltage output circuits<\/strong> (Figure 4) take feedback from the output point<\/strong>, meaning that external cables and sampling circuits<\/strong> can influence phase shift and gain. If their parameters change, the system may meet the oscillation conditions<\/strong>, leading to instability.<\/p>

            5.Conclusion<\/strong><\/h3>

            Voltage-type output signals can oscillate when:
            \u2714 Parasitic components introduce a 180\u00b0 phase shift.<\/strong>
            \u2714 Loop gain remains greater than 1 at this phase shift.<\/strong><\/p>

            To avoid oscillation:
            \u2714 Ensure loop gain is reduced below 0 dB before phase shift reaches 180\u00b0.<\/strong>
            \u2714 Minimize parasitic capacitance\/inductance in transmission cables.<\/strong>
            \u2714 Use appropriate filtering to stabilize the feedback loop.<\/strong><\/p>

            In contrast, current-type outputs<\/strong> are more stable since their feedback loop is less affected by external load conditions<\/strong>.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"

            \u00a0 1.\u56f3 1 \u306b\u793a\u3059\u3088\u3046\u306b\u3001\u96fb\u5727\u4fe1\u53f7\u4f1d\u9001\u30b7\u30b9\u30c6\u30e0\u306f\u6b21\u306e 3 \u3064\u306e\u4e3b\u8981\u30b3\u30f3\u30dd\u30fc\u30cd\u30f3\u30c8\u3067\u69cb\u6210\u3055\u308c\u307e\u3059\uff1a\u8ca0\u5e30\u9084\u96fb\u5727\u51fa\u529b\u56de\u8def \u4fe1\u53f7\u4f1d\u9001\u30b1\u30fc\u30d6\u30eb \u5bc4\u751f\u5bb9\u91cf\uff08C1\uff09\u3068\u5bc4\u751f\u30a4\u30f3\u30c0\u30af\u30bf\u30f3\u30b9\uff08L1\uff09\u3092\u542b\u3080\u3002\u5165\u529b\u4fe1\u53f7\u53d6\u5f97\u56de\u8def \u30ed\u30fc\u30d1\u30b9\u30d5\u30a3\u30eb\u30bf\uff08L2\u3001C2\uff09\u3092\u542b\u3080\u5834\u5408\u304c\u3042\u308b\u30022.\u767a\u632f\u306f\u306a\u305c\u8d77\u3053\u308b\u306e\u304b\uff1f\u8ca0\u5e30\u9084\u96fb\u5727\u4fe1\u53f7\u30b7\u30b9\u30c6\u30e0\u3067\u306f\u3001\u5165\u529b\u4fe1\u53f7\u306f\u4ee5\u4e0b\u306e\u3088\u3046\u306b\u8a08\u7b97\u3055\u308c\u308b\uff1a\u2223Xi\u2032\u2223=\u2223Xi\u2223-\u2223Xf\u2223|X_i'| = |X_i| - |X_f| \u3057\u304b\u3057\u3001\u30d5\u30a3\u30fc\u30c9\u30d0\u30c3\u30af\u4fe1\u53f7(|Xf|)\u3092 180\u00b0\u4f4d\u76f8\u30b7\u30d5\u30c8\u3059\u308b\u3068\u3001\u5f0f\u306f\u6b21\u306e\u3088\u3046\u306b\u5909\u308f\u308b\uff1a\u2223Xi\u2032\u2223=\u2223Xi\u2223+\u2223Xf\u2223|X_i'| = |X_i| + |X_f| \u3053\u308c\u306f\u3001\u5165\u529b\u4fe1\u53f7\u304c\u5370\u52a0\u3055\u308c\u306a\u304f\u3066\u3082\uff08|Xi| = [...]...<\/p>","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"elementor_theme","format":"standard","meta":{"_acf_changed":true,"footnotes":""},"categories":[8],"tags":[49],"class_list":["post-5380","post","type-post","status-publish","format-standard","hentry","category-ta-acutator","tag-input-signal"],"acf":[],"_links":{"self":[{"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/posts\/5380","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/comments?post=5380"}],"version-history":[{"count":0,"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/posts\/5380\/revisions"}],"wp:attachment":[{"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/media?parent=5380"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/categories?post=5380"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dclcontrols.com\/ja_jp\/wp-json\/wp\/v2\/tags?post=5380"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}