diff --git a/notes/pics/modulation.png b/notes/pics/modulation.png
new file mode 100644
index 00000000..5bdc389a
Binary files /dev/null and b/notes/pics/modulation.png differ
diff --git a/notes/计算机网络 - 物理层.md b/notes/计算机网络 - 物理层.md
index 2a10290f..b751e396 100644
--- a/notes/计算机网络 - 物理层.md	
+++ b/notes/计算机网络 - 物理层.md	
@@ -18,5 +18,9 @@
 
 模拟信号是连续的信号，数字信号是离散的信号。带通调制把数字信号转换为模拟信号。
 
-<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/c34f4503-f62c-4043-9dc6-3e03288657df.jpg" width="500"/> </div><br>
+基本的调制方法有调幅、调频和调相三种，分别用载波的振幅、频率和相位的变化来表示 0 和 1：
+
+<div align="center"> <img src="pics/modulation.png" width="720px"/> </div><br>
+
+实际的系统中还会把振幅和相位组合起来使用，例如正交振幅调制（Quadrature Amplitude Modulation, QAM）用 16 种不同的振幅和相位组合表示 4 个比特，一个码元就能传输更多的数据。
 
diff --git a/tools/diagram/src/modulation.svg b/tools/diagram/src/modulation.svg
new file mode 100644
index 00000000..11cb2c28
--- /dev/null
+++ b/tools/diagram/src/modulation.svg
@@ -0,0 +1,73 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="720" height="420" viewBox="0 0 720 420">
+  <style>
+    text { font-family: "PingFang SC", "Microsoft YaHei", Helvetica, Arial, sans-serif; fill: #3F3F3F; font-size: 12px; }
+    .title { font-weight: bold; font-size: 15px; fill: #333; text-anchor: middle; }
+    .b { font-weight: bold; }
+    .c { text-anchor: middle; }
+    .e { text-anchor: end; }
+    .sub { font-size: 11px; fill: #6E6E6E; }
+    .note { fill: #C0605A; }
+    .ok { fill: #6A9F68; }
+    .mono { font-family: Menlo, Consolas, monospace; }
+    .arrow { stroke: #555; stroke-width: 1.1; fill: none; marker-end: url(#a); }
+    .red { stroke: #C0605A; marker-end: url(#r); }
+    .green { stroke: #6A9F68; marker-end: url(#g); }
+    .dash { stroke-dasharray: 5 4; }
+    .line { stroke: #555; stroke-width: 1.1; fill: none; }
+    .frame { fill: none; stroke: #999; stroke-width: 1; stroke-dasharray: 4 3; }
+    .shadow { fill: #DADADA; }
+  </style>
+  <defs>
+    <marker id="a" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M1,1.5 L9.5,5 L1,8.5 z" fill="#555"/>
+    </marker>
+    <marker id="r" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M1,1.5 L9.5,5 L1,8.5 z" fill="#C0605A"/>
+    </marker>
+    <marker id="g" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
+      <path d="M1,1.5 L9.5,5 L1,8.5 z" fill="#6A9F68"/>
+    </marker>
+  </defs>
+  <rect width="100%" height="100%" fill="#FFFFFF"/>
+
+  <text x="360" y="28" class="title">带通调制：用载波的振幅、频率或相位表示 0 和 1</text>
+  <line x1="150" y1="56" x2="150" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="216" y1="56" x2="216" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="282" y1="56" x2="282" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="348" y1="56" x2="348" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="414" y1="56" x2="414" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="480" y1="56" x2="480" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="546" y1="56" x2="546" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="612" y1="56" x2="612" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <line x1="678" y1="56" x2="678" y2="364" stroke="#E2E2E2" stroke-dasharray="3 3"/>
+  <text x="183" y="52" class="c b mono">0</text>
+  <text x="249" y="52" class="c b mono">1</text>
+  <text x="315" y="52" class="c b mono">0</text>
+  <text x="381" y="52" class="c b mono">1</text>
+  <text x="447" y="52" class="c b mono">1</text>
+  <text x="513" y="52" class="c b mono">0</text>
+  <text x="579" y="52" class="c b mono">0</text>
+  <text x="645" y="52" class="c b mono">1</text>
+  <text x="30" y="90" class="b">数字信号</text>
+  <text x="30" y="106" class="sub">基带信号</text>
+  <line x1="150" y1="92" x2="678" y2="92" stroke="#D0D0D0"/>
+  <text x="30" y="170" class="b">调幅 AM</text>
+  <text x="30" y="186" class="sub">1 有载波，0 没有</text>
+  <line x1="150" y1="172" x2="678" y2="172" stroke="#D0D0D0"/>
+  <text x="30" y="250" class="b">调频 FM</text>
+  <text x="30" y="266" class="sub">1 频率高，0 频率低</text>
+  <line x1="150" y1="252" x2="678" y2="252" stroke="#D0D0D0"/>
+  <text x="30" y="330" class="b">调相 PM</text>
+  <text x="30" y="346" class="sub">0 和 1 的相位差 180°</text>
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+  <text x="30" y="400" class="sub">调相中圆圈标出了 0 和 1 交替处的相位跳变。示意图，每一位画 2 个载波周期（调频中 1 画 4 个）。</text>
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