diff --git a/docs/devlog.md b/docs/devlog.md
index 4112272..acc17e2 100644
--- a/docs/devlog.md
+++ b/docs/devlog.md
@@ -29,6 +29,8 @@ DLD154V4 线圈振荡两级频率（33/66nF 拨码，拨码不引 IO）。现场
 - ENV_RESYNC_WAIT 6000→1000（5s 预判 → 10s 学习 → ~16s 替换 → 空闲 5s 确认灭，总 ~21s）
 - TaskLoop.h：COIL_F_OK_MIN_KHZ=20 / COIL_F_OK_MAX_KHZ=120 / COIL_DEF_C_NF=66
 - 行尾修复：V4.24~25 编辑期间源文件行尾被写成多 CR（\r\r\r\r\n），本次统一规范化回标准 CRLF
+- DBG 周期打印（2s）增加 **L 粗估字段**（coil_l_uH(): Xn 精确反推，默认 66nF 档；拨 33nF 档结果 ×2）：
+  `DBG|t=2000ms f=16kHz L=1445uH Xn=3681 ...`；EVT|coil_fault 同步改用 Xn 算法（精度优于 f 整数反推）
 
 ### 验证
 
diff --git a/utilities/at32f421_freertos_demo/src/TaskLoop.c b/utilities/at32f421_freertos_demo/src/TaskLoop.c
index 6cedfbc..e20348b 100644
--- a/utilities/at32f421_freertos_demo/src/TaskLoop.c
+++ b/utilities/at32f421_freertos_demo/src/TaskLoop.c
@@ -800,6 +800,16 @@ static void env_resync_apply(uint32_t new_origin)
 #endif
     env_resync_abort();
 }
+/*===========================================================================
+ * V4.26: 线圈电感量粗估 (默认低频档 C=66nF; 拨码 33nF 档结果 ×2)
+ *   f = 60MHz/Xn → L_uH = Xn^2 / (4π^2 × 60MHz^2×1e-9... 化简为 Xn^2/(142×C_nF)
+ *   例: Xn=3681(f=16.3k) → L≈1445µH; Xn=685(f=87.6k) → L≈50µH
+ *===========================================================================*/
+static uint32_t coil_l_uH(uint32_t xn)
+{
+    return (uint32_t)(((uint64_t)xn * xn) / ((uint64_t)COIL_DEF_C_NF * 142u));
+}
+
 /*===========================================================================
  * V4.26: 线圈电感量合理范围监视
  *   拍板规格: 默认低频档口径, 空场频率合理窗 20~120kHz, 出界即判定电感量不合理
@@ -827,8 +837,8 @@ static void coil_monitor(void)
         g_coil_bad_cnt++;
         if (g_coil_bad_cnt >= 100) {
             if (g_coil_fault == 0)
-                PRINT("EVT|coil_fault f:%d kHz L~%lu uH(C66/×2C33) %s\n",
-                      f_khz, 25330000UL / (uint32_t)(f_khz * f_khz * COIL_DEF_C_NF),
+                PRINT("EVT|coil_fault f:%d kHz L~%lu uH(C66/×2C33) Xn:%d %s\n",
+                      f_khz, coil_l_uH(loop1_Xn), loop1_Xn,
                       want == 1 ? "too_large" : "too_small");
             else
                 PRINT("EVT|coil_ok f:%d kHz\n", f_khz);
@@ -1296,10 +1306,11 @@ void loop_task_function(void *pvParameters)
             if (_dbg_cnt >= 200) {      // 200 × 10ms = 2s
                 uint32_t _t_ms = _dbg_cnt * 10;   // V4.20: 先取时间再归零
                 _dbg_cnt = 0;
-                PRINT("DBG|t=%ums f=%dkHz Xn=%d LPCNT=%d Value=%d CAPVD=%d slow=%d "
+                PRINT("DBG|t=%ums f=%dkHz L=%luuH Xn=%d LPCNT=%d Value=%d CAPVD=%d slow=%d "
                       "Origin=%d dlt=%d SENS=%d VD=%d FRZ=%d/%d LIM=%d LOOP=%d STBL=%d\n",
                       _t_ms,
                       calc_freq_khz(loop1_CAPVD, loop1_LPCNT),
+                      coil_l_uH(loop1_Xn),
                       loop1_Xn, loop1_LPCNT, loop1_Value, loop1_CAPVD, loop1_CAPVD_slow,
                       loop1_Origin, loop1_dlt_ORG, loop1_SensLevel, loop1_VD_FLAG,
                       loop1_freeze_cnt, loop1_freeze_ref, g_slope_limit_cnt,