diff --git a/main.c b/main.c
index 99459cf..e728f7a 100644
--- a/main.c
+++ b/main.c
@@ -76,14 +76,17 @@ int32_t frequency_offset = 5000;
 int32_t frequency = 10000000;
 uint8_t drive_strength = SI5351_CLK_DRIVE_STRENGTH_2MA;
 
-void set_frequency(int freq)
+int set_frequency(int freq)
 {
-    frequency = freq;
 #if 0
     si5351_set_frequency(0, freq + frequency_offset);
     si5351_set_frequency(1, freq);
+    frequency = freq;
 #else
-    si5351_set_frequency_with_offset(freq, frequency_offset, drive_strength);
+    int delay;
+    delay = si5351_set_frequency_with_offset(freq, frequency_offset, drive_strength);
+    frequency = freq;
+    return delay;
 #endif
 }
 
@@ -269,21 +272,30 @@ static void cmd_scan(BaseSequentialStream *chp, int argc, char *argv[])
 {
   int i;
   int len = 100;
+  int32_t freq, cur_freq, step;
+  int delay;
   (void)argc;
   (void)argv;
 
-  for (i = 1; i <= len; i++) {
-    int32_t freq = i * 3000000L;
-    wait_count = 4;
+  freq = 3000000;
+  step = 3000000;
+  delay = set_frequency(freq);
+  delay += 2;
+  for (i = 0; i < len; i++) {
+    cur_freq = freq;
+    freq = freq + step;
+    wait_count = delay;
     while (wait_count)
       ;
-    dsp_disabled = TRUE;
-    set_frequency(freq);
+    //dsp_disabled = TRUE;
+    __disable_irq();
+    delay = set_frequency(freq);
     palClearPad(GPIOC, GPIOC_LED);
     calclate_gamma();
     palSetPad(GPIOC, GPIOC_LED);
-    dsp_disabled = FALSE;
-    chprintf(chp, "%d %d %d\r\n", freq, gamma_real, gamma_imag);
+    //dsp_disabled = FALSE;
+    __enable_irq();
+    chprintf(chp, "%d %d %d\r\n", cur_freq, gamma_real, gamma_imag);
   }
 }
 
@@ -300,7 +312,7 @@ static void cmd_test(BaseSequentialStream *chp, int argc, char *argv[])
     chThdSleepMilliseconds(50);
 
     palClearPad(GPIOC, GPIOC_LED);
-    set_frequency(100000000);
+    set_frequency(90000000);
     palSetPad(GPIOC, GPIOC_LED);
     chThdSleepMilliseconds(50);
   }
diff --git a/nanovna.h b/nanovna.h
index b1db0a4..9289b98 100644
--- a/nanovna.h
+++ b/nanovna.h
@@ -43,4 +43,4 @@ extern int32_t gamma_imag;
 void dsp_process(int16_t *src, size_t len);
 void calclate_gamma(void);
 
-void si5351_set_frequency_with_offset(int freq, int offset, uint8_t drive_strength);
+int si5351_set_frequency_with_offset(int freq, int offset, uint8_t drive_strength);
diff --git a/si5351.c b/si5351.c
index 97708f0..5394a0b 100644
--- a/si5351.c
+++ b/si5351.c
@@ -273,6 +273,9 @@ si5351_set_frequency(int channel, int freq, uint8_t drive_strength)
   }
 }
 
+
+int current_band = -1;
+
 /*
  * configure output as follows:
  * CLK0: frequency + offset
@@ -280,33 +283,63 @@ si5351_set_frequency(int channel, int freq, uint8_t drive_strength)
  * CLK2: fixed 8MHz
  */
 #define CLK2_FREQUENCY 8000000L
-void
+int
 si5351_set_frequency_with_offset(int freq, int offset, uint8_t drive_strength)
 {
-  si5351_disable_output();
+  int band;
+  int delay = 1;
   if (freq <= 100000000) {
+    band = 0;
+  } else if (freq < 150000000) {
+    band = 1;
+  } else {
+    band = 2;
+  }
+
+  if (current_band != band)
+    si5351_disable_output();
+
+  switch (band) {
+  case 0:
     // fractional divider mode. only PLL A is used.
-    si5351_setupPLL(SI5351_PLL_A, 32, 0, 1);
+    //if (current_band != 0)
+      si5351_setupPLL(SI5351_PLL_A, 32, 0, 1);
     si5351_set_frequency_fixedpll(0, SI5351_PLL_A, PLLFREQ, freq + offset,
                                   SI5351_CLK_DRIVE_STRENGTH_2MA);
     si5351_set_frequency_fixedpll(1, SI5351_PLL_A, PLLFREQ, freq, drive_strength);
-    si5351_set_frequency_fixedpll(2, SI5351_PLL_A, PLLFREQ, CLK2_FREQUENCY,
-                                  SI5351_CLK_DRIVE_STRENGTH_2MA);
-  } else if (freq < 150000000) {
+    //if (current_band != 0)
+      si5351_set_frequency_fixedpll(2, SI5351_PLL_A, PLLFREQ, CLK2_FREQUENCY,
+                                    SI5351_CLK_DRIVE_STRENGTH_2MA);
+    //delay = 1;
+    delay = 2;
+    break;
+
+  case 1:
     // div by 6 mode. both PLL A and B are dedicated for CLK0, CLK1
     si5351_set_frequency_fixeddiv(0, SI5351_PLL_A, freq + offset, 6,
                                   SI5351_CLK_DRIVE_STRENGTH_2MA);
     si5351_set_frequency_fixeddiv(1, SI5351_PLL_B, freq, 6, drive_strength);
     si5351_set_frequency_fixedpll(2, SI5351_PLL_B, freq * 6, CLK2_FREQUENCY,
                                   SI5351_CLK_DRIVE_STRENGTH_2MA);
-  } else {
+    delay = 2;
+    break;
+
+  case 2:
     // div by 4 mode. both PLL A and B are dedicated for CLK0, CLK1
-    si5351_set_frequency_fixeddiv(0, SI5351_PLL_A, freq + offset, 4,
-                                  SI5351_CLK_DRIVE_STRENGTH_2MA);
     si5351_set_frequency_fixeddiv(1, SI5351_PLL_B, freq, 4, drive_strength);
     si5351_set_frequency_fixedpll(2, SI5351_PLL_B, freq * 4, CLK2_FREQUENCY,
                                   SI5351_CLK_DRIVE_STRENGTH_2MA);
+    si5351_set_frequency_fixeddiv(0, SI5351_PLL_A, freq + offset, 4,
+                                  SI5351_CLK_DRIVE_STRENGTH_2MA);
+    delay = 2;
+    break;
   }
-  si5351_reset_pll();
-  si5351_enable_output();
+
+  if (current_band != band) {
+    si5351_reset_pll();
+    si5351_enable_output();
+    delay += 5;
+  }
+  current_band = band;
+  return delay;
 }