diff --git a/main.c b/main.c
index 6d52c88..49fdf6f 100644
--- a/main.c
+++ b/main.c
@@ -783,14 +783,12 @@ void
 update_frequencies(void)
 {
   uint32_t start, stop;
-  if (frequency1 > 0) {
+  if (frequency0 < frequency1) {
     start = frequency0;
     stop = frequency1;
   } else {
-    int32_t center = frequency0;
-    int32_t span = -frequency1;
-    start = center - span/2;
-    stop = center + span/2;
+    start = frequency1;
+    stop = frequency0;
   }
 
   set_frequencies(start, stop, sweep_points);
@@ -805,39 +803,36 @@ update_frequencies(void)
 void
 freq_mode_startstop(void)
 {
-  if (frequency1 <= 0) {
-    int center = frequency0;
-    int span = -frequency1;
+  if (frequency0 > frequency1) {
     ensure_edit_config();
-    frequency0 = center - span/2;
-    frequency1 = center + span/2;
+    uint32_t f = frequency1;
+    frequency1 = frequency0;
+    frequency0 = f;
   }
 }
 
 void
 freq_mode_centerspan(void)
 {
-  if (frequency1 > 0) {
-    int start = frequency0;
-    int stop = frequency1;
+  if (frequency0 <= frequency1) {
     ensure_edit_config();
-    frequency0 = (start + stop)/2; // center
-    frequency1 = -(stop - start); // span
+    uint32_t f = frequency1;
+    frequency1 = frequency0;
+    frequency0 = f;
   }
 }
 
 
 #define START_MIN 50000
-//#define STOP_MAX 900000000
-#define STOP_MAX 1500000000
+#define STOP_MAX 2700000000U
 
 void
-set_sweep_frequency(int type, int32_t freq)
+set_sweep_frequency(int type, uint32_t freq)
 {
   int cal_applied = cal_status & CALSTAT_APPLY;
-  // negative value indicate overflow, do nothing
+/*  // negative value indicate overflow, do nothing
   if (freq < 0)
-    return;
+    return;*/
   switch (type) {
   case ST_START:
     freq_mode_startstop();
@@ -872,46 +867,47 @@ set_sweep_frequency(int type, int32_t freq)
   case ST_CENTER:
     ensure_edit_config();
     freq_mode_centerspan();
-    if (frequency0 != freq) {
+    uint32_t center = frequency0/2 + frequency1/2;
+    if (center != freq) {
+      uint32_t span = frequency0 - frequency1;
       ensure_edit_config();
-      frequency0 = freq;
-      int center = frequency0;
-      int span = -frequency1;
-      if (center-span/2 < START_MIN) {
-        span = (center - START_MIN) * 2;
-        frequency1 = -span;
+      frequency0 = freq + span/2;
+      frequency1 = freq - span/2;
+      if (frequency1 < START_MIN) {
+        frequency0 -= START_MIN - frequency1;
+        frequency1 = START_MIN;
       }
-      if (center+span/2 > STOP_MAX) {
-        span = (STOP_MAX - center) * 2;
-        frequency1 = -span;
+      if (frequency0 > STOP_MAX) {
+        frequency1 += frequency0 - STOP_MAX;
+        frequency0 = STOP_MAX;
       }
       update_frequencies();
     }
     break;
   case ST_SPAN:
     freq_mode_centerspan();
-    if (frequency1 != -freq) {
+    if (frequency0 - frequency1 != freq) {
       ensure_edit_config();
-      frequency1 = -freq;
-      int center = frequency0;
-      int span = -frequency1;
-      if (center-span/2 < START_MIN) {
-        center = START_MIN + span/2;
-        frequency0 = center;
+      uint32_t center = frequency0/2 + frequency1/2;
+      frequency1 = center - freq/2;
+      frequency0 = center + freq/2;
+      if (frequency1 < START_MIN) {
+        frequency0 -= START_MIN - frequency1;
+        frequency1 = START_MIN;
       }
-      if (center+span/2 > STOP_MAX) {
-        center = STOP_MAX - span/2;
-        frequency0 = center;
+      if (frequency0 > STOP_MAX) {
+        frequency1 += frequency0 - STOP_MAX;
+        frequency0 = STOP_MAX;
       }
       update_frequencies();
     }
     break;
   case ST_CW:
     freq_mode_centerspan();
-    if (frequency0 != freq || frequency1 != 0) {
+    if (frequency0 != freq || frequency1 != freq) {
       ensure_edit_config();
       frequency0 = freq;
-      frequency1 = 0;
+      frequency1 = freq;
       update_frequencies();
     }
     break;
@@ -924,21 +920,21 @@ set_sweep_frequency(int type, int32_t freq)
 uint32_t
 get_sweep_frequency(int type)
 {
-  if (frequency1 >= 0) {
+  if (frequency0 <= frequency1) {
     switch (type) {
     case ST_START: return frequency0;
     case ST_STOP: return frequency1;
-    case ST_CENTER: return (frequency0 + frequency1)/2;
+    case ST_CENTER: return frequency0/2 + frequency1/2;
     case ST_SPAN: return frequency1 - frequency0;
-    case ST_CW: return (frequency0 + frequency1)/2;
+    case ST_CW: return frequency0/2 + frequency1/2;
     }
   } else {
     switch (type) {
-    case ST_START: return frequency0 + frequency1/2;
-    case ST_STOP: return frequency0 - frequency1/2;
-    case ST_CENTER: return frequency0;
-    case ST_SPAN: return -frequency1;
-    case ST_CW: return frequency0;
+    case ST_START: return frequency1;
+    case ST_STOP: return frequency0;
+    case ST_CENTER: return frequency0/2 + frequency1/2;
+    case ST_SPAN: return frequency0 - frequency1;
+    case ST_CW: return frequency0/2 + frequency1/2;
     }
   }
   return 0;
diff --git a/nanovna.h b/nanovna.h
index 7028228..f4d2c9b 100644
--- a/nanovna.h
+++ b/nanovna.h
@@ -71,7 +71,7 @@ enum {
   ST_START, ST_STOP, ST_CENTER, ST_SPAN, ST_CW
 };
 
-void set_sweep_frequency(int type, int32_t frequency);
+void set_sweep_frequency(int type, uint32_t frequency);
 uint32_t get_sweep_frequency(int type);
 
 float my_atof(const char *p);
@@ -292,9 +292,9 @@ void ili9341_read_memory_continue(int len, uint16_t* out);
 #define SAVEAREA_MAX 5
 
 typedef struct {
-  int32_t magic;
-  int32_t _frequency0; // start or center
-  int32_t _frequency1; // stop or span
+  uint32_t magic;
+  uint32_t _frequency0; // start or center
+  uint32_t _frequency1; // stop or span
   int16_t _sweep_points;
   uint16_t _cal_status;
 
diff --git a/plot.c b/plot.c
index af18742..0760ae8 100644
--- a/plot.c
+++ b/plot.c
@@ -8,7 +8,7 @@
 #define SWAP(x,y) do { int z=x; x = y; y = z; } while(0)
 
 static void cell_draw_marker_info(int m, int n, int w, int h);
-void frequency_string(char *buf, size_t len, int32_t freq);
+void frequency_string(char *buf, size_t len, uint32_t freq);
 void frequency_string_short(char *buf, size_t len, int32_t freq, char prefix);
 void markmap_all_markers(void);
 
@@ -1693,24 +1693,24 @@ cell_draw_marker_info(int m, int n, int w, int h)
 }
 
 void
-frequency_string(char *buf, size_t len, int32_t freq)
+frequency_string(char *buf, size_t len, uint32_t freq)
 {
-  if (freq < 0) {
+/*  if (freq < 0) {
     freq = -freq;
     *buf++ = '-';
     len -= 1;
-  }
+  }*/
   if (freq < 1000) {
     chsnprintf(buf, len, "%d Hz", (int)freq);
-  } else if (freq < 1000000) {
+  } else if (freq < 1000000U) {
     chsnprintf(buf, len, "%d.%03d kHz",
-             (int)(freq / 1000),
-             (int)(freq % 1000));
+             (freq / 1000U),
+             (freq % 1000U));
   } else {
     chsnprintf(buf, len, "%d.%03d %03d MHz",
-             (int)(freq / 1000000),
-             (int)((freq / 1000) % 1000),
-             (int)(freq % 1000));
+             (freq / 1000000U),
+             ((freq / 1000U) % 1000U),
+             (freq % 1000U));
   }
 }
 
@@ -1746,20 +1746,18 @@ draw_frequencies(void)
 {
   char buf[24];
   if ((domain_mode & DOMAIN_MODE) == DOMAIN_FREQ) {
-      if (frequency1 > 0) {
-        int start = frequency0;
-        int stop = frequency1;
+      if (frequency0 < frequency1) {
         strcpy(buf, "START ");
-        frequency_string(buf+6, 24-6, start);
+        frequency_string(buf+6, 24-6, frequency0);
         strcat(buf, "    ");
         ili9341_drawstring_5x7(buf, OFFSETX, 233, 0xffff, 0x0000);
         strcpy(buf, "STOP ");
-        frequency_string(buf+5, 24-5, stop);
+        frequency_string(buf+5, 24-5, frequency1);
         strcat(buf, "    ");
         ili9341_drawstring_5x7(buf, 205, 233, 0xffff, 0x0000);
-      } else if (frequency1 < 0) {
-        int fcenter = frequency0;
-        int fspan = -frequency1;
+      } else if (frequency0 > frequency1) {
+        uint32_t fcenter = frequency0/2 + frequency1/2;
+        uint32_t fspan = frequency0 - frequency1;
         int x = OFFSETX;
         strcpy(buf, "CENTER");
         ili9341_drawstring_5x7_inv(buf, x, 233, 0xffff, 0x0000, uistat.lever_mode == LM_CENTER);
diff --git a/si5351.c b/si5351.c
index 148a227..8c548a3 100644
--- a/si5351.c
+++ b/si5351.c
@@ -227,7 +227,7 @@ si5351_setupMultisynth(uint8_t     output,
 #define PLLFREQ (XTALFREQ * PLL_N)
 
 void
-si5351_set_frequency_fixedpll(int channel, int pll, int pllfreq, int freq,
+si5351_set_frequency_fixedpll(int channel, int pll, uint32_t pllfreq, uint32_t freq,
                               int rdiv, uint8_t drive_strength, int mul)
 {
     int denom = freq;
@@ -256,7 +256,7 @@ si5351_set_frequency_fixedpll(int channel, int pll, int pllfreq, int freq,
 }
 
 void
-si5351_set_frequency_fixeddiv(int channel, int pll, int freq, int div,
+si5351_set_frequency_fixeddiv(int channel, int pll, uint32_t freq, int div,
                               uint8_t drive_strength, int mul)
 {
     int denom = XTALFREQ * mul;
@@ -324,25 +324,31 @@ si5351_set_frequency_with_offset(uint32_t freq, int offset, uint8_t drive_streng
   int band;
   int delay = DELAY_NORMAL;
   uint32_t ofreq = freq + offset;
-  int mul = 1, omul = 1;
+  uint32_t mul = 1, omul = 1;
   uint32_t rdiv = SI5351_R_DIV_1;
-  if (freq >= config.harmonic_freq_threshold * 3) {
+  if (freq >= config.harmonic_freq_threshold * 7U) {
+    mul = 9;
+    omul = 11;
+  } else if (freq >= config.harmonic_freq_threshold * 5U) {
+    mul = 7;
+    omul = 9;
+  } else if (freq >= config.harmonic_freq_threshold * 3U) {
     mul = 5;
     omul = 7;
   } else if (freq >= config.harmonic_freq_threshold) {
     mul = 3;
     omul = 5;
   }
-  if ((freq / mul) < 100000000) {
+  if ((freq / mul) < 100000000U) {
     band = 0;
-  } else if ((freq / mul) < 150000000) {
+  } else if ((freq / mul) < 150000000U) {
     band = 1;
   } else {
     band = 2;
   }
-  if (freq <= 500000) {
+  if (freq <= 500000U) {
     rdiv = SI5351_R_DIV_64;
-  } else if (freq <= 4000000) {
+  } else if (freq <= 4000000U) {
     rdiv = SI5351_R_DIV_8;
   }