diff --git a/firmware/command_ctrl.ino b/firmware/command_ctrl.ino
index a83fa53..12a348e 100644
--- a/firmware/command_ctrl.ino
+++ b/firmware/command_ctrl.ino
@@ -188,97 +188,95 @@ void cc_startMeasurement()
   //    MSG_TYPE_CONFIG
   cc_getConfig();
 
-  // 2. start a for loop from the frequence to start to the end frequence
-  if (start_freq > 0 && start_freq <= 150000000 &&
-      end_freq > 0 && end_freq <= 150000000 &&
-      start_freq < end_freq &&
-      intervall > 0 &&
-      step_freq > 0)
+  if (start_freq == 0 || start_freq > 150000000 ||
+      end_freq   == 0 || end_freq   > 150000000 ||
+      step_freq  == 0 || step_freq  > 150000000 ||
+      start_freq >= end_freq ||
+      intervall  == 0)
   {
-    si5351.drive_strength(SI5351_CLK0, drive_str); // 2 4 6 8ma
-
-    uint8_t t = 0;
-    for (t = 0; t < 100; t++)
-    {
-      uint16_t tmp = analogRead(A0);
-               tmp = analogRead(A1);
-    }
-
-    uint32_t freq = 0;
-    for (freq = start_freq; freq <= end_freq; freq += step_freq)
-    {
-      if (freq > end_freq)
-      {
-        // prevent to step over the end frequency
-        // maybe the user is not allowed to send data in the frequency band
-        freq = end_freq;
-      }
-      uint32_t a0_sum  = 0;
-      uint32_t a1_sum  = 0;
-      uint16_t i = 0;
-
-      si5351.set_freq((uint64_t)freq * 100, SI5351_PLL_FIXED, SI5351_CLK0);
-      si5351.output_enable(SI5351_CLK0, 1); // enable clock output 0
-      delay(1);
-
-      for (i = 0; i < intervall; i++)
-      {
-        // 3. on every loop read the analog input A0 and A1 for the in intervall (milliseconds)
-        //    and generate the average value, read the ADC value every milli second.
-        uint8_t t = 0;
-        uint16_t ta0 = 0;
-        uint16_t ta1 = 0;
-        for (t = 0; t < MEAS_LOOP_CNT; t++)
-        {
-          ta0 += analogRead(A0);
-          ta1 += analogRead(A1);
-        }
-        a0_sum += (ta0 / MEAS_LOOP_CNT);
-        a1_sum += (ta1 / MEAS_LOOP_CNT);
-
-        delay(1);
-      }
-
-      a0_sum = a0_sum / intervall;
-      a1_sum = a1_sum / intervall;
-
-      // 4. send the current output frequency, the drive strength and the measured ADC values from A0 and A1 to the host
-      //    MSG_TYPE_MEAS_FREQ_INFO
-      sendSOM();
-      Serial.write(MSG_TYPE_MEAS_FREQ_INFO);
-      Serial.write((uint8_t)((freq & 0xff000000) >> 24));
-      Serial.write((uint8_t)((freq & 0x00ff0000) >> 16));
-      Serial.write((uint8_t)((freq & 0x0000ff00) >>  8));
-      Serial.write((uint8_t) (freq & 0x000000ff));
-      Serial.write((uint8_t)((a0_sum & 0xff00) >> 8));
-      Serial.write((uint8_t) (a0_sum & 0x00ff));
-      Serial.write((uint8_t)((a1_sum & 0xff00) >> 8));
-      Serial.write((uint8_t) (a1_sum & 0x00ff));
-      sendEOM();
-
-      si5351.output_enable(SI5351_CLK0, 0); // disable clock output 0
-
-      if (freq >= end_freq)
-        break; // abort the loop because all is done
-    }
-
-    // 5. send a measurement end message to the host
-    //    MSG_TYPE_MEAS_END_INFO
-    char* tmp = "     ";
-    sprintf(tmp, "%c%c%c%c%c", MSG_SOM1, MSG_SOM2,
-                               MSG_TYPE_MEAS_END_INFO,
-                               MSG_EOM1, MSG_EOM2);
-    Serial.write(tmp);
-
-  } else {
     // on error
-    char* tmp = "     ";
-    sprintf(tmp, "%c%c%c%c%c", MSG_SOM1, MSG_SOM2,
-                               MSG_TYPE_ANSWER_NOK,
-                               MSG_EOM1, MSG_EOM2);
-    Serial.write(tmp);
+
+    sendSOM();
+    Serial.write(MSG_TYPE_ANSWER_NOK);
+    sendEOM();
+
+    return;
   }
 
+  // 2. start a for loop from the frequence to start to the end frequence
+  si5351.drive_strength(SI5351_CLK0, drive_str); // 2 4 6 8ma
+
+  uint8_t t = 0;
+  for (t = 0; t < 100; t++)
+  {
+    uint16_t tmp = analogRead(A0);
+             tmp = analogRead(A1);
+  }
+
+  uint32_t freq = 0;
+  for (freq = start_freq; freq <= end_freq; freq += step_freq)
+  {
+    if (freq > end_freq)
+    {
+      // prevent to step over the end frequency
+      // maybe the user is not allowed to send data in the frequency band
+      freq = end_freq;
+    }
+    uint32_t a0_sum  = 0;
+    uint32_t a1_sum  = 0;
+    uint16_t i = 0;
+
+    si5351.set_freq((uint64_t)freq * 100, SI5351_PLL_FIXED, SI5351_CLK0);
+    si5351.output_enable(SI5351_CLK0, 1); // enable clock output 0
+    delay(1);
+
+    for (i = 0; i < intervall; i++)
+    {
+      // 3. on every loop read the analog input A0 and A1 for the in intervall (milliseconds)
+      //    and generate the average value, read the ADC value every milli second.
+      uint8_t t = 0;
+      uint16_t ta0 = 0;
+      uint16_t ta1 = 0;
+      for (t = 0; t < MEAS_LOOP_CNT; t++)
+      {
+        ta0 += analogRead(A0);
+        ta1 += analogRead(A1);
+      }
+      a0_sum += (ta0 / MEAS_LOOP_CNT);
+      a1_sum += (ta1 / MEAS_LOOP_CNT);
+
+      delay(1);
+    }
+
+    a0_sum = a0_sum / intervall;
+    a1_sum = a1_sum / intervall;
+
+    // 4. send the current output frequency, the drive strength and the measured ADC values from A0 and A1 to the host
+    //    MSG_TYPE_MEAS_FREQ_INFO
+    sendSOM();
+    Serial.write(MSG_TYPE_MEAS_FREQ_INFO);
+    Serial.write((uint8_t)((freq & 0xff000000) >> 24));
+    Serial.write((uint8_t)((freq & 0x00ff0000) >> 16));
+    Serial.write((uint8_t)((freq & 0x0000ff00) >>  8));
+    Serial.write((uint8_t) (freq & 0x000000ff));
+    Serial.write((uint8_t)((a0_sum & 0xff00) >> 8));
+    Serial.write((uint8_t) (a0_sum & 0x00ff));
+    Serial.write((uint8_t)((a1_sum & 0xff00) >> 8));
+    Serial.write((uint8_t) (a1_sum & 0x00ff));
+    sendEOM();
+
+    si5351.output_enable(SI5351_CLK0, 0); // disable clock output 0
+
+    if (freq >= end_freq)
+      break; // abort the loop because all is done
+  }
+
+  // 5. send a measurement end message to the host
+  //    MSG_TYPE_MEAS_END_INFO
+  sendSOM();
+  Serial.write(MSG_TYPE_MEAS_END_INFO);
+  sendEOM();
+
   si5351.output_enable(SI5351_CLK0, 0); // disable clock output 0
 
 }