diff --git a/firmware/common/hackrf_core.c b/firmware/common/hackrf_core.c
index 64254c5d..a42a06ae 100644
--- a/firmware/common/hackrf_core.c
+++ b/firmware/common/hackrf_core.c
@@ -712,6 +712,15 @@ void cpu_clock_init(void)
 #endif
 }
 
+void activate_best_clock_source(void)
+{
+	if (si5351c_clkin_signal_valid(&clock_gen)) {
+		si5351c_set_clock_source(&clock_gen, PLL_SOURCE_CLKIN);
+	} else {
+		si5351c_set_clock_source(&clock_gen, PLL_SOURCE_XTAL);
+	}
+}
+
 void ssp1_set_mode_max2837(void)
 {
 	spi_bus_start(max2837.bus, &ssp_config_max2837);
diff --git a/firmware/common/hackrf_core.h b/firmware/common/hackrf_core.h
index 3b2f7982..e6e4e811 100644
--- a/firmware/common/hackrf_core.h
+++ b/firmware/common/hackrf_core.h
@@ -294,6 +294,8 @@ bool sample_rate_frac_set(uint32_t rate_num, uint32_t rate_denom);
 bool sample_rate_set(const uint32_t sampling_rate_hz);
 bool baseband_filter_bandwidth_set(const uint32_t bandwidth_hz);
 
+void activate_best_clock_source(void);
+
 #if (defined HACKRF_ONE || defined RAD1O)
 void enable_rf_power(void);
 void disable_rf_power(void);
diff --git a/firmware/common/si5351c.c b/firmware/common/si5351c.c
index 64a3dda1..a7364241 100644
--- a/firmware/common/si5351c.c
+++ b/firmware/common/si5351c.c
@@ -22,7 +22,7 @@
 
 #include "si5351c.h"
 
-enum pll_sources active_clock_source;
+enum pll_sources active_clock_source = PLL_SOURCE_UNINITIALIZED;
 
 /* write to single register */
 void si5351c_write_single(si5351c_driver_t* const drv, uint8_t reg, uint8_t val)
@@ -239,25 +239,14 @@ void si5351c_set_int_mode(si5351c_driver_t* const drv, const uint_fast8_t ms_num
 
 void si5351c_set_clock_source(si5351c_driver_t* const drv, const enum pll_sources source)
 {
-	si5351c_configure_clock_control(drv, source);
-	active_clock_source = source;
+	if( source != active_clock_source ) {
+		si5351c_configure_clock_control(drv, source);
+		active_clock_source = source;
+	}
 }
 
-void si5351c_activate_best_clock_source(si5351c_driver_t* const drv)
-{
-	uint8_t device_status = si5351c_read_single(drv, 0);
-
-	if (device_status & SI5351C_LOS) {
-		/* CLKIN not detected */
-		if (active_clock_source == PLL_SOURCE_CLKIN) {
-			si5351c_set_clock_source(drv, PLL_SOURCE_XTAL);
-		}
-	} else {
-		/* CLKIN detected */
-		if (active_clock_source == PLL_SOURCE_XTAL) {
-			si5351c_set_clock_source(drv, PLL_SOURCE_CLKIN);
-		}
-	}
+bool si5351c_clkin_signal_valid(si5351c_driver_t* const drv) {
+	return (si5351c_read_single(drv, 0) & SI5351C_LOS) == 0;
 }
 
 void si5351c_clkout_enable(si5351c_driver_t* const drv, uint8_t enable)
diff --git a/firmware/common/si5351c.h b/firmware/common/si5351c.h
index 533c5830..03e82260 100644
--- a/firmware/common/si5351c.h
+++ b/firmware/common/si5351c.h
@@ -29,6 +29,7 @@ extern "C"
 #endif
 
 #include <stdint.h>
+#include <stdbool.h>
 
 #include "i2c_bus.h"
 
@@ -59,6 +60,7 @@ extern "C"
 #define SI5351C_LOS (1<<4)
 
 enum pll_sources {
+	PLL_SOURCE_UNINITIALIZED = -1,
 	PLL_SOURCE_XTAL = 0,
 	PLL_SOURCE_CLKIN = 1,
 };
@@ -84,7 +86,7 @@ void si5351c_configure_clock_control(si5351c_driver_t* const drv, const enum pll
 void si5351c_enable_clock_outputs(si5351c_driver_t* const drv);
 void si5351c_set_int_mode(si5351c_driver_t* const drv, const uint_fast8_t ms_number, const uint_fast8_t on);
 void si5351c_set_clock_source(si5351c_driver_t* const drv, const enum pll_sources source);
-void si5351c_activate_best_clock_source(si5351c_driver_t* const drv);
+bool si5351c_clkin_signal_valid(si5351c_driver_t* const drv);
 
 void si5351c_write_single(si5351c_driver_t* const drv, uint8_t reg, uint8_t val);
 uint8_t si5351c_read_single(si5351c_driver_t* const drv, uint8_t reg);
diff --git a/firmware/hackrf_usb/usb_api_transceiver.c b/firmware/hackrf_usb/usb_api_transceiver.c
index fda9b6a9..3d64d7f9 100644
--- a/firmware/hackrf_usb/usb_api_transceiver.c
+++ b/firmware/hackrf_usb/usb_api_transceiver.c
@@ -273,7 +273,7 @@ void set_transceiver_mode(const transceiver_mode_t new_transceiver_mode) {
 
 
 	if( _transceiver_mode != TRANSCEIVER_MODE_OFF ) {
-		si5351c_activate_best_clock_source(&clock_gen);
+		activate_best_clock_source();
 
         hw_sync_enable(_hw_sync_mode);