diff --git a/BUILD b/BUILD
index 47e9030..1b39a51 100644
--- a/BUILD
+++ b/BUILD
@@ -1,6 +1,11 @@
 Install required packages:
   sudo apt-get install git g++ make grep mawk ntp
 
+Make sure you are using the latest kernel by updating your system. The latest
+kernel includes fixes wich improve NTP ppm measurement accuracy:
+  sudo apt-get update
+  sudo apt-get dist-upgrade
+
 Get code/ compile:
   rm -rf WsprryPi
   git clone https://github.com/JamesP6000/WsprryPi.git
diff --git a/README b/README
index 384d30c..bb8370e 100644
--- a/README
+++ b/README
@@ -7,10 +7,16 @@ port to Antenna (and LPF), operates on LF, MF, HF and VHF bands from
 ******
 Installation / update:
 ******
-  sudo apt-get install git
-  git clone https://github.com/JamesP6000/WsprryPi.git
-  cd WsprryPi
-  make
+  Make sure you are using the latest kernel by updating your system. The latest
+  kernel includes fixes wich improve NTP ppm measurement accuracy:
+    sudo apt-get update
+    sudo apt-get dist-upgrade
+
+  Download and compile code:
+    sudo apt-get install git
+    git clone https://github.com/JamesP6000/WsprryPi.git
+    cd WsprryPi
+    make
 
   See the accompanying BUILD file for more details.
 
diff --git a/wspr.cpp b/wspr.cpp
index 8d85100..d7a48aa 100644
--- a/wspr.cpp
+++ b/wspr.cpp
@@ -49,7 +49,7 @@ using namespace std;
 // Empirical PLLD clock frequency which minimizes the discrepancy between
 // the ntp measured ppm error and the ppm error measured with an external
 // frequency counter.
-#define F_PLLD_CLK   (500005000.0)
+#define F_PLLD_CLK   (500000000.0)
 // Empirical value for F_PWM_CLK that produces WSPR symbols that are 'close' to
 // 0.682s long. For some reason, despite the use of DMA, the load on the PI
 // affects the TX length of the symbols. However, the varying symbol length is