Updated refclock docs for USRP1, USRP2 and N2XX

    Added power range limits
    Added external refclock notes for USRP1

2 files changed, 35 insertions, 5 deletions

diff --git a/host/docs/usrp1.rst b/host/docs/usrp1.rst
index be684e20e..9038616a8 100644
--- a/host/docs/usrp1.rst
+++ b/host/docs/usrp1.rst
@@ -89,3 +89,25 @@ Windows - install driver
 On Windows, a driver must be installed the first time the USRP1 is attached to the host computer.
 A download link for this driver can be found on the UHD wiki page.
 Download and unpack the driver, and direct the Windows driver install wizard to the .inf file.
+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Hardware setup notes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+------------------------------------------------------------------------
+External Clock Modification
+------------------------------------------------------------------------
+The USRP can be modified to accept an external clock reference instead of the 64MHz onboard reference.
+ * Solder SMA (LTI-SASF54GT) connector to J2001
+ * Move 0 ohm 0603 resistor R2029 to R2930
+ * Move 0.01uF 0603 capacitor C929 to C926
+ * Remove 0.01uF 0603 capacitor C924
+
+The new external clock needs to be a square wave between +7dBm and +15dBm
+
+To configure UHD for the new reference clock, modify host/lib/usrp/usrp1/clock_ctrl.cpp:
+
+::
+
+    static const double master_clock_rate = <YOUR_NEW_REFERENCE_FREQUENCY>;
+
diff --git a/host/docs/usrp_nxxx.rst b/host/docs/usrp_nxxx.rst
index 617555b56..df8c6fcb0 100644
--- a/host/docs/usrp_nxxx.rst
+++ b/host/docs/usrp_nxxx.rst
@@ -228,19 +228,27 @@ The LEDs reveal the following about the state of the device:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Ref Clock - 10MHz
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Using an external 10MHz reference clock requires a signal level between
-+5dBm and +20dBm at 10MHz applied to the Ref Clock SMA port on the front panel.
+Using an external 10MHz reference clock, square wave will offer the best phase
+noise performance, but sinusoid is acceptable.  The reference clock requires the following power level:
+
+* **USRP2** 5 to 15dBm
+* **N2XX** 0 to 15dBm
 
 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 PPS - Pulse Per Second
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Using a PPS signal for timestamp synchronization requires a 5Vpp square wave signal
+Using a PPS signal for timestamp synchronization requires a square wave signal with the following amplitude:
+
+* **USRP2** 5Vpp
+* **N2XX** 3.3 to 5Vpp
+
+Test the PPS input with the following app:
+
+* <args> are device address arguments (optional if only one USRP is on your machine)
 
-Test the PPS input of the USRP2 with the following app:
 ::
 
     cd <prefix>/share/uhd/examples
     ./test_pps_input --args=<args>
 
-* <args> are device address arguments (optional if only one USRP is on your machine)