[hpsdr] preferred 122.88 MHz clock connections for coherent Mercury operation

[Joe Martin K5SO]

All,

After comparing Rx performance with multiple, coherent Mercury  
receiver boards using various 122.88 MHz clock interconnection  
methods, I recommend using the LVDS header J1 on the Mercury boards  
with twisted-wire pair connections between the multiple Mercury boards  
to provide a differential 122.88 MHz clock signal to the "slave"  
Mercury boards rather than my earlier suggestion of using coaxial  
interconnections directly to the single-ended clock signal of the  
122.88 MHz oscillator module on the "master" Mercury board.  The LVDS  
twisted-wire pair inter-connection method is cleaner.

I have modified the wiki page that describes how to configure HPSDR  
for multiple, coherent Mercury operations and I have also changed my  
website photos showing the recommended clock connections and jumpers  
for multiple, coherent Mercury operations.  Thanks, John W7FU, for  
providing me with the website photos.

73,  Joe K5SO