[hpsdr] Yet more on Alex

[John Ackermann N8UR]

While I was looking at the Alex test results, I found this and thought 
it might be interesting: http://www.febo.com/pages/alex/grounding.png

This plot shows the complete Alex frequency response from antenna input 
to RX output with Alex configured for 10 meters, putting the 10-12M TX 
LPF, 55 MHz RX LPF, and 20MHZ RX HPF in the signal path.  It was made 
using the original toroids.

In addition to showing the overall frequency response of Alex, it shows 
the importance of providing a ground strap between the Atlas backplane, 
the Alex enclosure, and the Alex circuit boards.  The "before" trace 
just relied on the coax interconnects and Pandora mounting hardware for 
grounding.

There's a greater than 20dB difference in stop-band attenuation at some 
frequencies (and not shown on the plot is the fact that the hump in the 
"before" response moved around if you moved the coax jumpers or changed 
their length).

John