[hpsdr] 10Mhz reference

[Tom McDermott]

Hi Glen,

The problem you describe can happen when the external 10 MHz input
reference to Hermes is not
actually toggling the logic.  This causes the Hermes FPGA to fall back into
a mode where it senses
that it has no 10 MHz clock of any kind and thus uses one of the on-board
oscillators to establish
10 MHz.  On my Hermes board when I tried an external reference is was
off-frequency by just
about the amount you describe.

There is a known problem with the Hermes external reference clock input
buffer.  I ended up having to
build a circuit to buffer that clock with a CMOS gate and bypass the
two-transistor buffer on the board.

Dick, K9IVB did a design that provides auto-detection and switching.  I
ordered some of his boards
from OSH Park, and built it up.  I did need to tweak one level on the board
to get it to work properly,
but (it's been a couple years) it was just a resistor change or something
similarly easy. Works great.
http://www.k9ivb.net/Hermes/index.htm


-- Tom, N5EG
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