[hpsdr] 10Mhz reference

[Dick Faust]

There was a thread back in 2014 about R141 on Hermes.  This is the 
emitter resistor on the diff amp pair of Q3 & Q4.  The resistor needs to 
be 50 Ohms not 100 as in the original build.  Just parallel another 100 
Ohm resistor across R141 is the easiest way to correct.


Dick K9IVB


On 3/3/2017 8:33 AM, Tom McDermott wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>
>
> 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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