[hpsdr] 20MHz ref. for Horton LO

[Grant Hodgson]

Some comments about generating the 800kHz comparison frequency for the Horton
uW LO :-

I would have thought that if Horton needs a 20MHz reference signal, then it
should be Horton that generates it - not Gibraltar - it would seem to be a
much neater engineering solution to keep the 20MHz signal on the Horton PCB only.

There are a couple of easy ways to double the 10MHz signal :-
1) use a transformer and a dual-schottky diode pair to make a full-wave
rectifier.  You could buy one from the likes of Mini-circuits, but this
would be a very expensive solution, a couple of discrete components is a very
cheap, but effective solution.  A buffer may be required for the 20MHz signal,
but most PLL ICs have one on-chip already.  The output doesn't really need
much filtering - the odd harmonics should be fairly well down if the
transformer is balanced and the diodes are matched, and the even harmonics
don't matter - the PLL IC would be just as happy with a 20MHz square-wave input.

2) Use the internal reference doubler in one of the frac-N PLL ICs such as the
LMX2486.  The doubler adds negligable phase noise.  You wouldn't need to use
the fractional part of the dividers within this IC; you could use it just like
a normal integer-N PLL with no additional fractional spurs.  

The LMX2486 has a few other features as well that I'm investigating for use on
Horton.

I wouldn't try to synthesise a 20MHz signal from 10MHz - far too complicated
with worse performance and more cost.

regards

Grant   G8UBN