If the signal is on the high side of the oscillator,
Q should lead I after the mixer,
s(t) = sin(wc * t); // Signal, wc = 2 * pi * Fc
i(t) = sin(wc * t) * cos( wo * t); // LO, wo = 2 * pi * Fo
q(t) = sin(wc * t) * sin( wo * t);
expands to
i(t) = 1/2 * { sin((wc+wo)*t) + sin((wc-wo)*t) }
q(t) = 1/2 * { cos((wc-wo)*t) - cos((wc+wo)*t) }
which after LPF to remove the wc+wo component leaves
i(t) ~= 1/2 * sin((wc-wo)*t)
q(t) ~= 1/2 * cos((wc-wo)*t)
which has q(t) leading i(t) by 90 degrees.
The above are the polarities used by vlfrx-tools and ebnaut.
On the scope, signal at 77500 Hz and LO at 77490 Hz looks like
http://abelian.org/vlf/tmp/151213a.gif
with Q (the 2nd channel) peaking 90 deg before I.
--
Paul Nicholson
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