--- In [email protected], "Fred Krom" <[EMAIL PROTECTED]> wrote:
>[...]
> I find a strong (+/- 45db) signal (DC) on the 0 Hz point, I know
that there is some more noise at 0Hz but it also give a beat signal.
> Is that normal with a Tayloe mixer (direct conversion) or is it
something in my build of the mixer?
> [...]
Fred,
that can be or cannot be considered normal...try to switch off the
Local Oscillator feeding your Tayloe QSD and see if that peak goes
away. If it is still there, then it is caused by 1/f noise in the
sound card. I have observed much varying behavior from different sound
cards. The better cards had almost no, or very weak, peaking at 0 Hz,
while the mainboard integrated chipset performed much poorer in this
respect.
If, on the contrary, the peak disappears when you switch off the LO,
then probably you have a leakage from the LO to the input circuitry of
your radio. The LO signal mixes with itself, and causes a DC signal.
Normally the sound cards do not have response down to DC, but what
you see is the phase noise of the oscillator, which surrounds the DC
point.
Hey, this could be a good method to measure the phase noise of an
oscillator !
If this latter is actually your case, then a better shielding or a
better balancing of the mixer is in order.
73 Alberto I2PHD
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