[minima] Anything special about T3's capacitor to ground?

  • From: Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
  • To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
  • Date: Sat, 22 Feb 2014 19:48:26 -0800

So I used the AD9850 dds to generate a 10 mhz signal and track that down in
my minima. I think I found my problem...

LPF, KISS mixer, RF pre-amp and BFO all seem to work.

However, I have absolutely no signal at the output of the BFO mixer. But if
I remove the 0.1 uF cap to ground, I suddenly have a beautiful signal
modulated with the difference of the BFO and the IF. If I switch my BFO to
the alternate frequency (LSB), the modulation frequency also changes.

So I can get a 4.1 or 6.8 kHz (very precisely my BFOs minus 20mhz) but of
course it still is carried over the 20 mhz. I understand the cap to ground
is supposed to remove that.

*On the schematic, this specific cap (below T3) is marked as a polarized
component. Should I use some specific type of capacitor here?*

This is what the output of t3 looks like into my scope, 50 ohm terminated -
*without* the capacitor.


[image: Inline image 1]

This looks right to me.

Any recommandation for this specific capacitor?

thanks!
thomas

PNG image

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