The octo-soundcard seems to cause QRM in the GHZ range during operation:
I had successful GPS reception after booting the Raspi. As soon as i
initialized the soundcard, the blue PPS LED remained dark!
Tried to identify the mechanism:
I monitored the supply voltage, found nothing suspicious.
Added common mode chokes on the coax leading the PPS signal, even
disconnected that cable to break any eventual ground loops, but still no
GPS reception.
In a next step i removed the DC/DC converter which was mounted quite
close to the GPS antenna on my plate. Still lost GPS reception during an
active soundcard.
Added 470 uH in series and 1 uF in parallel to the supply voltage. Still
the same problem.
Applied a grounded shield (against E fields) between Raspi+soundcard,
still the same problem.
Disconnected the RXD TXD cables of the serial interface, still the same.
Used a separate supply voltage (9V block + 78L05) to power the GPS
module. So there was conducting connection any more between the GPS
module/antenna which is located in 6 cm distance to the Raspi (it has
never been a problem in the past). => Still the GPS reception was lost
when activating the soundcard!
Even WLAN access is lost when running the soundcard. First i thought it
is due to the real time priority of vlfrx tools. But now i have some
doubts...
When placing the GPS module about 6 cm more distant to the soundcard,
GPS reception is restored!
So now i have to apply a better shield against E and H fields coming
from the board.
73, Stefan
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