Another capture, now with the card running at 48 kS/s, showing the 1
kHz large signal experiment. Now that's a very good result, i am quite
relaxed now and can look forward to the next steps.
Still need to manage to set the clok by gpsd, but then the
configurations are complete and i can focus on winding coils :-)
73, Stefan
Am 21.07.2017 15:53, schrieb DK7FC:
Hi Paul, VLF,
The observed effect is clearly a funtion of the temperature of the
chip, which reaches abt 50 °C (human estimation) when in operation!
Meanwhile i managed to stream audio data from the Raspi+Octo using
vtvorbis (which is BTW running over a terminal only, no GUI installed!)
to another Linux-computer via SSH and also to an Icecast2 server, so i
can put it to SpecLab to get a scrolling spectrogram, more or less
'live'. So now i am able to search for the strange behaviour of the
soundcard.
In the middle of the spectrogram, see attachment, showing CH1(top) and
CH2 (bottom) i sprayed some cooling spray on the chip. The dark
vertical lines (unwanted effect) disappear, the whole noise level drops
on both channels. When the higher case temperature recovers, the lines
come back again. When spraying onto other components on the board, i
can't see the lines disappearing.
Putting a small heat sink on the chip reduces the 'dark line'
intensity. Probably it can be fully avoided by using proper heat sinks.
The effect maybe happens d to the Affenhitze in my office, about 25 °C.
This all applies to 96 kS/s here.
But it is also possible to run the card at 48 kS/s. Then, the
CPU load is lower, the temperature is lower and the effect is gone, on
all channels! And the noise seems to be even lower, with some frequency
response though, whereas the soundcard noise has been different on
different channels at 96 kS/s, which is another strange effect!
So i conclude that either cooling of the chip helps, or running the
card at 48 kS/s helps.
I want to decide to run the card at 48 kS/s. Or would you say that this
will lead to a significant inaccuracy of the timing/precision?
During my last ULF recordings i run the card at 24 kS/s and was quite
happy with the results.
BTW, meanwhile i am using a more efficient DC/DC converter which has
about 95% efficiency.
Incredible!: Now, when running the Raspi + the GPS module and
recording two antenna inputs to an USB stick (up to nearly 4 days, for
portable postprocessable experiments), the power consumption
is 13.9 V * 170 mA = 2.4 W !!!
The first time it allows to record from 2 antennas (e.g. two loops or a
loop and an E field antenna) to form an optimised cardioid antenna by
software. The recording process is locked to an atomic standard and
accurately resampled to the expected sample rate.
Looks like it is time for a real test soon!
73, Stefan
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