in this video series we are building a
ground station for a geostationary
satellite and try to talk to a guy in
antarctica
this is a bonus series and not directly
related to the regular topics of this
channel
it is strongly suggested to start
watching at the beginning of the
playlist now showing in the top right
gritsy youtubers here is the guy with
the swiss accent with a new episode and
fresh ideas around sensors and
microcontrollers
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always sit in the first row
the last two weeks were rainy and wet
here in switzerland
i have the antenna outside and i did not
want to go there with my delicate
equipment
that is why i still was not able to
transmit a signal and receiving my voice
from space
but i was not lazy
i built a 25 megahertz filter for the
local oscillator i played dentist and
pulled a crystal out of the lnb
it did not hurt
i added the reference signal injection
path to the lnb i assembled the whole
station on a wood port
reprogrammed the addon pluto to work
with a 25 megahertz reference
and maybe killed one of them
so let's start if you remember we need a
stable 25 megahertz local oscillator
signal for the lnb
and the sdr receiver needs a stable 40
megahertz clock signal
i decided to buy this gpsdo from
leobotner because it has two outputs
but this device delivers a square
waveish signal not a sine wave because
it uses a quartz crystal the original
oscillator in the lnb produces a sine
wave
so how can we transform a square wave
into a sine
the solution is simple if we look at the
spectrum of the square wave it has
harmonics at megahertz 75 megahertz and
even higher frequencies
if we were able to filter these
harmonics we would have a pure sine wave
but how can we build such a filter
easy peasy google is your best friend
and you find for example this page
we want a low pass filter which does not
hurt all frequencies below 25 megahertz
but kills everything above it
so low pass 25 megahertz cutoff and 50
ohms on both sides are evident
but the rest
for example the type
these names sound very exotic
which one to choose it does not matter
too much for this application so we
choose butterworth maybe because of
sympathy with our british colleagues
which suffered a lot because of corona
order four
standard values and e12 because i do not
want to wind the coils myself
by the way what is e12
it is a system of preferred numbers for
electronic components and consists of 12
values per decade
you immediately recognize it because all
our resistors for example are numbered
like that
there are also e24 e48 and even e96
series available
but already an e24 series of resistors
needs a lot of space
this is why i only have an e12 series of
smd capacitors resistors and inductances
now we press compute and get a beautiful
drawing of our filter
and also this curve
relevant for us is s21 which is the
attenuation between input and output
it is around 2.5 db at 25 megahertz and
36 db at 75 megahertz
the 50 megahertz harmonics anyway was
smaller and therefore needs a little
less attenuation
with this filter we should get a decent
sine wave
and here is the result
all soldered under the binocular
connected to the spectrum analyzer we
see a similar curve as simulated
the attenuation at 75 megahertz is a
little lower than expected but still
okay
and if we insert the filter into the gp
sdo signal we get a nearly pure sine
wave cool
so we should be able to insert it into
the lnb
next i had to take out the crystal and
add a possibility to inject the 25
megahertz into the lnb
i used this twin lnb and desorted one of
its outputs
i found this small network to inject the
signal and i used through-hole
components because i think it is easier
to solder them in place
ready is the lnb i hope i can test it
next week the weather forecast is
promising
i also found that we can change the
reference frequency of the addon pluto
using this command
just copy paste
if i choose 25 megahertz i can use a
single frequency gpsdo in the future for
the lnb and the sdr the signal produced
by the original oscillator in the pluto
is around 2.2 volts and we have to match
it more or less using attenuators
and it is somehow square so we do not
need a filter for this path now it was
time to put all components in place
i used the wood board concept where i
glue the main parts in place and connect
them using coax and other cables
i also added a power distribution box
for 12 volts
and because this project is closely
related to rockets i thought i add an
appropriate main switch
let's look at the connections and start
with the pluto the rx path currently is
quite simple just the bias t which
inserts the 12 volts to power the lnb
maybe later i have to add an attenuator
because the signal from the lnb is quite
strong
the transmission path goes via the
pre-amplifier to the power amplifier and
to the potty antenna
one channel of the gpsto is connected to
the atom pluto and the other via the
filter is connected to the lnb
unfortunately my modified addon pluto
stopped working with sdr console it is
still visible on the computers though
i hope i did not fry one of the chips
during my extended experiments
one last thing
here is the gift of my wife for two
hundred thousand subscribers
it is a yambik here from begali
a beautiful piece of mechanics in an
otherwise electronics lab
long time subscribers know that i once
was a radio telegrapher for the red
cross
with age i found out that morse is like
driving a bicycle if you were able to do
it once on a certain level you do not
lose it
it only gets rusty
because the keyer does not produce a
tone i had to build this beautiful clear
project from k3 ng
now i hear what i keep
pure nostalgia
maybe i will be able to do my first qso
with it on qo100
as always you find all the relevant
links in the description
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interesting for you
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