when we started our bonus project to
communicate via the geostationary
satellite q0100
we created a to-do list
a high-power 2.4 gigahertz transmitter a
sensitive receiver for 10 gigahertz and
an internet connection to the adam pluto
in the last video we built a prototype
for the transmitter we just need an
antenna to complete the task so this
week we will make a cheap first version
of the receiver and hopefully listen to
the first signals from space
and you will learn pll
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10 gigahertz is a very high frequency
not easy to handle and usually the
equipment is costly because we do not
have a lot of money in this video we
will
find out how we can receive such highest
frequency signals for only a few dollars
we will learn how to reduce frequencies
and why this is good for our wallet
we will learn how stable highest
frequencies are created
we will see the advantages of mass
production and we will try to get a
signal from space
first of all this experiment is only
possible because we are very fortunate
why
for a long time we can receive satellite
tv with standard tv sets these
satellites broadcast at 10.7 to 12.75
gigahertz the q0100 is a spare
transponder of such a tv satellite
called eshil2
because satellites are costly they
contain additional transponders in case
one of the main transponders dies
the amateur radio community is allowed
to use one of those transponders as long
as the main transponders work
so the q0100 transponder also has to use
similar frequencies but of course not
the same as tv stations fortunately also
satellite dishes work on the frequencies
used by qr100 so we should be able to
repurpose such a dish
but how does such a dish work
where i live the satellite beam comes
from around 32 degrees above the horizon
and from 156 degrees which is nearly
south the dish concentrates the energy
to one focal point where we install the
first part of our receiver the lnb
we can use 40 60 80 centimeters or even
bigger dishes on this chart you see that
the gain of such dishes is considerable
the 80 centimeter dish i plan to use has
a gain of around 38db
do you remember the db numbers of the
last video
38db is
6300 times
the 31db of the 40 centimeter dish still
is 1200 times so the satellite signal is
heavily amplified before it reaches the
lnb
but what is an lnb
it is a low noise block down converter
according to wikipedia such a device has
lots of functions the lnb is a
combination of
low noise amplifier frequency mixer
local oscillator
intermediate frequency or if amplifier
it serves as the rf front end of the
satellite receiver receiving the
microwave signal from the satellite
collected by the dish amplifying it and
then converting the block of frequencies
to a lower block of intermediate
frequencies
and you know what
the dish including the lnb cost me only
around 35 dollars shipped
incredible
the fruit of mass production
this is hacking i like
repurposed mass-produced products for
something meaningful
here i have such an lnb and of course i
had to take it apart
it is not a valuable one and could not
be used for the q0100 as we will later
see
its main ic is a cxnb
4202
this is a specialized chip for lnbs and
in its datasheet we find the block
diagram of a typical lnb as wikipedia
said we find low noise amplifiers a
mixer two local oscillators and an if
amplifier two things are new here first
lnbs have two antennas one horizontally
and one vertically polarized
we can switch between these two antennas
by applying 12 or 16 volts to the lnb
and they have two local oscillators one
for the low and one for the high band we
can switch the high band local
oscillator on by sending a 22 kilohertz
signal to the lnb
but we do not need the high band and qo
100 is vertically oriented so we just
feed our lnb with 12 volts
fortunately these values are
standardized and the power is sent
through the coax cable this is why we
have to inject it somewhere along the
line
let's travel with the signal from the
antenna to the output
the first step is the horn where waves
from space enter the lnb
next it travels via the antenna to a low
noise amplifier on 10 gigahertz
the disassembled lnb uses these small
v75 chips which are quite cheap for what
they can do
the next is a frequency mixer viewers of
video number 286 know
that if we mix the input frequency with
the frequency of the local oscillator we
get the sum and the difference of these
frequencies as an output the difference
is always lower than the two signals so
this is a promising path to lower cost
we will later have a closer look at
those local oscillators because they are
essential for frequency stability
after the mixer we have the if amplifier
of course we need filters everywhere
between these stages
a closer look at the pcb shows that
microwave pcbs look very different from
standard pcbs
they look a little artistic with strange
structures
for the layman they look like crop
circles or other structures made by
extraterrestrials the expert however
sees everywhere filters consisting of
inductances and capacitances
and the accountant sees that such
filters can be produced cheaply and in
masses because they are just part of the
pcb
that is a big part of the secret that
such an lnp costs less than ten dollars
the packaging of the lnb says that the
low lo frequency is 9.75 gigahertz so we
can calculate where we can receive the
signal coming out of the lnb
the q0100 band is at
10.489 gigahertz minus 9.75 gigahertz
equals 739 megahertz this is a frequency
we can handle with our sdr receivers
good news
we also can use cheap cables to connect
the lnb to the receiver because the
signal was amplified and cable losses on
739 megahertz are much lower than on 10
gigahertz
these cheap lnbs use dielectric
resonator stabilized local oscillators
short dros
compared with quartz crystals a dro is
relatively unstable with temperature and
frequency variations can be as much as
plus minus 2 megahertz at 10 gigahertz
not good if the signals we want to
listen to are only 2.5 kilohertz wide
so we need something much better even to
start with
we need to be able to use crystal
oscillators which are much more stable
unfortunately we do not get such parts
for 10 gigahertz
but ingenious engineers found a way to
get the job done
they invented the pll or phase locked
loop
this is such an essential concept that
we will pause for a moment with our
satellite project and have a closer look
i use a
cd4046 chip to build such a pll and show
you how it works it consists of three
parts
a voltage controlled oscillator or vco
a phase comparator
and a loaf pass filter
optionally you can add a frequency
divider
the vco produces a frequency which is
influenced by a voltage
the higher the voltage the higher the
frequency
this cd4046 can create frequencies up to
around one megahertz of course nothing
for our 10 gigahertz project but good
enough to show you the principle the
phase comparator has two inputs and one
output it is basically an xor gate and
it creates a one if the two signals are
not the same
if the two signals are exactly the same
its output is zero
i now feed a one kilohertz signal to one
input and connect the output of the vco
to the other input of the phase
comparator
what would you expect because the two
signals are completely different we see
a fluctuating signal
we also see that the vco all the time
changes its frequency
and now comes the trick if i connect the
output signal of the comparator to the
input of the vco
everything is stabilized nearly
immediately and the output signal is
nearly the same as the input signal cool
the input voltage of the vco is around
400 millivolt if i increase the
frequency of the input signal the output
of the vco follows closely and the blue
voltage increases slowly up to 4.15
volts at 1.1 megahertz still here the
input and the output frequency are the
same
as soon as one of the two signals change
the phase detector creates a different
voltage and adjusts the frequency
but the main question remains
why should i build such a stupid machine
in the best case it reproduces the input
signal
you might think i steal your time
but wait for a second
i reduce the input signal to one
kilohertz again and add a cd 4040
counter chip to the game
this chip divides frequencies by power
of two
we start with divided by two
what happens the output frequency
doubles
interesting
because the output signal is divided
the phase comparator detects differences
until the vco has exactly double the
frequency
now both signals are the same again
and the comparator is happy
and what happens if i use the divided by
4 output of the 4040
we get 4 kilohertz
we just invented a simple frequency
multiplier and of course we could go on
till we get the maximum frequency of the
4046 chip
really at the 2 to the power of 10
output it is
1.024 megahertz one kilohertz times 2 to
the power of 10. cool
and because the ratio is locked this
frequency is nearly as stable as the
input signal
why only nearly because of course we
also multiply the frequency variations
of the input with a multiplication
factor
and i want to show you another effect
which becomes important phase noise
at one kilohertz the output signals
jitters a little if we compare it with
the input
which means its frequency oscillates
around one kilohertz
if used as a local oscillator this noise
quickly transfers into our receiving
signal and this is obviously not good
fortunately it can be reduced as we see
on one megahertz but this is stuff for
other discussions
good lnbs use precisely the same
principle for their local oscillators
they have a 25 megahertz resonator and
boost it to 9.75 gigahertz the factor
can easily be calculated 390.
some lnbs have 27 megahertz resonators
then the factor is 361.
all in all we need a very stable 25 or
27 megahertz signal to start with
which is not provided by the standard 10
lmbs and which is not needed for tv
satellite reception because tv signals
are much broader and satellite receivers
can automatically adjust their receiving
frequency if the lnb drifts
but this is a topic for another video
the last question remains does it work
yes i heard signals from q0100
okay
[Music]
but pay attention
the opening angle of the 80 centimeter
dish is smaller than i expected
i put the receiver to
739.75 megahertz and the span of 550
kilohertz moved the dish very slowly and
watched the stations appear on the
spectrum
this was all for today next week i hope
we can start with frequency
stabilization efforts
or with building the 2.4 gigahertz
antenna
because even without a receiver i still
could try my transmitter and use the
online satellite receiver in cornwall to
check if i hear myself
as always you find the relevant links in
the description
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