I thought of following systems in order to build devices I
needed. Some of them are classical solutions.
1. The sign detector
A receiving antenna is a device that produces a weak electric
signal. That signal is intended to be fed to the receiving
electronics.
The trick is to first input the antenna signal into a sign
detector. That is a device that outputs a binary signal: 0 if
the antenna signal is negative, 1 if it is positive. A
comparator, an operational amplifier with a high gain, a c-mos
TTL gate or a saturating receiving antenna amplifier may be
used for that purpose.
The fact the sophisticated antenna signal becomes a simple
binary signal allows the rest of the receiving electronics to
be mainly a rudimentary 1-bit digital circuit.
Take for example a superheterodyne FM receiver where the
multiplicator is a precisely adjusted MOS-FET transistor used
in his resistive functioning area. Thanks to the sign detector
MOS-FET and passive components can be replaced by a simple X-OR
gate. One input of the gate receives the binary antenna signal,
the other gate receives a digital clock signal with a frequency
slightly different from the signal frequency. The X-OR gate
output may be directly fed into a rudimentary low frequency
filter.
Imagine the use of a microcontroler to be an AM receiver.
Thanks to the signal detector only one digital microcontroler
pin will be necessary to input the signal. And only simple
1-bit instructions will be used to increment and decrement the
two receiving accumulators, with no fear for saturation or
counter loop.
This system may make you fear it implies a loss of
information. There will indeed be a loss of information for
receptions with a high signal/noise ratio. But there will be no
loss for receptions with a weak signal/noise ratio, and that's
what matters.
The sign detector offers three main advantages:
The receiving electronics becomes a lot more simple and
cheaper to build.
The receiver is no more sensitive to variations in antenna
signal intensity. Indeed, only the sign of the signal matters,
not it's amplitude.
In the case of an AM receiver, the receiver will directly
calculate the signal/noise ratio.
And four drawbacks:
The sign detector may not be used for extremely high
frequencies, because there exists no digital devices capable of
operating at those frequencies.
If an AM signal is received too well, stronger than the
radio noise, the sign detector will not be able to allow the
measurement of the AM signal amplitude. Unless an artificial
strong noise signal is intentionally added to the antenna
signal before the sign detector!
Every electronic circuit generates some radio noise.
Especially if it is the circuit of a receiver it should be more
or less shielded. Otherwise that noise will be received by the
antenna and will lower the performances of the receiver.
Digital circuits are extremely noisy circuits and need
therefore a more sophisticated shielding.
I've once been responsible for the crash of a drone build by
friends because one circuit used a microcontroler. Although
that microcontroler was completely surrounded by grounded
metal, the high frequencies it produced have disturbed the
radio link with the pilot. The high frequencies traveled to the
drone's radio receiver through the Vcc and signal wires. The
solutions have been the following ones (depending on the
application some solutions may be pointless):
The noisy electronic circuit and it's shielding should be
as little as possible.
All wires should come out of the shielding through one
sole hole inside that shielding.
To prevent high frequencies from escaping out of the
shielding through the wires, the following can be done or
should not:
Common selfs and coils let the high frequencies through,
because of a capacitive or inductive effect between input and
output loops. That's thus not a good solution. It is possible
to use selfs yet they would either be very long or use too thin
wire. Maybe selfs can be purchased intended for this purpose
yet I could not find any (or better said: I found a lot but
none did the job).
Common optocouplers are not a good solution because of the
capacitive effect between input and output. High frequencies
cross an optocoupler with no problem. A short fibre optics
should be used or special optocouplers with a very low
capacitive effect.
Dc/dc converters may be used for Vcc, provided there is no
capacitive effect between input and output. Such devices too
cannot be found in common stores.
High impedance metalfilm resistors are a very good
solution for signal wires. Something like 100 k or
1 M. High frequencies are blockaded and weakened heavily
when they must pass through such resistor (any signal will be),
and virtually disappear at the output. The resistors should be
located near the output hole inside the shielding. (Please note
for security against noise and glitches the c-mos inputs
receiving a signal that went through such a resistor should be
latched to ground through a little condensator.)
Little common condensators, a few nanofarads, provided
they are suited for high frequencies, are a very good solution:
they should be used to latch the wires to the ground. Best
place is where the wire is coming out of the shielding. For
some applications the condensators gave such a good result the
shielding was useless. (Because condensators are a short-cut
for high frequencies, the outputs that are latched to the
ground that way should first go through a resistor; in order
not to make the output gates heat up and use too much current.)
A sign detector for weak radio signals may be difficult to
build:
One solution is to use a high gain receiving antenna
amplifier circuit (possibly two amplifiers put in serial) and
cut out the output signal with a resistor and two diodes
(nothing looks more like a digital signal than a saturated
analogic signal).
The input should not have any sort of hysteresis.
The circuit should not produce a stronger internal noise
than the noise received by the antenna. If you want to verify
the quantity of noise produced by the circuit itself, try to
measure the signal before it is converted into a digital signal
and see what difference you get when the input is latched to
ground. When the input is latched to ground, the noise you
measure is the one produced by the circuit itself.
Some circuits have an unpredictable input level, or a
level uneasy to find. For example a HC gate may switch around
2.49 Volts while another may switch around
2.50 Volts. That's of no importance for common digital or
half-analogic purposes, yet for a weak radio signal it is a
real problem. Provided the circuit is an inverting gate, the
solution is to latch the input to the output through a high
impedance resistor. That way, should the input for example be
too low, the output will be set to 5 Volts and will load
the input condensators until the input level becomes high
enough. (The resistor can be calculated in order to filter away
very efficiently strong low frequencies like the 50 Hz or
60 Hz AC net current.)
The current supply for such circuit should often be
perfectly regulated. Do thus not forget to put a good
condensator between Vcc and ground. Sometimes a good solution
is to build such circuit with their own dedicated voltage
regulator.
2. The modulators and demodulators chain
A common radio emitter contains one modulator. That is
a device that produces a high frequency signal whose amplitude
(or frequency, or phase shift) is influenced by the signal you
want to transmit. For example: full amplitude to transmit a
binary 1 signal, zero amplitude to transmit a binary 0 signal.
A common radio receiver contains one demodulator. That
is a device that inputs a high frequency signal and outputs a
usable signal. For example: a binary 1 is output if a high
frequency signal is detected, a binary 0 is output if no high
frequency is detected
Suppose we send 1,000 binary bits per second. And we use a
high frequency of 1 Mhz. Then we need clocks with a
precision better than 1/1,000 th.
Suppose we decide to use a higher frequency; 100 MHz.
That frequency being 100 times higher, we are supposed to be
able to send the signal 10 times further. (This
is not true in fact: because there is less environmental noise
at higher frequencies we will have more than a 10 times
increase. Unless there are obstacles like a hill or the earth
curvature, then low frequencies, by diffracting around
obstacles and bouncing back on the ionosphere, can travel a lot
further than high frequency signals. But here we neglect these
phenomenons.)
Yet this requires our clocks to have a precision of
1/100,000 th.
Suppose we do not own clocks with such a precision. We only
have clocks with a precision of 1/1,000 th. Then, although
we transmit at 100 MHz, we will have no increase in
transmission distance.
A solution is to you use intermediate modulators and
demodulators. (This is commonly done for signal
encryption or transmission of several signals on a unique radio
frequency. An example: the stereo difference signal, for
standard FM receivers, is encoded on a subfrequency of
30 kHz that is added to the common mono signal. Another
example: RC systems for modelists use several modulators in
parallel, called "ways", using frequencies in the order of
10 kHz, to transmit commands to different actuators inside
the model plane, ship or car. The modulators signals are added
before being fed into the radio modulator. Inside the model the
received signal is given to all demodulators and each one of
them, having his own frequency, extracts the subfrequency
intended for him. But here we do it to increase the
transmission distance.)
We use our binary data signal at 1 kHz to modulate a
binary frequency at 1 MHz, then we use that modulated
1 MHz signal to modulate a radio signal at 100 MHz.
The receiver will contain a demodulator with an output at
1 MHz that will be fed into a second demodulator that will
render the 1 kHz digital binary data signal.
That way, we will able to transmit further while using low
precision clocks!
The distance increase will not be of 10, it will be less.
There is a factor two difference. Yet the result is still
important.
A chain of more than two modulators and demodulators can be
used. They may be three, four, five... That way it is
theoretically possible to transmit a signal up to any distance,
whatever the clock precision.
It is possible, for example, to transmit one bit per hour with
a 1 GHz radio signal thanks to two modulators using little
quartz crystals with a precision of 1/1,000,000 th
(available in common electronic components stores for $2).
That's very interesting for special purposes like space probes
or measurement and activation systems in very noisy
environments. Would such a transmission system have been
programmed into the processor of the Pioneer 10 probe, we would
still be in contact with it while using a much cheaper antenna
system. Maybe the transmission rate would be of a few bytes per
year, yet that's enough to receive some key information.
Most funny with this system is the fact at big distances the
first demodulator only feeds a noise to the second demodulator.
Yet the second demodulator manages to detect the signal out of
that noise!
As mentioned above, this system is not strictly equivalent to
using a single modulator and demodulator stage with high
precision clocks. There is a loss (comparable to the
particle/wave packet detection in quantum mechanics). Therefore
the number of stages should be kept as little as possible and
best possible clocks should be used.
Another problem is the fact the radio signal will use a
bandwidth a lot broader than strictly necessary. This may be
partially compensated by the fact several transmissions may
work at the same time, using the same main radio frequency, but
using different subfrequencies. If one emitter emits at
100 MHz a signal modulated by a signal at 1 MHz that
is modulated by a signal at 1 kHz, it will not be heard by
a receiver that receives also at 100 MHz but then
demodulates at 2 MHz to produce also a signal at
1 kHz. That way the bandwidth can be used effectively,
without too much wasting. It is simply another way of sharing
the bandwidth.
Other advantages are:
The possibility to build little and low-cost devices
because no very high precision clocks are needed.
Telecommunication systems that will for example be a lot
less sensitive to the Doppler effect.
The possibility to consume a continuous and very weak
amount of electric current, without the problems associated
with the emission of radio "flashes" that consume a lot of
electricity an a short time (this is another trick to emit far
with low precision clocks).
3. The fractal signal
The system described above implies information is transmitted
at a given rate; 1 kHz in the examples.
Suppose now you just want to transmit a "ON" signal. To call
for help, for example.
The problem is it is difficult to choose at what speed you
want that signal to be transmitted. Suppose you choose 1
second. Well that's quick. Okay, but what if you are very far
from the receiver? Your signal will not be heard. Unless you
transmit slower, say on one hour. That way you can transmit 60
times further. But that's stupid if you were close to the
receiver. Why wait an hour before the rescuers are warned?
When using only one modulation and demodulation stage, the
solution is simple: just let the receivers' accumulators
accumulate information on a one hour basis, but look at their
content every second. If a strong signal arrives it will make
the accumulators increment very fast. That will be noticeable
and will allow the system to react instantly.
But what if a demodulators chain is used ?
The solution is to allow each demodulator stage to have also
the property to detect the signal at his frequency, instead of
only feeding it to the next demodulator. That way, if the
signal is very strong, the first demodulator will instantly
react. "I hear it!". If it is weaker, the first demodulator
will only hear a noise. That noise is fed to the second
demodulator stage that will detect the signal and react, yet
his reaction will be slower. If the signal is too weak for that
second demodulator too, the noise fed to the third demodulator
will perhaps make him react, yet still again a lot slower than
the second. And so on, you may use a guirland of
demodulators/detectors, allowing the signal to be detected in a
few milliseconds or in a year...
At each stage there must always be a signal at any time. For
example there must always be a frequency of x Hz or
2 x Hz. The way the signal changes between the two
frequencies is used to feed the next demodulator. But the
demodulator will react if any of the two frequencies is
detected.
When you plot such a modulated signal, you get a one
dimensional fractal pattern. Previous system also had a fractal
look, yet here the full identity of a fractal appears: the
fractality can be developed up to a virtually infinitely low
frequency, and every stage has the same purpose: make a
demodulator react.
4. The noise link
In general the basic signal is a sinusoid. That sinusoid is
modulated in order to transmit information.
You may use anything else instead of a sinusoid. You may even
use a noise; a totally non-repetitive random signal.
The only condition is that the receiver must know that noise.
It must know any detail of it and know when it is emitted.
Precise clocks and time shift adjustment techniques must be
used.
For secret transmissions, the advantage of using a noise is
obvious. It is totally unhearable for receivers who do not know
what noise exactly is emitted. There are just three drawbacks:
Output power must be reasonably weak. Otherwise the
emitter can be heard and localized just because it is emitting
a strong noise.
There must not be too much radio echo. Sine waves can deal
relatively well with echo because the random sum of sine waves
is always a sine wave. The noise wave, on the contrary, will
destroy itself if there is too much echo.
The receivers electronics, in order to find the time shift
of the signal, then follow the time shifts in order to remain
locked, needs some calculation power and clock precision. (Yet
nothing too tremendous.) What's more, the random numbers
generators used by the emitter and the receiver must be of good
quality; non-repetitive and unpredictable, yet both producing
the same numbers. (They may use a combination of calculated
random numbers and shared lists of real random numbers.)
Best way to modulate the noise signal is to invert it. For
example it is emitted non inverted for digital 1 and inverted
for digital 0. The receiver just has to multiply the antenna
signal with the noise and sum the results to get big numbers
that will indicate digital 1 if they are positive and digital 0
if they are negative. That way the emitter emits constantly
with the same power. (This method of multiplicating the signals
and summing the results is called a correlation.)
Again, the time shift is a serious problem. If the receiver
is not locked on the emitter, it must try out all possible time
shifts until the signal is found. Then the signal must be
followed carefully in order not to be lost.
One way to emit a noise-like signal while being closer to
usual techniques is to emit the sum of a wide range of
ultraweak sinusoids. The receiver will also contain a wide
range of demodulators, one for each frequency, yet all
demodulators use the same shared accumulators to sum their sine
periods. Those single accumulators, containing the sum of all
sums of all demodulators, will then be used to tell if there
was an emission or not. (Such a set of demodulators can also be
used to determine if for example an emitter is emitting a
continuous series of Dirac wave patterns.)
5. Modulate several bits together
At first hand the method to transmit bits
along a very noisy channel is to modulate the status
of each bit and send each bit the one after the
other. Yet there is a drawback. If say a hundred or
a thousand modulation periods are used for each
bit then the transmission yield will be quite far
away from the maximum baudrate allowed by the Shannon
theorem. A better yield is achieved when a lot more
modulation periods can be used for each bit. Say ten times
up to hundred times more. Yet this would imply a slower
baudrate. the solution is to transmit several bits
together, each with its own modulation. If ten bits
are transmitted together, then the number of signal
periods available for each bit will be multiplicated
by about ten.