SX and RF
william chan
Posts: 1,326
To all RF experts,
Can the SX running at 50Mhz be used to directly drive an antenna to transmit radio waves of any frequency between 1 to 20 Mhz?
Can another SX using only simple discreet components detect this radio wave directly?
A simple On / Off signaling would suffice.
Thanks.
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Can the SX running at 50Mhz be used to directly drive an antenna to transmit radio waves of any frequency between 1 to 20 Mhz?
Can another SX using only simple discreet components detect this radio wave directly?
A simple On / Off signaling would suffice.
Thanks.
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Comments
While I would not want to pass myself off an expert, I do have experience at what you are asking about.
1. A cautious "yes", but totally illegal, and unapprovable as it would also transmit a tremendous amount of interference, sending out tons of harmonics. Also, at the higher end of the frequencies you were asking about, you would have no hope of modulating anything but carrier(s) on/off. Frequency stability would be horrid.
2. Not a chance unless you were in VERY close proximity to the transmitter, and only then at the lower frequencies, and employing some trickery. You'd be better off interfacing an SX to an AM radio receiver.
Cheers,
Peter (pjv)
If the square wave coming off from the SX's IO pin is filtered using RC filters to get sine waves, amplified by transistors before
going into the antenna, wouldn't that reduce or eliminate the harmonics?
I think Frequency Stability should be good considering the stability of 50Mhz ceramic resonators.
How about using a simple LC filter to detect the carrier and just light up an led when the carrier is detected?
In you opinion, what freq would be the best/easiest to manage?
Thanks.
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Yes, in concept you are correct, however the practical implementation of that is not so simple. At the lower frequencies (say 1 MHz) you can probably get a transmitter working, but you'll have a bear of a time getting rid of the harmonics. At higher frequencies it's a lot tougher, and you'd (probably) never meet the emission requirements.
For the using an SX as a receiver, the sensitivity is just not there for reliably picking up a few (or a few tens) of microvolts. Impossible ?, depending on your willingness to water-down your requirements and bend the rules, probably not. But absolutely not practical. There are much easier approaches.
As for stability of the resonator, it all depends on the Q of your detection circuit. The higher the Q, the better the sensitivity, but also the tighter the bandwidth.
That said, I'm sure someone will have a go at this just to prove it can be done. And if they succeed, I'll say good for them because they will have learned a whole bunch through the process...... experimentation IS a lot of fun.
Cheers,
Peter (pjv)
it is definitely possible to let an SX generate RF signals to be transmitted after filtering out the unwanted harmonics, and you could also build some SX-based receiver, with a pre-amplifier, and an LC circuit resonant on the transmitting frequency. Nevertheless, doing so leads you into illegality. The frequencies you might be able to use are somewhere in the range from a couple of MHz up to 20 MHz, i.e. somewhere in the shortwave bands. Most of these bands are reserved for special purposes. As licensed radio amateur (DK4TT), I naturally have a close eye on the 20, 15, and 10m bands reserved for ham radio, and I don't like to see "SX coms" there
My professor for radio communications once explained the difference between wired, and wireless communications like this: For wired communications, think of a long dachshund with his head in LA, and the tail in NY. When you step on his tail in NY, he will yawl in LA. Wireless communications is exatly the same - just without the dachshund.
I remembered this explanation again when I did some experiments with the XBee, and XBeePro modules - they really make it easy to handle wireless communications, and they perfectly replace the dachshund
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Greetings from Germany,
G
As to reception, it would be interesting to try with a REALLY big coil antenna and something like a charge pump to see if a signal could be detected by the SX input pin. Maybe "charge pump" is the wrong word. I'm thinking tuned RF (tank and cap) with a series diode and a cap from the pin to ground. Maybe something like the sigma-delta A2D systems? Pull the pin down for a second and then wait for it to float back up; faster rise time means more RF energy was being recieved at the tuned frequency. The rise would be slower when the transmitter was off.
IANAL, but all the legal issues go away if the power is low enough. Transmit on the walky-talky frequencies at milliwatt power levels. How can they complain?
Something that works for in-house, very slow, wireless signalling would be very useful. I'm thinking remote thermostate, yard water controllers / water sensors, sump alarm, greenhouse temp, etc...
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Sorry guys, but the levels of harmonics required to comply with regulations without employing spread spectrum techniques is so low that it borders on being unachievable, and that is at permitted frequencies. On most bands are simply not permitted to be an "intentional radiator" no matter how low your power. That is if you want to stay legal.......
As far as receiving low level signals, I have -so far unsuccessfully, more work to be done- tried to use an SX, biased at the switching threshold, to detect the 60 KHz time signal from Boulder Colorado. I used a synchronous detection method where I would sample twice per 60 KHz cycle, and accumulate into a "positive" and "negative" bins to detect average excursions in the noise, and hopefully detect a signal. I used a VERY accurate·clock source to ensure there was no drifting of sample frequency, at least none that I would not be able to notice. Considering that the receive amplitude on the 60 KHz tuned coil is only a few microvolts, the noise was simply too overwhelming, and my algorithm was unable to pull the signal out of the noise.
So, in making your own SX based transmitter at frequency of a couple of Megahertz, your radiating efficiencies are very low unless yous use proper antennas, many tens of feet long (one wavelength at 10 MHz is approximately 100 feet), it just isn't practial. And if you don't radiate properly, and still require a similar sized antenna to receive the signal, and then considering the obligatory 36 dB path loss that you can do nothing about, your received signal is in the weeds!
If wireless data is your goal, go buy some approved modules, there are lots to choose from, and inexpensive to boot. If experimentation is your goal, then have at it and enjoy. There is LOTS to learn in this field!
Cheers,
Peter (pjv)
Post Edited (pjv) : 10/8/2007 9:19:03 PM GMT
Are you sure two samples per cycle are enough? If you're spot on the frequency, but happen to be sampling at the zero crossings, you won't get anything. That's why quadrature detection is often employed in direct conversion systems. This entails four samples per cycle and two accumulators. The amplitude will be the square root of the sum-of-squares of the two accumulators.
-Phil
How about we use only a 1/4 wave antenna for transmitting and receiving?
That would reduce the antenna length right?
Let's say we don't use a SX for receiving but just a simple LC resonant circuit to detect the carrier.
Is it possible?
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For receiving, you should at least use a diode detector (with a capacitive filter after it). That way, the SX would just have to see if the input voltage rises above about 1V (roughly).
Another type of antenna to consider is a tuned loop, both for transmitting and receiving.
Bean.
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Phil thanks for your interest.... two samples in quadrature is my method. But to this point the levels of a few microvolts are just too tough to ferret out. I need to spend more time on this as I believe it can be done. When I put the tuned detector coil on my Tektronix spectrum ananlyzer (9kHz to 8 GHz) I did not see any signal at all, and I temporarily shelved the experiments. As far as frequency is concerned, I have a very precise generator, and I can choose to be bang on frequency, or very slowly let the sample and carrier in question·drift past each other while observing the accumulations in real time. As I said, more time on this later.
William..... depending on your frequency choice, think of 1/4 wave being 25 feet for the transmitter, and the same for the receiver. I doubt that is practical for your scenario? As far as a simple LC resonant circuit is concerned, the resulting levels are still very low..... you'll need an amplifier or at least some good SX tricks.
Mike .... the levels to expect are a few (perhaps a few tens of) microvolts unless the antennas are very close to each other. I believe William did not have that idea in mind. One might as well make them touch in that case, and then what was the point of RF?
Again, my opinion is: you'll never be legal..... other solutions are cheap and off the shelf...... but experiment away because the pursuit and learning is well worth the effort.
Cheers,
Peter (pjv)
Post Edited (pjv) : 10/9/2007 4:47:16 AM GMT
That would solve the problem of longish antenna's right?
Should one end of the coil antenna be connected to ground or left unconnected?
So, if the receiver also uses a 100feet coiled antenna, a LC resonant circuit with a diode detector and an OpAmp to amplify the detected dc voltage before feeding into an SX, a simple communication system should be achievable.
Will the coil antenna be highly directional?
Sorry for so many questions. I am an amateur when it comes to RF.
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In another thread, Beau related having used a 4069 inverter section as a receiver front end, with the output fed back to the input via a 10M resistor to form a high-gain amp. By AC couplng to that with a ferrite antenna and cap tuned to 60KHz, you might pull that time signal out of the mud yet!
-Phil
If I were to use the OSC2 pin, buffer it, filter it before being fed into the antenna, I would be able to transmit at 50Mhz right.
That would make the antenna much shorter.
Do you think it would be feasible?
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While very small signals can be localized and might·not bother many people, the main restriction is that you ought to get a HAM license and stay within the approprite Amature bands if you don't want to have neighbors complaining and angry officials visiting you. Once you start putting up antennas, you can easily be found out.
There are radio frequency bands just for experimenters - use those with proper permission.· But also realize that one must use sine waves, not square waves to stay in the appropriate band.
In sum, there is a right way to do this and a wrong way. The right way will open a lot of opportunities to meet friends and learn to combine analog electronics with digital.
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Post Edited (Kramer) : 10/9/2007 11:26:32 AM GMT
http://www.al7fs.us/AL7FS2.html
google---qrp pixie
ok now for the cautionary statements:
these little radios are designed to work in the amateur radio bands...If you are not licensed to operate in these bands then it is illegal to transmit. Even if you are licensed there are only certain segments of the bands that can be used for experimentation. Even at the extremely low power of a few milliwatts these radios under the right conditions can transmit a signal hundreds or possibly thousands of miles.
My suggestion would be to·browse the numerous schematics online and adapt them to your project. Also you may want to look online for info on unlicensed experimental bands that you can use without getting into any trouble.
the radios tranismit CW (really just a tone) you could adapt the idea to made a digital type mode such as PSK31 or you own design.
personally i'd just go spend a few bucks at digi-key or your local shop and get the linx technologies tx/rx.
they are fully legal and easy to implement
http://www.linxtechnologies.com/
the around $7 a piece
these guy also have an interesting offering
http://www.radiometrix.co.uk/
Im sure there are may more out there.
Dan
·****EDIT*****
Also anything you do in the Amateur bands if for non-commercial purposes!!!!
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DTQ
Post Edited (kb2hap) : 10/9/2007 4:34:16 PM GMT
One is an amateur radio device especially designed for certain bands = but you need to have your own licensed call letters in all transmissions to use it == and the other is stick with radio bands that are set aside for the general public; like R/C airplance, CB radio, commercial walkie-talkie and respect the community that is using them. Blue-tooth and WIFI are newer innovations that can sort out all the collisions in traffic while letting you remain blissful ignorant of whoelse is on the same frequency.
The above discussion implied the real problem, but I'm not sure it was clearly explained.
A sine wave is just one frequency, that is used as the carrier of the data. That is the beauty of it and radio transmission relies on the purity of it. But a square wave is really a construction of several frequencies - the primary one, and several higher harmonics. When you transmit a pure square wave, you really end up with transmission on several different frequencies at the same time. The transmission goes places it never was intended to go.
So the question is when and how to best convert digital square wave to analog sine waves. It seems silly to me to use a microprocessor as the oscillator. Just build a tuned circuit of some sore, like a VCO, a voltage controlled oscillator, to keep the purity. Then mix the two through appropriate means.
BTW, while audio enthusiast seem to worship the beauty of sine waves in their Hi-fi gear, but audio too is full of harmonics and impure wave forms. They really don't understand the original need for pure sine wave came from radio transmission. Good Hi-fi copies all the mathimatically impure qualities and faithful copies them without adding noise. So, audio radio is actually imposing a very messy signal on a much faster pure sine way carrier through modulation of the amplitude or the frequency. But the air waves just see the sine way. It some ways, that is quite ironic.
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William..... the answer is NO NO NO.
1. Just coiling up wire does not an antenna make!
2. And where will you find an op amp that will give you 50 or more dB of gain at 10 MHz and also not provide tons of noise?
3. You cant hook anyting to the OSC pins of the SX.... it will stop oscillating.
If you don't believe it, then build it to try.
Phil..... interesting approach; I wonder if I could bias the internal comparator to get at the 300K gain speced for it. Need to try!
I know you can bias a standard SX input at the switching point and get quite a bit of gain.... certainly detect 100 microvolts.
Cheers,
Peter (pjv)
A coiled 100 ft wire is not the same as one extended to its full length. There are interactions among adjacent wires and the interaction between the incoming radio wave and the wire is different. Usually the coiled wire is used as a loop antenna and forms the inductive part of a tuned circuit with a capacitor connected across the ends of the wire. One end of the loop is connected to the circuit ground and the other is connected to the input terminal of the circuit. The Wikipedia has a good explanation (en.wikipedia.org/wiki/Loop_antenna).
This is exactly the circuit used for many low frequency receivers including simple AM radio receivers. In that case, the coil was smaller (with less wire) and formed around a ferrite bar to increase the inductance. There was a variable capacitor connected in parallel to form a parallel resonant tuned circuit at the desired frequency. A diode was connected to one end of the tuned circuit and the other end was grounded (to the circuit ground). The output of the diode was bypassed (for RF) to ground by a small capacitor and a headphone or audio amplifier was connected to the diode output. In your case, you'd use an op-amp (or the SX's comparator) to detect a voltage threshold.
Short loop antennas are bidirectional. They're sensitive to signals coming in along the coil axis and fairly insensitive to signals coming in perpendicular to the coil axis. Long loop antennas (see the Wikipedia article for the differences) are sensitive to signals coming in along the plane of the loop and insensitive to signals along the loop axis.
Post Edited (Mike Green) : 10/9/2007 4:45:05 PM GMT