mugzzzzy

hi
i need help to build a fsk transmitter....i m using 10Mhz for logical 1 and 6mHz for logical 0... i want to build a system that can transmite binary ..any one has any design or hint for me is welcome

ty

pike

Dude, you can't broadcast on those frequencies, they're reserved for air services. Wouldn't it be easier to buy LIPD (low interference potential device) module???

It's alot easier to buy a premade module. And circuits like these are hard to come by.

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mugzzzzy

Hi,

sorry i forgot to mention that its a school project, i m allowed to use theses frequency becuz the range between my transmitter and my receiver is not more than 10 meter... i have to build from scratch with out using chips or any device

well my main probleme is that i donno how to combine those 2 frequency in one signal depanding if its a logical 1 or 0 ...that will goto the antenna

i m building a system that will send binary (ASCII) to an other device

i've tryed to use analog switch SPST and SPDT...both didnt give the effect i needed.

;-)

Nigel Goodwin

I don't think the power matters a great deal, transmitting on an illegal frequency is still illegal. Unless you are in a screened metal room you won't be limiting those frequencies to only 10 meters.

My main concern if the frequency shift you are suggesting, from 10MHz to 6MHz is crazy!. In FSK you would normally shift a very small amount, perhaps 1KHz or so - why are you suggesting such a huge shift?.

stevez

Radio amateurs, among others, have used FSK for years. As previously mentioned the amount of shift is usually small but I suppose that doesn't mean that you can't do what you propose (with due regard for the law and interference). You might look for ideas in amateur radio publications.

__________________
stevez

Roff

You can do the FSK with a simple data selector if you have the two frequencies and the data available as logic levels. See below.

Attached Images

FSK.GIF (7.0 KB, 481 views)

mugzzzzy

hi,

i gotta admit thats a good idea...i dont understand how i didnt thought of that before

Thanks a lot Ron

P.S. at 10Mhz its a ammateur band zone so its ok....but i might have some probleme with the 6Mhz ...but its ok for me to use it becuz its for education purposes...and i m not trying to decode their information

:-)

jem

While Ron's dircuit would indeed produce the results you desire, it would be non phase coherent FSK. Also, to have a receiver with a bandwidth of 6MHz to 10MHz is not easy, if not downright impossible. You would need two receivers, and then synchronize the two output streams to give you the required data. Both receivers would have to have some form of squelch circuit to cut one off during periods when the other is transmitting. A typical FSK, as mentioned before, have frequencies that are fairly close together.

Roff

I'm no communications expert, but why would you even want phase coherence with FSK, unless perhaps the data rate were a significant percentage of the carrier frequencies? While I agree that trying to transmit and receive this (6MHz/10MHz) by conventional radio techniques is difficult, it could easily modulate a solid state laser and be transmitted over a considerable distance and still be received and demodulated. I'm not defending the choice of carrier frequencies - just pointing out that, under some circumstances, they will work. I agree that the frequency deviation should be a small percentage of the carrier frequency if RF techniques are to be used.
Regarding the need for squelch mentioned above - over short distances, where good S/N is available, AGC would be unnecessary (most homemade, short range receivers don't have AGC). Without AGC, why would squelch be necessary?

mugzzzzy

hi,
ya i tought about it while busy with nature...lol...and realise that indeed it will have probleme with it becuz out of the AND gates the signal are TTL level...therefore i will only get the positive half of the oscilator...I m thiking maybe with a summing op-amp will do the job....

i m not very good in telecom stuff...but just wondering whats a AGC and squelch...

U mentioned that the frequency of the carrier should have a ratio with the data rate....well i m not sure yet ..its either 19200bps or 9600bps... and also i m not sure yet if i should use the TTL level for the data or the RS-232 bipolar data(-9 to +9)...

:?: :!: :?: :!: :?: :!: :?: :!: :?:

jem

There are good reasons for phase coherence in FSK, especially if a PLL is going to be used for the decoding. Without phase coherence, as in the case above, the system is no better than OOK. In other words, why bother to use 10 MHz, and 6MHz? One could simply transmit 10MHz for logical 1, and nothing for logical 0 (OOK). Also, even in that kind of system, one would need something to "mute" the output when nothing (no 10 MHz signal) is being received; otherwise, the data slicer would have a hard time following the 1 - 0 transitions.

Just my humble opinions, of course!

Jem

Roff

I'm trying to understand this. Why would phase coherence aid in demodulating FSK using a PLL?

jem

Once a PLL has "locked" on a signal, it will track the signal provided there are nondiscontinuous phase changes, and the signal remains within its tracking range. In the worst of cases, a PLL will take quite some time to lock on a signal even though the signal frequency is within its capture range depending on the phase of its internal VCO at the time in relation to the phase of the input signal (also on the characteristics of the LPF). The result of this delay is that the detected pulse width is smaller than the actual pulse that was sent, by a random (but bounded) amount. The bottom line is the data rate suffers. It is quite possible to send a tone burst, and detect it by using a tone detector such as the 567, but if the tone burst duration is small, sometimes (worst case about 14 cycles) the 567 will not detect it.
Anyway, this is at least my understanding of it. I haven't been a commications engineer since four jobs, and about 16 years, ago!

Jem

pike

have you considered infra-red ???my tv remote just covers 13 metres (39 feet). I think it would be a good option compared to using a prebuilt wireless module that your not allowed to use.

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mugzzzzy

Hi,

i figure out how to make it work with the SPDT (DG403)...for some reason it doesnt work with high frquecy oscilator...the data sheet only talks about the clock and more detail bout it...but not the frequency of the inputs...anyway i got it to work with smaller frquency

Yes i do really need 2 diferent frequency cuz if logical 0 is represented by no frequency and if i have nothing to send is also no frequency...if my fisrt data bit is 0 ...then i m screwd the data

No infra red..gotta be wireless....and once again its not illegal what i m doing...its for educational purposes...and the power of my antenna wont effect the information of who ever is using this frequency

:-)