RF data link

[Ayne]

i need a RF data link circuit cuz i want to connect two computers about one kilo meter far from each other..

i don't want to buy wireless modems ........ i want to make circuit my self

i want to use serial data transfer
Baud rate = 115200 BPS
oprating frequency 20 to 200 Mhz.
Any one can help..... Any idea, any circuit, etc.


below is the circuit
i want to use it for data modulation(Some thing like keyed modulation )
it will be run?
using 555 timer as a voltage control oscillator
input pulse frequency is 115 KHz and it's voltages are 0-5V
and out put is shown..
plz share ur ideas on this circuit also..

 

[Nigel Goodwin]

You have many problems to over come, first off is the legal aspects, what you are wanting to do is probably illegal almost anywhere - unless you can obtain a licence to do it?.

Next your frequency, this is probably far too low for the bandwidth you're looking for - you need to go much higher.

Distance - 1km is a fairly long way, again it's unlikely that you can get a licence to do it!.

Protocol - you can't just squirt plain RS232 across a radio link.

I would suggets you look at licence free radio modules, you can get some in the km range, but not that will do 115kbps.

The 555 idea is far too crude!.

 

[Ayne]

i will apply for licence soon

 

[JimB]

OK I will try and explain why it wont work, and why the radio licencing authorities will have a good laugh when they see your application.

Consider the Bell 202 standard, an ancient standard for modems using a telephone line. It used two tones 1200hz and 2200hz to send data at 1200bps.
It was a form of audio frequency shift keying. One of the tones represented binary 0 the other reperesented binary 1.

If you wanted to use a similar technique to send 115kbps, roughly 100 times the data rate, you would need tones of 220khz and 120khz.
Now try and modulate this onto a radio signal and you will have a signal which is at least 440khz wide. In other words occupying the same amount of radio spectrum as 35 voice channels (UK specification).

Sorry, but your scheme will just not fly.

JimB

 

[Ayne]

Is there any way for doing this??? else buying a ready made circuit

 

[Ayne]

My idea

i want some thing like this
for transmitting digital 1 we will transmit carrier and for transmitting digital 0 we will transmit nothing............. as shown in diagram.
need a circuit or guidence for doing this.

i need ur guidence.. really i want to make a data link my self

data rate can also be decrease.......... it can be 57600Bps..

 

[Papabravo]

You still don't seem to have absorbed what several people have tried to explain. To transmit data at high speed takes considerable RF bandwidth. This bandwidth is not available at low to medium frequencies. When you go to higher frequencies the design of the RF Receiver and Transmitter become much more difficult, the reliable line of sight distance goes down and you need to solve a host of problems that you appear to be ill equipped to solve.

The last point is that you do not want your input to be a choice between DC and some frequency. Trust me on this, it is possibly the worst data modulation scheme immaginable.

For some positive suggestions, investigate Manchester encoding, run length limited codes, and bit stuffing.

 

[zkt_PiratesDen]

This outta do it: Not all that easy to use tho
http://www.nvlsi.no/index.cfm?obj=product&act=display&pro=89

 

[Papabravo]

The output of 0 dBm or 1 milliwatt for the nvlis module is not going to propagate very far. I'd be surprised if you can get 30 feet. It certainly will not go a kilometer which was the original requirement.

 

[RadioRon]

While i agree with Papabravo that it will be difficult to get this to work at 1Km but must disagree with the range of 30 feet. If he can transmit at 0 dBm and include a directive antenna for a bit of extra gain, it is likely that the path loss budget available is 80 dB or more. This can potentially reach several hundred metres purely line of sight. The bigger problem is that our original poster is not likely to know how to get everything to work properly and optimally, so perhaps <30 feet is the likely outcome anyways.

 

[Papabravo]

Radio Ron is correct about the benefits of directional antennas. I was thinking of a 2.4 GHz system similar to a Linksys Router which has a pair of vertical antennas and seems to have a range of 30 feet or so. Good for room to room but useless outside the house at any distance.

The other point is that as the signal level goes down the Bit Error Rate goes up and the probability of a retransmission or the necessity to use Error Correcting Codes goes up. Both of these strategies will have an effect on the overall throughput.