Specific question related to radio

[owenpauljames]

Hi all, this is my first post probably of many.

This question is for those of you that have more experience of radio electronics

I want to transfer a digital signal about 400m. It needs to be relitively low power 2-3 AA batteries will be used, I am not sure of the data rate of the signal yet but it is the output of a USB webcam is that about 12Kb/s. I plan on taking the D+ and D- outputs of the webcam then transmitting just D+ (is this wise or is there a way i can transmit both D+ and D- so they pick up the same noise {like in a USB cable})at the reciver I will amplify the signalk to teh correct level and then recreate D- from D+ and then send that along the USB data lines as normal (this should mean me avoiding having to do any programming i can just use the webcam software).

What frequency would you suggest i transmit on for that range (and how did you get your answer any equations would be welcome, i have had a difficult time trying to find this kind of thing out). It has to be a legal UK transmission frequency, i dont want no thought police knocking on my door and telling me not to invent things .

And what kind of transmission should i use, obviously SSB/SC is normally best for analogue but quite tricky to make a reciver and i am transmitting digital which shoudl be easier. What if i were to just transmit a pulse of the carrier frequency for each toggle of D+, this would save loads of power correct? or is there an even better way?

Do you forsee any obvious problems with what i am trying to do?

Thank you very much,

Owen

 

[RadioRon]

I'm wondering what your motivation might be for considering designing and building such a thing yourself. Is this data link a one-off, something that you will do for yourself only and just the once? Or is it something that you want to build as a product for sale to others? I ask because what you are trying to do is not trivial, and you may be much further ahead buying a ready-made RF transceiver module of some sort rather than building something yourself. Even if your goal is simply to learn how to do it, you may still be better off with something modular.

The question of frequency remains, but with off-the-shelf modules at least you have a limited choice. The range that you get will be a function of several key variables:
- the transmit power
- the transmit antenna
- the modulation type and bandwidth (which determines the minimum SNR ratio needed at the receiver to decode correctly)
- the receive antenna
- the location or orientation of the antennas, including height above ground
- the frequency

So, as you can see, even if you must choose a module from a limited selection, a choice that effectively locks in your choice of power, modulation and frequency, at least you can still play with antennas in order to maximize the range.

The regulatory rules dramatically limit what you can do. For example, in most countries there are license-free ISM bands which you can use, but they limit your choice of maximum power and modulation types. For example, here in Canada you can choose 915 MHz but are limited to 1 Watt of power, and only if you use some sort of spread spectrum modulation. That's just an example. Many experimenters choose to use an ISM band. If you are a licensed Radio Amateur, you have many other band choices and powers available. Perhaps you could consider earning an Amateur License as a step to more elaborate experiments.

 

[kchriste]

If you want to buy one, get a wireless webcam and router. You are not going to be able to transmit the D+ and D- USB signals over the air easily because of the very wide bandwidth required, NRZ issues, among many other things.

 

[owenpauljames]

Thanks for your quick replys.

I wanted to make it my self to save money and so i can have the exact specifications i want, and yes I do plan on selling these things once i make them.

How come there would be NRZ issues? This is why i want to do it myself, I would transmit a signal that is not the same as the USB signal, and recreate the USB signal at the other end, that way i can make sure that the 1 and 0 values are the ones my computer will expect.

A wireless webcam does not have the range I want, I think thats because of the high frequency it transmits at it uses the normal 802.11 standard.

If you know of a web site where i can get cheap small 0.2-0.4W transmitters with a wide range of frequencies, that would be really helpfull. I wasn't set on building it all myself i just thought that would be much cheaper and easier to get the specs i wanted.

thank you

 

[tkbits]

USB does not transmit in strict binary code. For 12 Mbps (not 12 k!), there is a third state used for signalling a USB reset and for signalling the end of a packet. You will not be able to re-create the third state by transmitting only D+ information.

It sounds like you expect to need only one way communication. USB is bidirectional with the host (PC) initiating all data transactions. There is a tight turnaround time requirement for acknowledging data packets. Demodulation may add an undesirable amount of delay.

And if you are transmitting and receiving only D+/D- information, then you will need to simulate a plug-in, for the case where you turn the remote device on after you've plugged in the PC transceiver. This is because the PC will not talk to your device until it detects that it is plugged in, and then it will immediately attempt to talk to it.