On November 27, 2005 02:09 am, Vitaliy wrote:
> > Radio passes through walls.
>
> Yes, I noticed. ;)

8-)

> > Your 100mW may still be overshadowed by a transmitter a mile away
> > pumping out 5W....
>
> Wow, you are right! I should have done my homework... this goes
> against my intuition:
>
> 100 mW / (10 m)^2 = 1 mW/m^2
> 5000 mW / (1609 m)^2 = 1.9 mW/m^2

Friend of mine was trying out some industrial controls. He managed to 
still control the system from over 15 miles away. He had to talk with 
his partner over a cell phone to make sure he didn't accidently run the 
machinery somewhere unwanted, but the distance was the interesting 
point. 5W, 450something MHz. Not theory, but actual fact.
Your calculations are valid for a point antenna but disregards the fact 
that directional antennas exist and may or might not cross your 
property or the site of interest.  ;-)
Like Gerhard points out, a directional antenna crossing the property 
would cause problems.... but reading your reply to Olin under EE, 
sounds like this is a project under the fun-factor, so no point in 
butting horns,  have fun.   ;-)

> > unless your frequency of choice is dedicated to
> > low-power stuff, for example garage-door openers or particular
> > bands like that. If you use those bands, then, yes, you're likely
> > okay.
>
> Alright. Where would I find a list of such frequencies, and how would
> I know what would make it legal to transmit at those frequencies
> (output power, antenna length)?

http://www.google.ca/search?hl=en&ie=ISO-8859-1&q=radio+frequency+chart+pdf&btnG=Google+Search&meta=

...some countries tend to follow the US but not necessarily.
fcc tends to look the other way for 100mW or less, unless you cause 
problems.

> > I'm not sure why you need freq stability.
>
> I haven't done anything wireless for a long time, but I do remember
> how I had to constantly adjust the trim cap (or stretch/compress the
> coil) to compensate for frequency shift due to differences in
> temperature. That's what I meant by frequency stability -- not having
> to tune the Tx/Rx modules every time I need to use them.

Makes sense, thanks.
One place I used to work used thermistors to aid for compensation, but 
that tended to be a custom solution. If this is a one-of-a-kind, you 
could probably go that way, but if you are building many, you might go 
the way of using a varactor and a temperature chart if building many.
Apply more/less voltage to a varactor to tune your LC. PIC holds the 
chart and could bleed in the varactor voltage thru a 1meg resistor.

> > AM is what I thought you meant from your original posting.
> > FM FSK PSK is much much better, so I'm glad you chose it.
>
> Yes, I meant "AM modulated carrier."

FSK would be preferable if you have to deal with long strings of 1s or 
0s, this way you tend to have 50%high/50%low. AM tends to drift to 
either high or low (depending on how you tuned your circuit). For 
example, say your circuit is usually 5v for nothing sent, then when you 
send a long string of 0s, you'll note your signal drifts back towards 
5v despite fact you continue to send 0s.  I noticed in Olin's reply 
(tag changed to EE he speaks of manchester encoding, but it's still a 
form of 50%high/50%low which is what you would want to do, but as this 
is a project under the fun-factor, choose what you like.
For what you want to do, you may find it simple to send FSK, maybe 
2400hz for a 0 and 4800hz for a 1. Watch for 0->1 or 1->0 crossings.

> > Well, it is the piclist, so it was a guess a pic was needed here,
> > but something in your circuit needs to deal with radio noise, be it
> > the black-box, the pic, or the terminal at the end of the 1200baud.
>
> I was surprised to find that there isn't much choice when it comes to
> 1200 baud modems. TCM3105 seems to be the easiest one to use,
> unfortunately the datasheet says "Full Duplex up to 1200 Tx and 150
> Rx"

I sort of expected that without bothering to look.
Something this "fine" you are unlikely to find as open source ideas and 
you are more likely going to have to roll your own solution or ask 
someone to code a black box for you.

> I was planning to use it with a circuit which automatically echoes
> back whatever it receives, so I need FD at 1200 baud both ways. It
> seems that my only choice at this point is to use XR2206 and XR2211.
>
> --- took a break to do more Googling ---
>
> What if I use an rfPIC, and a stand-alone RF receiver (e.g.,
> rfRXD0420)?

rfpic has got some good open source ideas in the pdfs you can follow. 
Looks like approx $3 in parts starting from your regen output (or 
standalone receiver) going to your rfpic 1200 baud output. The rfpic 
should be able to clean up the signal for you and present a nice 
1200baud solution for you to use, assuming the transmitter sends a 
decent packet of information (555555-data-checksum).
If you have your own AGC and it's cheap, you might try a seperate pic 
and your AGC circuit, that may bring you down to $1 for the pic and 
whatever your AGC circuit costs, maybe less than $2 altogether.
In standalone mode, you might want the rfpic to also send a carrier 
detect or RTS, or maybe better, a DTR, to your terminal.
DTR may be better, since if your receiving pic can't receive a signal, 
then there is a good chance you can't transmit the other way either 
until the radio path is okay for transmission again.

...just food for thought, and have fun with your project.
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