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'[EE] simple AM radio for data transmission'
2007\03\09@195054 by Andrei Yurkevich

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Hi,

I'm working on a simple RF data communication system that would allow  
one-way radio communication between two PICs, some kind of serial-
over-RF. I'm a total dummy when it comes to analog circuitry, so I  
definitely need some advice here...
Currently, I've got a 1MHz carrier generator built with a 74hc14 and  
a quartz xtal, that is AM-modulated with an output from PIC - logic  
high is full amplitude, logic low is something about 1/3, data rate  
is going to be about 100~200 bits/sec. I can perfectly hear this  
transmitter when I tune in with my stereo in another room :)
The question is what kind of a receiver should I use. Can someone  
point me to a suitable design that is easy to understand/implement in  
order to achieve this goal? Maybe some "classic" circuit that can be  
easily modified to receive such kind of data transmission? And yes,  
the receiver is going to be mobile so I need a solution that is  
rather compact and portable.

cheers,
Andrei

2007\03\10@021232 by Richard Prosser

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Look at super-regenerative receivers.
They can be a bit tricky to get going however. Also,  1MHz is a bit
low , you might want to head into the 10's of MHz region.

Therwise, for short range, you might be able to get a TRF receiver
working for simple on-off keying & using a highish speed opamp.

RP

On 10/03/07, Andrei Yurkevich <spam_OUTkervichTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

> -

2007\03\10@072847 by Rich

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Unless you have a very low data rate you might want to try around 100 MHz.
You can use a simple MPF102 circuit to transmit and Phillips and I think
National make receiver chips both AM and FM.  If you use AM you will need to
be concerned with ambient EM noise that would not affect FM.
{Original Message removed}

2007\03\10@131935 by Dr Skip

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If you're in the US, the FCC may take issue with willy-nilly frequency
choices. I've seen BIG fines for such, and they take the equipment.
Other countries probably have the same situation. There are bands
assigned for low power operation however. Your signal is square wave,
which means it has lots of harmonics, and at a relatively low frequency,
you'll need a large antenna to effectively radiate on that frequency.
You're probably transmitting from the loop itself on the board rather
than any wire you might have for an antenna, and are not directional or
in control of the harmonics. That means you could easily cause
interference to licensed services in many bands. I do know of a few of
those groups of licensees that regularly use sensitive receivers and
make it their business to track done and turn in unlicensed sources...

Pick a legal frequency as high as possible. Don't use a square wave
carrier. Shield the RF in its own box. Put a narrow filter on the
output. Create a directional antenna tuned for the frequency if
possible. Stay within the power limits the law allows for that
frequency. Look at other data modulation formats out there like PSK31 or
JT65. Simple AM serial type transmission won't work in real life - noise
from motors, effects of distance, interference from licensed sources on
the freq you choose (if you don't use the ones you should) would cause
all sorts of receive problems and probably require more time in ECC code
than would be spent in some other mode. Plus, there's the whole problem
of determining high and low at different distances (different signal
strengths). If set during receive, noise bursts would mess up the level
setting, and if static, transmitter power would have to go up as you
moved the receivers out.

It may be easier to just get one of the many transmitter/receiver
modules out there already FCC approved and in an appropriate band. They
usually take digital in and should handle the rest.

-Skip

Richard Prosser wrote:
{Quote hidden}

2007\03\11@210104 by David VanHorn

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>
> It may be easier to just get one of the many transmitter/receiver
> modules out there already FCC approved and in an appropriate band. They
> usually take digital in and should handle the rest.


There are a bunch of vendors out there, with single chip modules on 915 and
2.4 GHz,  Modular approvals under part 15, ready to go.

2007\03\11@222134 by Harold Hallikainen

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I have an application where I need an extremely cheap RF transceiver that
talks to a more expensive one. I'm thinking someone might make an RFID tag
with a serial port that could be connected to a PIC. An RFID reader would
have an EIA232 port on it that could be connected to a host computer. The
host would poll the tag which would respond (half duplex). Anyone know of
an RFID tag with a serial port?

THANKS!

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2007\03\11@234921 by Richard Prosser

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Harold,

On 12/03/07, Harold Hallikainen <haroldspamKILLspamhallikainen.org> wrote:
> I have an application where I need an extremely cheap RF transceiver that
> talks to a more expensive one. I'm thinking someone might make an RFID tag
> with a serial port that could be connected to a PIC. An RFID reader would
> have an EIA232 port on it that could be connected to a host computer. The
> host would poll the tag which would respond (half duplex). Anyone know of
> an RFID tag with a serial port?
>
> THANKS!
>
> Harold
>
>
> --
> FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
> opportunities available!
> -

2007\03\12@084748 by Gerhard Fiedler

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David VanHorn wrote:

> There are a bunch of vendors out there, with single chip modules on 915 and
> 2.4 GHz,  Modular approvals under part 15, ready to go.

I'm not sure what you mean with "modular approvals under part 15", but if
you mean an FCC number that allows to use the module in a product and sell
it (as long as you don't modify the module) -- I haven't found that many
(MaxStream is one, and another one slipped my mind right now).

Most module manufacturers I found claim that their modules are compliant,
but don't have their own number -- which seems to mean that they imply that
you can easily pass the required tests, but that you have to do them
anyway.

Or am I missing something?

Gerhard

2007\03\12@100211 by Jamesp

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All,

It seems to me, and correct me if I'm wrong, but it has always been my
understanding that in the FCC rules, it states that virtually anyone can
transmit on almost any frequency provided that....

1).  The input to the transmitter final RF Amp stage does not exceed 100
     milliwatt.

2).  The antenna used does not excced 1 meter in length


3).  No harmful interference is cause to FCC approved equipment.


And from the sound of the description of the application described here,
it would fall under this umbrella.  Unless the FCC rules have changed
regarding this, you should be okay.  Be aware that I am not giving legal
advice here.   Just an idea for thought and research from an old amateur
radio operator that seems to remember somethign to this effect.  It's up
to you to find the actual FCC regulation and interpret it.


                                           Regards,

                                             Jim  (KA9QHR)





{Quote hidden}

>> --

2007\03\12@120748 by Dr Skip

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Part 15 of the FCC rules, which this would fall under, specifies field
strength limits, measured at various distances, all which differ by
frequency. As of 1989, the 100mW limit is no longer applied. There are
strict limits on bandwidth in any case, and some bands are off limits
entirely, some can be used by educational institutions for experiments
with notification to the FCC, some for industrial usage, etc. This is an
intentional radiator, but there are also rules about unintentional
radiators as well, like from an unshielded oscillator, which can further
the headache.

And from the FCC OET bulletin on such things:

Furthermore, if the Commission determines
that the operator of such a transmitter has not attempted to ensure
compliance with the
Part 15 technical standards by employing good engineering practices then
that operator
may be fined up to $10,000 for each violation and $75,000 for a repeat
or continuing
violation.

Home built devices (up to 5 a year and not for sale) have to follow the
field strength rules, but don't have to be certified. Prototypes of
commercial devices and commercial ones (all but hobbyist-home ones) have
to be certified. So, under the more recent rules, you need to make sure
you use good engineering practices (shielding, filtering, etc), stay out
of the bands that are off limits, limit field strength in the band you
choose (and are allowed in) to the allowed amount, don't go outside your
band with harmonics, and don't cause interference. The 'good engineering
practice' part is the biggest shield to the home builder (and a contrite
attitude when caught).

Wasn't there a post that said all that? ;)

Hack something together in poor form, do no calculations or measurements
on field strength, cause interference - these are good ways to lose
money, even if you've only got 1 mW! Show you used the right practices
(know what they are too), that you made every effort to limit radiation
(per the restrictions) and field strength, that you were below allowed
amounts, and shut it down when someone else hears you and you'll get a
warning letter instead. The whole idea is no one should hear you...

-Skip

EraseMEjamespspam_OUTspamTakeThisOuTintertex.net wrote:
{Quote hidden}

2007\03\12@122343 by PAUL James

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Dr. skip,

Thanks for the lesson in updated FCC rules.  I was aware of the rules
change for part 97 dealing with amateur
Radio, but wasn't aware that other parts had changed as well.   Maybe I
should have been more aware of this,
But I have my hands full with the changes in the Amateur service.  I
will look into this further so I better understand these changes.  It
can't hurt, right.

And to all, what is said below about the 100mw limit.....never mind.



       
Thanks and Regards,

       
Jim

{Original Message removed}

2007\03\12@123337 by David VanHorn

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On 3/12/07, Gerhard Fiedler <listsspamspam_OUTconnectionbrazil.com> wrote:
>
> David VanHorn wrote:
>
> > There are a bunch of vendors out there, with single chip modules on 915
> and
> > 2.4 GHz,  Modular approvals under part 15, ready to go.
>
> I'm not sure what you mean with "modular approvals under part 15", but if
> you mean an FCC number that allows to use the module in a product and sell
> it (as long as you don't modify the module) -- I haven't found that many
> (MaxStream is one, and another one slipped my mind right now).


RF digital products are approved this way..
Standard disclaimers apply, you might wonder why they are full of AVRs. :)

2007\03\12@145651 by Harold Hallikainen

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That 100mW limit applies only to the AM broadcast band. You can see
today's version of part 15 at http://www.hallikainen.com/FccRules/2007/15/

Harold


{Quote hidden}

> {Original Message removed}

2007\03\13@082124 by Gerhard Fiedler

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David VanHorn wrote:

>>> There are a bunch of vendors out there, with single chip modules on 915
>>> and 2.4 GHz,  Modular approvals under part 15, ready to go.
>>
>> I'm not sure what you mean with "modular approvals under part 15", but
>> if you mean an FCC number that allows to use the module in a product
>> and sell it (as long as you don't modify the module) -- I haven't found
>> that many (MaxStream is one, and another one slipped my mind right
>> now).
>
> RF digital products are approved this way..

Right... they are the other one I couldn't remember. That makes only two
vendors of reasonably small modules that have their own FCC number.

Gerhard

2007\03\13@111351 by David VanHorn

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>
> Right... they are the other one I couldn't remember. That makes only two
> vendors of reasonably small modules that have their own FCC number.


I'm pretty sure Linx (lynx?) also does.

2007\03\13@181001 by Gerhard Fiedler

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David VanHorn wrote:

>> Right... they are the other one I couldn't remember. That makes only two
>> vendors of reasonably small modules that have their own FCC number.
>
> I'm pretty sure Linx (lynx?) also does.

Linx Technologies.

Yes and no. They have their line of FCC-certified "OEM Products"
<www.linxtechnologies.com/index.php?section=products&category=oem_products>,
but these are not modules for integration into PCBs, they are complete
keyfobs and other "big" stuff in cases.

The nice small "RF Modules" they sell
<www.linxtechnologies.com/index.php?section=products&category=rf_modules>
are AFAIK not FCC-certified.

I imagine this is because the modules don't come with a built-in antenna.
Adding an antenna would invalidate any FCC certification anyway, right?

Gerhard

2007\03\13@183032 by David VanHorn

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>
>
> I imagine this is because the modules don't come with a built-in antenna.
> Adding an antenna would invalidate any FCC certification anyway, right?


Yup.

Hmm.. Well RFD has them with internal antennas, and certified.
Somehow I thought the market was more crowded than that.

I buy my RF connectors there too, a separate site:
http://rfconnectors.rfdigital.com/

I just finished a major antenna project, nine feedlines up the tower.
One FSJ1-50, two half inch hardline, and six LMR-400.
The ones that go above the rotor have flex sections of  9913F7.
N feedthrus at the window, and N to whatever jumpers inside from the spare
9913F7 and FSJ1-50 as appropriate.
N connectors on everything unless the antenna end needed PLs, and all the
connectors came from that site.

I was rather surprised at how the connectors multiply :)
For the lines above the rotor, that's six connectors per line, plus two
barrels.
The other ones have "only" four, and a single barrel.

I used all silver-teflon connectors, and they are all nicely weatherproofed.

It's nice to finally be hot on 20M through 915 MHz.
(plus a 2.4 dish,  but that's wifi.)

2007\03\13@195955 by Tech

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> Linx Technologies.
>
> Yes and no. They have their line of FCC-certified "OEM Products"
> <www.linxtechnologies.com/index.php?section=products&category=oem_products>,
> but these are not modules for integration into PCBs, they are complete
> keyfobs and other "big" stuff in cases.

Linx used to have a 900MHz SC-PA module with built-in RPSMA antenna connector
that was pre FCC certified, but they dropped that a while back. All of their pre certified
products now are OEM like keychains, hand-helds, function relay boxes, etc,,...

> The nice small "RF Modules" they sell
> <www.linxtechnologies.com/index.php?section=products&category=rf_modules>
> are AFAIK not FCC-certified.

Yep. They are not pre certified.

> I imagine this is because the modules don't come with a built-in antenna.
> Adding an antenna would invalidate any FCC certification anyway, right?

Most pre certified gear requires you to use the antenna it was originally certified
with to stay within spec.

Linx RF modules like the LR, ES, KH, HP3, etc, are not pre certified, but I believe
http://www.celectronics.com/ still offers developers a discount on the certification
process, if you're using Linx parts. They're an accredited TCB so they can test &
certify a product for you -- if you decide to go that route.

Linx has some outstanding products, and the LR series are hard to beat.

Regards,

Bruce




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