Questions about FSK

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I am building a multi-drop communication system that will communicate
over power lines (12V DC).
After some discussion and research, I've decided to use FSK.
The system will have 1 master and multiple slaves. I want it to be
full duplex (so the master can always be transmiting and 1 slave at a
time can be responding). The master will transmit at one frequency and
the slaves on another, I want to couple the outputs onto the +12V
line. I'm going to use center frequencies of ~100k & ~150k.
I'm going to use a CD4046 (74xx4046) PLL chip.

My question is what is the best way to overcome the loads on the power
line. The loads I have are the power source (which will most likely be
a battery), the voltage regulators (at each of the slaves), the FSK
receivers, and the FSK transmitters.
From some testing that I've done, it appears that the output impedance
from the PLL chip is about 1k. From the data sheets it looks like the
input resistance to the receiver is about 400k ohms, and the input
sensitivity is about 1/2 volt. (the input resistance goes down and the
sensitivity goes up when the supply voltage increases)

So far I have come up with the following solution (this is where I'm
open to suggestions).
I can isolate the battery with an inductor (not isolate, but increase
the impedance looking towards the battery) and I can do the same with
the voltage regulators. (but this has a downside if there is a quick
demand for more current, and I would need to use large inductors to
get any effective resistance).
The output of the VCO can be turned off, but I don't know what it's
impedance is (if it goes high it would help, but if it stays at 1k,
then I might need to add some other kind of buffer).

I would like to be able to have a lot of nodes on the line (maybe as
many as 99), so if anybody has any suggestions, please post.

thanks
-jeff

ps. - I've seen other companies that sell a multi-drop FSK modem that
can couple on to any lines (power, pbx, etc.) and they don't seem to
have any problems with loading, so there must be a way to do this.

Re: Questions about FSK


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Why reinventing the wheel and not just using x10 protocol, etc?

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There is no way to overcome the load of the power line. Power line
behaves close to the short circuit at 100...150kHz.


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In order to be operable, your system has to accept the input signals
with the level of 1mV or below. It is not a big problem to achieve that
with the analog filter and amplifier.


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That does not look like a good solution.


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Just increase the sensitivity of your receiver.


Vladimir Vassilevsky, Ph.D.

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Re: Questions about FSK

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That's pretty common.  There's a whack of a lot of industrial
process control stuff in the field that talks Bell-202 (1200
baud FSK) on top of DC supply lines.

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FSK on top of DC power is _very_ common.  There's way more of
it out there than X10.

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You use either a real inductor or a "simulated" inductor that's
built out of active components so that your power supply and
other devices are all high impedance at signalling frequencies.

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You never heard of a choke?

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Not sure what that has to do with anything.  The PLL is the
recieving half of it.

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You're going to need analog driver/receiver circuitry talored
to the type of wiring and the blocking impedance you choose.

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Why not?  It's done all the time in industrial gear.

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You put a blocking inductor in your power supply and in your
devices so that they all present a reasonably high impedence at
signalling frequencies.

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Wrong.


Ah.  That explains it.

--
Grant Edwards                   grante             Yow!  Who wants some
                                  at               OYSTERS with SEN-SEN an'
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Re: Questions about FSK


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Bell-202 implies signaling in the voice band.
Good luck fighting interferrence.    

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The power line is intended to deliver power. Nobody is going to do you a
favor of placing chokes everywhere.

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Because it increases the impedance.
How about a choke which can handle 100A current?

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Power line communication works indeed despite of what you said.

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You got a lot to learn, dear fellow.



Vladimir Vassilevsky, Ph.D.

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Re: Questions about FSK
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Like I said, there are hundreds of thousands of units out there
in chemical plants, refineries, sewage plants, breweries and
pharmiceutical plants that have been working fine for 10+
years.

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Yes they will.  You either design them into the power supply
and devices themselves, or you spec them in the system
schematic and make sure the electricians install them when the
devices are installed.

We're not talking about AC mains you know.  We're talking about
a two-wire 12VDC bus installed specifically for the devices in
question.

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What on earth are you talking about?  These devices typically
draw 10 or 20 mA, and there are rarely more than a dozen or so
on a line.  We're talking an Amp or two max at the power supply.

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I don't think you're talking about the topic at hand.

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Anybody who thinks he doesn't is a fool.

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Never trust anybody outside of academia who insists on putting
Ph.D. after his name...

--
Grant Edwards                   grante             Yow!  ANN JILLIAN'S HAIR
                                  at               makes LONI ANDERSON'S
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Re: Questions about FSK
Just to clarify things a bit.
This system is isolated from everything else. The master will have the
power source that will be used to power the rest of the system. Most
of the cable runs will be short (5-15 feet between slaves, entire run
probably not more than 100-200 feet. Occasionally maybe longer runs of
a few hundred feet).

I was planning on putting a choke between the source (battery) and
where the signal will be coupled onto the line. I will probably also
put a choke on the slave boxes before the voltage regulator (does
anybody know what the input impedance of a linear, say a 7805,
regulator is?)
I don't expect the current supplied for the entire system to be over a
couple of AMPs (each of the slaves will have an AVR microcontroller
and a few other chips).

Seems like most of the discussion is things I've already thought
about, I guess I should just break down and build a handfull of
receivers and make some measurements as I add more to the line.

-jeff

snipped-for-privacy@engineer.com (Jeff Hendrix) wrote in message
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Re: Questions about FSK
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The 8705 requires a cap on the input.
Nevertheless, you can add a series inductor before that.

If possible, have the signal feed symmetrical, meaning
a set of compensated chokes for isolation and a transformer
for coupling the sender.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net


Re: Questions about FSK


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You may want to take a look at the Exar XR2211. I used it for a similar
application (many years ago), it has a wide input range, and is pretty
cheap.

-Hershel

Re: Questions about FSK


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FSK (or other modulation) transceiver at the speeds of up to 2400 baud
can be easily implemented with a microcontroller like PIC or AVR with
minimum glue parts.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Re: Questions about FSK
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If the CPU clock is fast enough. If it's a low-power
application, the CPU may only be running at a few hundred KHz.

--
Grant Edwards                   grante             Yow!  I am a traffic light,
                                  at               and Alan Ginzberg kidnapped
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Re: Questions about FSK
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The other thing is that you mustn't try using the frequency-phase detector
(PD-II) in
the 4046 for this.  Any time you get a noise pulse that crosses the logic
threshold,
your PLL will lose lock and have to reacquire.  Multiplying phase detectors,
e.g. the
XOR PD-I in the 4046, are much better at rejecting noise.

Cheers,

Phil Hobbs

Re: Questions about FSK
Hi there,

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I have also used the 2211 chip in a FSK system and found it very good,
by good I mean a great balance of price/performance. I used it with
the XR2206 sine wave generator as the Tx part and a transformer to
step it up in amlitude a bit.

The system I used as the network was an electric fence, so I had
similar requirments to powerline modems. I was able to get up to 5km
distance between Tx and Rx on this lossy wire, not nice copper like
house wire, with this EXAR pair. The devices had to appear as high
impedance but still allow my communications signal in and out. The key
was the coupling circuit. I think you should investigate "resonant"
circuits. If you can't seperate, spectrally, your comms signal from
other high amplitude signals on the line, in order to filter them,
then you have to look at using a type of filter with really steep
rolloff.

My favorate powerline modem design is the motorola DSP one, here is a
link to the site which has the code and reference circuit.
http://e-www.motorola.com/webapp/sps/site/prod_summary.jsp?code=RDDSP56F8PLMOD&nodeId01%27953905

In the document DRM035.pdf there is a section on coupling to the power
line. I also think the ST app notes on this topic are not to bad.


good luck,
Tony McKay.

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