Help with cleaning up noisy 10 bps communication

>>>> then there are frequencies up to 10MHz or more that matter. 
>>Erm, no. That would be a million times the baud rate.
>>
>>As a rule of thumb, fit two bits into one cycle (look at the fundamental
>>component of a 010101... sequence). Analog Telefax is the classical
>>textbook example. Some excess bandwidth is OK, a factor of 10 maybe.
>>
>>Bandwidth must be limited, otherwise the cable becomes the antenna of a
>>long range thunderstorm detector.
>>	 
>That is something we were just talking about, limiting the rise and fall
>time of the signal (at the driver), which is not being done now.  This
>would reduce the ringing I believe.  But we are very reluctant to add
>active components because of reliability issues.  We are working at high
>temps.
>
>The cable is underground, vertical, to a well motor, so I don't believe
it
>can act as an antenna.
>Ron	 

It sounds like you don't have proper termination at the reciever and are
fighting transmission line effects of your long cable. Seems like there
should be a simple solution to that......	 

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On Wed, 13 Aug 2014 08:39:34 -0500, BasicPoke wrote:

>>>> then there are frequencies up to 10MHz or more that matter. 
>>Erm, no. That would be a million times the baud rate.
>>
>>As a rule of thumb, fit two bits into one cycle (look at the fundamental
>>component of a 010101... sequence). Analog Telefax is the classical
>>textbook example. Some excess bandwidth is OK, a factor of 10 maybe.
>>
>>Bandwidth must be limited, otherwise the cable becomes the antenna of a
>>long range thunderstorm detector.
>>	 
> That is something we were just talking about, limiting the rise and fall
> time of the signal (at the driver), which is not being done now.  This
> would reduce the ringing I believe.  But we are very reluctant to add
> active components because of reliability issues.  We are working at high
> temps.
> 
> The cable is underground, vertical, to a well motor, so I don't believe it
> can act as an antenna.
> Ron	 
> 
> _____________________________		
> Posted through www.DSPRelated.com

This "ringing" is at such a low frequency that I suspect that you have some
reactive elements other than just the connecting cable.  Could the motor
somehow be part of the circuit?  Is your signaling over differential lines
(which might help guard against various problems), or single-ended WRT some
'ground' that might actually be different on receiving and transmitting ends?

You should be able to control risetimes with a simple passive circuit.  While
you haven't AFAICR said how long your line is, using differential signalling
would help deal with many potential problems.

>This "ringing" is at such a low frequency that I suspect that you have
some
>reactive elements other than just the connecting cable.  Could the motor
>somehow be part of the circuit?  Is your signaling over differential
lines
>(which might help guard against various problems), or single-ended WRT
some
>'ground' that might actually be different on receiving and transmitting
ends?
>
>You should be able to control risetimes with a simple passive circuit. 
While
>you haven't AFAICR said how long your line is, using differential
signalling
>would help deal with many potential problems.
>
>
The 3-phase motor windings are part of the circuit, in series with 10k feet
of the motor's power cable.  The signal passes over the power cable.  There
are other reactive components.  It is not a simple things to terminate the
cable apparently.  We do not know the type of motor that will be installed,
the type of cable, or the length of cable.  It is not an option to use a
different type of cable to optimize communications.  The primary purpose of
the cable is to power the motor.

I have used an active low pass filter, using an op amp, and got a much
cleaner signal, so I think that will have to be part of my solution.  But
we have yet to see how this filter will work with the motor running, which
we will do today.  This may be ugly.
Ron	 

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On 13/08/14 17:35, BasicPoke wrote:
>> This "ringing" is at such a low frequency that I suspect that you have
> some
>> reactive elements other than just the connecting cable.  Could the motor
>> somehow be part of the circuit?  Is your signaling over differential
> lines
>> (which might help guard against various problems), or single-ended WRT
> some
>> 'ground' that might actually be different on receiving and transmitting
> ends?
>>
>> You should be able to control risetimes with a simple passive circuit.
> While
>> you haven't AFAICR said how long your line is, using differential
> signalling
>> would help deal with many potential problems.
>>
>>
> The 3-phase motor windings are part of the circuit, in series with 10k feet
> of the motor's power cable.  The signal passes over the power cable.  There
> are other reactive components.  It is not a simple things to terminate the
> cable apparently.  We do not know the type of motor that will be installed,
> the type of cable, or the length of cable.  It is not an option to use a
> different type of cable to optimize communications.  The primary purpose of
> the cable is to power the motor.

General rule of thumb: if you can "optimise" do so, but if you can't
then "randomise".

Classic reasons you might not be able to optimise are that parameters
are unknown and/or changing.

Randomise, in this case, implies spread spectrum, either
slow frequency hopping or, arguably more likely, direct sequence.

Hi,

the driver bandwidth is not the problem here, fortunately. Bandwidth is
largely a receiver design decision.

Try the CIC filter I lined out earlier. It's simple to implement and seems
like the most straightforward solution to limit the bandwidth. The
(digital) filter will simply reject high frequency components, which is
here as good as not creating them in the first place.

If., properly filtered, the ringing causes intolerable intersymbol
interference, an equalizer would seem the next logical step, as was
mentioned. From a practical perspective, this is quite a step up in
complexity, though.

	 

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On Wed, 13 Aug 2014 11:35:49 -0500, "BasicPoke" <101269@dsprelated>
wrote:

>>This "ringing" is at such a low frequency that I suspect that you have
>some
>>reactive elements other than just the connecting cable.  Could the motor
>>somehow be part of the circuit?  Is your signaling over differential
>lines
>>(which might help guard against various problems), or single-ended WRT
>some
>>'ground' that might actually be different on receiving and transmitting
>ends?
>>
>>You should be able to control risetimes with a simple passive circuit. 
>While
>>you haven't AFAICR said how long your line is, using differential
>signalling
>>would help deal with many potential problems.
>>
>>
>The 3-phase motor windings are part of the circuit, in series with 10k feet
>of the motor's power cable.  The signal passes over the power cable.  There
>are other reactive components.  It is not a simple things to terminate the
>cable apparently.  We do not know the type of motor that will be installed,
>the type of cable, or the length of cable.  It is not an option to use a
>different type of cable to optimize communications.  The primary purpose of
>the cable is to power the motor.
>
>I have used an active low pass filter, using an op amp, and got a much
>cleaner signal, so I think that will have to be part of my solution.  But
>we have yet to see how this filter will work with the motor running, which
>we will do today.  This may be ugly.
>Ron	 

If it were me, I'd find a way to characterise the channel with the
motor off and with the motor running.   You may be able to do this
with your modulator, or with a sweep circuit or something substituted
for the modulator.    That will give you a much more full picture of
what you're up against.

Are you transmitting the signal at baseband?   If so, and the
reactance of the cable/motor circuit is distorting the signal, it may
be possible to move the signal spectrum to a higher frequency and get
a much cleaner channel.   Again, this is the sort of thing you can
discover with channel characterization.   There may be a sweet spot
within the frequency range of your hardware that could significantly
improve the reliability of the link.    The good news for you is that
your signal spectrum is pretty narrow, so you don't need a very big
chunk of clean spectrum.

Characterizing the channel will also give you a good idea whether
equalization will be needed and how much.  Since your symbol rate is
so low, I'm now a bit skeptical as to whether an equalizer would do
you any good, but it is the sort of thing that becomes more clear once
the channel characteristics are known.

Also, characterizing the interference environment by turning the
modulator off and just listening to the noise, with the motor on and
off, will also help isolate portions of the available spectrum that
may be happier than others.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

On Wed, 13 Aug 2014 11:35:49 -0500, BasicPoke wrote:

>>This "ringing" is at such a low frequency that I suspect that you have
> some
>>reactive elements other than just the connecting cable.  Could the motor
>>somehow be part of the circuit?  Is your signaling over differential
> lines
>>(which might help guard against various problems), or single-ended WRT
> some
>>'ground' that might actually be different on receiving and transmitting
> ends?
>>
>>You should be able to control risetimes with a simple passive circuit.
> While
>>you haven't AFAICR said how long your line is, using differential
> signalling
>>would help deal with many potential problems.
>>
>>
> The 3-phase motor windings are part of the circuit, in series with 10k
> feet of the motor's power cable.  The signal passes over the power
> cable.  There are other reactive components.  It is not a simple things
> to terminate the cable apparently.  We do not know the type of motor
> that will be installed,
> the type of cable, or the length of cable.  It is not an option to use a
> different type of cable to optimize communications.  The primary purpose
> of the cable is to power the motor.
> 
> I have used an active low pass filter, using an op amp, and got a much
> cleaner signal, so I think that will have to be part of my solution. 
> But we have yet to see how this filter will work with the motor running,
> which we will do today.  This may be ugly.
> Ron

You must be modulating the signal onto some sort of a carrier.  I think 
we've all been assuming a separate pair of wires for the signal.

Details, please, unless you have NDA constraints.  Your problem just got 
hugely more complicated.

On the bright side, as long as your carrier frequency is well above line 
frequency I suspect that the motor won't change the line characteristics 
whether it's running or not.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

BasicPoke wrote:
>[*SNIP*]
>>
>>
> The 3-phase motor windings are part of the circuit, in series with 10k feet
> of the motor's power cable.  The signal passes over the power cable.  There
> are other reactive components.  ...

You *HAVE* problems!
You need someone competent dealing with low level signals in 
presence of interference orders of magnitude larger.

I have experience as an engineering tech.
I once was fired for refusing to pretend to be an engineer.
[On a subsequent job I got a positive review for same stance ;]
I'm past 3 score and 10, so I'm safe ;)

See thread on "good questions".

On Wed, 13 Aug 2014 19:23:50 -0500, Richard Owlett wrote:

> BasicPoke wrote:
>>[*SNIP*]
>>>
>>>
>> The 3-phase motor windings are part of the circuit, in series with 10k
>> feet of the motor's power cable.  The signal passes over the power
>> cable.  There are other reactive components.  ...
> 
> You *HAVE* problems!
> You need someone competent dealing with low level signals in presence of
> interference orders of magnitude larger.
> 
> I have experience as an engineering tech.
> I once was fired for refusing to pretend to be an engineer. [On a
> subsequent job I got a positive review for same stance ;]
> I'm past 3 score and 10, so I'm safe ;)
> 
> See thread on "good questions".

Line carrier communications have worked since the 1940's or earlier 
(apparently it's what some frustrated amateur radio operators did during 
WW-II -- there's a chapter on it in one of my wartime ARRL Handbooks).

AFAIK it's mostly done by choosing a frequency that's high enough so that 
the line frequency stuff is easy to filter out, and low enough that it's 
not horribly attenuated by the wires.  And then use a really high-power 
amp on the transmit side!

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On Wed, 13 Aug 2014 19:58:02 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Wed, 13 Aug 2014 19:23:50 -0500, Richard Owlett wrote:
>
>> BasicPoke wrote:
>>>[*SNIP*]
>>>>
>>>>
>>> The 3-phase motor windings are part of the circuit, in series with 10k
>>> feet of the motor's power cable.  The signal passes over the power
>>> cable.  There are other reactive components.  ...
>> 
>> You *HAVE* problems!
>> You need someone competent dealing with low level signals in presence of
>> interference orders of magnitude larger.
>> 
>> I have experience as an engineering tech.
>> I once was fired for refusing to pretend to be an engineer. [On a
>> subsequent job I got a positive review for same stance ;]
>> I'm past 3 score and 10, so I'm safe ;)
>> 
>> See thread on "good questions".
>
>Line carrier communications have worked since the 1940's or earlier 
>(apparently it's what some frustrated amateur radio operators did during 
>WW-II -- there's a chapter on it in one of my wartime ARRL Handbooks).
>
>AFAIK it's mostly done by choosing a frequency that's high enough so that 
>the line frequency stuff is easy to filter out, and low enough that it's 
>not horribly attenuated by the wires.  And then use a really high-power 
>amp on the transmit side!

I think he really needs to do a decent channel characterization.   He
has some advantages in that there should not be much rf interference
other than what their own system is generating, and his carrier is
very small spectrally.   So the chances are reasonable that there is,
or can be made to be, a suitable chunk of spectrum on the line within
their usable system bandwidth so that the signal survives reliably. 

Don't know how much info he can share, but I'd be interested in more
details about the architecture.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com