From:
http://www.qrpis.org/~k3ng/bpl.html
-----------

Q: What is intermodulation?

A: Intermodulation is the mixing of radio signals which produces new radio signals. Think of
it as radio waves having children. But just how do radio waves have children ?

This mixing is caused by what are called non-linearities. One non-linear electronic component
that you find in most any electronic device is a diode. When multiple radio signals are run
through the diode, they mix together. Let's say we have a 4 Mhz signal and a 6 Mhz signal
going into the diode. We would then get:

4 + 6 = 10 Mhz

6 ? 4 = 2 Mhz

4 and 6 Mhz had two ?children?, 2 and 10 Mhz !

Now, non-linearities are usually good. This phenomenon is used in just about every radio
device to either create a signal to be transmitted, or receive a signal that you hear or see.
But, non-linearities can occur where you don't want them and then in causes problems. One such
place is in power lines. Bad, corroded connections or dissimilar metals touching can create
natural diodes that act like mixers and produce this intermodulation.

So, let's take a BPL signal and for the sake of discussion, say it's a grossly simplified
consisting of radio signals at 1, 5, 8, 9, and 12 Mhz. Some of the intermodulation products
that could be created would be:

1 + 5 = 6 Mhz

8 + 9 = 17 Mhz

9 + 12 = 21 Mhz

12 ? 9 = 3 Mhz

But you could also have what is known as third order products:

1 + 9 + 12 = 22 Mhz

8 + 9 + 12 = 29 Mhz

8 ? 5 + 12 = 15 Mhz

Or even:

2 * 12 = 24 Mhz

(9 ? 5) * 12 = 48 Mhz

You can do the math and figure out each permutation, but you get the idea. If we took a real
BPL signal that has signals from 1 ? 80 Mhz the number of products and where they would fall
are mind-boggling. The resulting intermodulation products in a system could extend well above
the band BPL proponents want, falling into FM broadcast, VHF TV, Aeronautical, and more public
safety bands. This is just another reason why BPL is so problematic.

It's arguable that such non-linearities in power lines are exhibited as arcing connections,
something that most power companies are actively searching for these days as the RFI (radio
frequency interference) effects are well understood. These maintenance issues will be
addressed quickly by well run utilities. However, non-linear loads are common in homes, light
dimmers being the first devices that come to mind. Theoretically, these devices could create
intermodulation that would in turn be radiated by the house wiring and outside power cabling.

"Steve Stone" wrote in message
et...

From:
http://www.qrpis.org/~k3ng/bpl.html
-----------

Q: What is intermodulation?

A: Intermodulation is the mixing of radio signals which produces new
radio signals. Think of
it as radio waves having children. But just how do radio waves have
children ?


[snip]

Reality trumps theory. BPL is being tested. If BPL was making much in
the way of harmonics or IMD products, the low end of the BPL spectrum
will be interfering with the high end of the BPL spectrum. The high end
of the BPL spectrum would be interfering with TV and FM radio.

BPL's fundamentals are the confirmed problem for the radio hobbyist. Ed
Hare and other radio amateurs have done alot of work documenting the
interference levels on SW radio.

Frank Dresser

It's hard to generalize about all digital communication. I think
BPL is some kind of phase modulated OFDM as Frank says, so in that
case you could use essentially rectangular pulses (in practice there
is probably some roll-off and guard time to boot). Each individual
tone would actually occupy a bandwidth much greater than its keying
rate, but since each tone's keying rate is so low compared to the
total bandwidth, the net effect is minor, again exactly as Frank
says.

For single carrier high date rate systems however, the last thing you
want to use is rectangular pulses. The spectrum won't have discrete
harmonics but it will look like (sin(x)/x)^2 in frequency with
significant energy beyond the Nyquist frequency. In those applications a
waveform that falls off in time as t^2 is generally used, though there
are other options, like minimum-shift keying, which can be looked at
either as continuous phase FSK or QPSK using smooth shaped pulses.
Continuous phase modulation has some complications though.

Oz


Brenda Ann wrote:
"Frank Dresser" wrote in message
...


A square wave, itself, won't convey much information. It needs to be
modulated, and the modulation would have to effect the symmetry and
result in both odd and even harmonics.

I don't know what sort of modulation BPL is using. I can imagine
hundreds of low amplitude sine wave carriers from 2 to 60 Mhz, all of
them phase modulated. In that case, I don't think there would be much
harmonic output.


Digital comms are purely square waves. The modulation is FSK or similar
(generally)... in other words, the on-state is one frequency, the off state
is another. This creates a chain of square waves which themselves are not
modulated. The bandwidth, in this case 75 MHz, is how many on/off states
there are in one second. This is also concurrent with bitrate. Compression
schemes can raise the apparent bitrate, however the actual bitrate is the
same as the frequency used. I'm not sure how they do the band notching that
Japan tried before they tossed out the idea completely.

In article ,
"Frank Dresser" wrote:

"Telamon" wrote in message
...


Data communications occupy wider bandwidths than the stated clock
rate.
It is not unreasonable to expect harmonics 3 to 5 times the clock rate
because the signaling uses square waves and there is significant power
in the odd harmonics.

--
Telamon
Ventura, California

A square wave, itself, won't convey much information. It needs to be
modulated, and the modulation would have to effect the symmetry and
result in both odd and even harmonics.

I don't know what sort of modulation BPL is using. I can imagine
hundreds of low amplitude sine wave carriers from 2 to 60 Mhz, all of
them phase modulated. In that case, I don't think there would be much
harmonic output. This would certainly still be a big problem for the
radio hobbyist, but not so much for the FM/TV user. There have been
several BPL tests in various communities, and it doesn't seem to have
wiped out normal broadcast use.

If BPL caused enough bothersome interference to keep people in the test
communities from their TVs and radios, the National Association of
Broadcasters would have squashed it like a bug.


It is a common error to assume that digital communications are similar
to analog RF. One reason is very fast edge times are required to create
the most eye margin possible at the decoding end of a data stream so the
bandwidth required is much greater. A good rule of thumb is 3.7 times
the clock rate as a minimum. Usually the engineering shoots for the
fastest edge times practical.

An one/zero pattern and multiples thereof are square waves but I should
not have used that term because it looks like I just threw you off the
path of understanding.

--
Telamon
Ventura, California

In article ,
Larry Ozarow wrote:

It's hard to generalize about all digital communication. I think
BPL is some kind of phase modulated OFDM as Frank says, so in that
case you could use essentially rectangular pulses (in practice there
is probably some roll-off and guard time to boot). Each individual
tone would actually occupy a bandwidth much greater than its keying
rate, but since each tone's keying rate is so low compared to the
total bandwidth, the net effect is minor, again exactly as Frank
says.

For single carrier high date rate systems however, the last thing you
want to use is rectangular pulses. The spectrum won't have discrete
harmonics but it will look like (sin(x)/x)^2 in frequency with
significant energy beyond the Nyquist frequency. In those applications a
waveform that falls off in time as t^2 is generally used, though there
are other options, like minimum-shift keying, which can be looked at
either as continuous phase FSK or QPSK using smooth shaped pulses.
Continuous phase modulation has some complications though.

I haven't read how BPL is supposed to work but is it reasonable to
expect that a encoding scheme would be used that would shift the
spectrum requirements downward so that increased coupling would be
needed across the transformers in the power system?

--
Telamon
Ventura, California

"Telamon" wrote in message
...

It is a common error to assume that digital communications are similar
to analog RF. One reason is very fast edge times are required to
create
the most eye margin possible at the decoding end of a data stream so
the
bandwidth required is much greater. A good rule of thumb is 3.7 times
the clock rate as a minimum. Usually the engineering shoots for the
fastest edge times practical.

Well, I thought we started out with harmonics of the BPL carriers. I
don't see why there would have to be BPL harmonics due to the digital
modulation anymore than a RTTY transmission would have to have
harmonics.

If you're saying the sidebands of carriers will be spaced as far as 3.7
times as far as the data clock rate, sure, why not? I don't know any of
the specifics. But if the BPL carriers stop at 80 Mhz, I suppose the
total spectrum won't go much past 80 Mhz..



An one/zero pattern and multiples thereof are square waves but I
should
not have used that term because it looks like I just threw you off the
path of understanding.

--
Telamon
Ventura, California

That's the rocky path of understanding, for ya. Can't even take take my
shoes off when I need to count past 10.

Frank Dresser

In article
,
"Frank Dresser" wrote:

"Telamon" wrote in message
...

It is a common error to assume that digital communications are similar
to analog RF. One reason is very fast edge times are required to
create
the most eye margin possible at the decoding end of a data stream so
the
bandwidth required is much greater. A good rule of thumb is 3.7 times
the clock rate as a minimum. Usually the engineering shoots for the
fastest edge times practical.

Well, I thought we started out with harmonics of the BPL carriers. I
don't see why there would have to be BPL harmonics due to the digital
modulation anymore than a RTTY transmission would have to have
harmonics.

If you're saying the sidebands of carriers will be spaced as far as 3.7
times as far as the data clock rate, sure, why not? I don't know any of
the specifics. But if the BPL carriers stop at 80 Mhz, I suppose the
total spectrum won't go much past 80 Mhz..



An one/zero pattern and multiples thereof are square waves but I
should
not have used that term because it looks like I just threw you off the
path of understanding.

--
Telamon
Ventura, California

That's the rocky path of understanding, for ya. Can't even take take my
shoes off when I need to count past 10.

No it just that since the data is sent without a clock the data stream
regardless of the encoding need fast and precise (low jitter) edge
times. Faster edges provide more timing margin. Fast edges have most of
the energy in the odd harmonics 1, 3, 5, 7 etc. Most of the energy is in
the lowest odd harmonics 1, 3 and 5 being the most important. This
explanation only makes sense for a single carrier two level scheme. I do
not know what BPL employs but I expect a high frequency scheme be used
to reduce the coupling requirements across transformers in the power
system.

--
Telamon
Ventura, California

"Telamon" wrote in message
...

No it just that since the data is sent without a clock the data stream
regardless of the encoding need fast and precise (low jitter) edge
times. Faster edges provide more timing margin. Fast edges have most
of
the energy in the odd harmonics 1, 3, 5, 7 etc. Most of the energy is
in
the lowest odd harmonics 1, 3 and 5 being the most important. This
explanation only makes sense for a single carrier two level scheme.

Let's say one of the BPL carriers is at 10 Mhz. Let's say it's
modulated at 10 khz. If you're saying the modulation is making a
channel which covers something like 9.960 Mhz to 10.040 Mhz, that sounds
OK to me.

If you're saying the modulation creates harmonics at 20, 30, 40 Mhz, I
can't see how.


I do
not know what BPL employs but I expect a high frequency scheme be used
to reduce the coupling requirements across transformers in the power
system.

--
Telamon
Ventura, California

Power pole transformers should have a nice grounded copper
electrostactic shield between the primary and secondary windings. This
reduces capacitive coupling between the windings to almost zero. The
BPL company will have to bypass the shield with some sort of bandpass
coupling. I suppose something as simple as a capacitor would do the
job, but they probably have something more elaborate.

Maybe they're using a small ferrite transformer with enough insulation
to withstand the full primary voltage. Bypassing the power
transformer's internal shield would be a lightning hazard.

Frank Dresser

Unfortunately there's no readily available description of
how BPL works, so none of us can really claim he knows exactly
what he's talking about. My general understanding is that
BPL uses a set of spaced carriers, each modulated at some
reasonably low data rate. These carriers are arrayed, I think,
over a range like 2MHz to 75 Mz or maybe 2 - 40 or something
of that order of magnitude.

The transmitted signal is not a pure digital baseband signal -
anytime any wideband signal has to propagate over any appreciable
distance this can't be used because the medium be it wire or "ether"
has a response which varies in frequency, and is not flat enough
over the passband of the entire signal bandwidth. There are two
broad classes of approach to combatting this (very oversimplified).
A single carrier approach takes the baseband signal and uses
it to modulate a carrier. This moves the spectrum away from DC
which most media don't like (including power lines as you note in the
paragraph I've quoted below). The resulting signal would have a
natural bandwidth of a modest multiple of the symbol rate
as you have noted in another post. Even this is usually too wide
in most applications, so the baseband pulses are shaped in time
to concentrate them (around DC at baseband, but around the carrier
when using the signal to modulate one). This pulse shaping causes the
pulses to have very long time durations compared to the signalling
interval, but a matched filter is used at the receiver, to
re-concentrate the energy temporally.

The other broad approach is to use a bunch of evenly spaced RF
carriers (as Frank has suggested, and is what I think they actually
do). The tones are spaced at frequency intervals of 1/(signalling
interval), and the tradeoff between length and frequency spacing is
a design decision depending on medium - it does not affect the overall
data rate. You can key twice as fast but then carry only half as many
tones in a given bandwidth. This is OFDM. In this case, since the
signalling rate of each tone is only a small fraction of the total
bandwidth the fact that the effective bandwidth of a rectangular pulse
is 3 or 5 or whatever times the signalling frequency doesn't effect the
total bandwidth by much since the total bandwidth is that of hundreds or
thousands of tones. Again, like the case of single carrier, since each
tone modulates an RF carrier, there is no LF energy in the resulting
signal. Now the medium is roughly flat over the effective BW of each
individual tone, and all is basically well.

Lastly there was a confusion in the thread between harmonics and excess
bandwidth. A randomly modulated square wave does not have discrete
harmonics, because the modulation eliminates the periodicity. It does
have excess BW as per your discussion of the need for sharp edges.
Again in practice the pulses used are not rectangular so the spectrum
does not have the ideal (sin(x)/x)^2 roll-off, but something that falls
off much more quickly, but there is still significant energy beyond
the Nyquist frequency.

Oz



Telamon wrote:


I haven't read how BPL is supposed to work but is it reasonable to
expect that a encoding scheme would be used that would shift the
spectrum requirements downward so that increased coupling would be
needed across the transformers in the power system?

"Frank Dresser" wrote in message
...

Bypassing the power
transformer's internal shield would be a lightning hazard.


I meant to say that bypassing the internal shield in a simple way could
be a lightning hazard. I suppose some kind of opto-isolator would work
well.

Frank Dresser