Crude Powerline FSK comms literally shorts the power bus at a certain frequency?

C

cupoftea

Advanced Member level 6
Advanced Member level 6
Joined
Jun 13, 2021
Messages
3,282
Helped
65
Reputation
132
Reaction score
150
Trophy points
63
Activity points
17,024
Hi,
Is there such thing as a powerline FSK comms system where multiple points along a long cable (bus) are
supplied power by the cable, but also the cable is literally shorted/opened at a certain frequency so as to give an
FSK powerline comms system? All the load circuitry doesn't get affected by the bus getting shorted because the loads
are supplied by capacitor banks which are downstream of high resistances connected to the bus so that the capacitor bank is
effectively "isolated" from the bus.
Obviously there just needs to be some initiation time so that the capacitors can get charged up at power up time.
 

Sort by date Sort by votes
Powerline comms are short haul from what I've seen.
You throw C-blocked HF carrier onto the (effectively)
long-wire-L-blocked 50/60Hz line and you pick it
off C-blocked, elsewhere (before it's attenuated too
badly by L / distance).

So "AC short" (but at an AC frequency well
out-of-band). Since a useful modern data rate
is > MBPS carrier will be way up there, C will be
small, "short" will escape notice (just more
small signal noise, picocoulomb blips)
 
Upvote 0 Downvote
Thanks, i see what you mean , you are thinking of coupling the signal to the powerline via a blocking capacitor. I have seen that before too. Also, AYK, you get transformer coupling. However, the schem i have in fornt of me has none of that...it just has a BCX56 SOT89 NPN shorting the rail.....(24v rail)...well, its not shorting as its got a 75R in series with it.
But i guess due to the powerline L, that will give a bit of a dip in the rail which will be detected by the detector?
Have you seen that type of FSK system before?

I cannot see any blocking cap or signal transformer here.
The schem i have is on multiple pages with cross references going off all over the place but i deffo dont see any C or Transformer.
 

Upvote 0 Downvote
So this is DC power line I guess. Same basic idea only now you can have as big a DC choke as you please (properly situated outside TX / RX link).
 
Upvote 0 Downvote
Thanks, It seems odd to do it our way with FSK?....i would have thought just ON/OFF keying would be the one for the way we are doing it?
What frequency would you say (highest) is possible with our way?
 

Upvote 0 Downvote
Well a frequency detector would be one way
to trade bandwidth for spurious rejection (a
single pulse might be indistinguishable from
other line-crud)?
 

Upvote 0 Downvote
cupoftea said:
However, the schem i have in fornt of me has none of that...it just has a BCX56 SOT89 NPN shorting the rail.....(24v rail)...well, its not shorting as its got a 75R in series with it.
Click to expand...
Sounds like you are not talking about a powerline modem but a special bus powered communication interface, most likely base band rather than modulated, e.g. MBUS or similar. Bus power is current limited, and pulling down the voltage by a certain delta or even short it temporarily is intended operation.
 
Upvote 0 Downvote
Thanks , whereas with DALI comms you have a current clamped comms bus, and you literally pull it to ground with a transistor to do the signalling....i think the one we want to use just kind of puts some small dV ripple on the bus, which can be read somehow. The ripple is the bits so to speak. And since its only small ripple its done by pulsing at a certain frequency say 10 times for a ONE, and a different frequency ten times for a ZERO. Is there such a comms system as this?
Its good because it doesnt require the full current limiting, just a bit of current limiting.......and probably a decent size inductor just downstream of the PSU that supplies the bus.
 

Upvote 0 Downvote
Your "tone detector" ICs would "snap to" in about
10 cycles.

National used to have many "special function"
ICs, like DTMF encode / decode. Put the tones
on some HF carrier of convenience / legality,
put it to the wire, decode and detect elsewhere.

With enough selectivity in a "channelizer" you
could have FDM room for many "channels" on
one wire. Cable company manages it. They sure
are not using baseband shunt switches to do it.

How about everybody gets a "factory-set"
bandpass filter and matching oscillator and
PA w/ blocking/matching network? With a
home office control-head that has N of same,
in card-rack?
 

Upvote 0 Downvote
You must log in or register to reply here.

Similar threads