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Off Peak Control Tones

Discussion in 'Hobby Electronics' started by Phil Allison, Aug 27, 2012.

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  1. Phil Allison

    Phil Allison Guest

    ** Hi,

    following recent debate here and since midday Sunday, I set up a simple rig
    to monitor the incoming AC supply for "ripple" tones superimposed by the
    local energy supplier.

    The rig consists of a 12VAC, 1A plug pack with a series tuned LC filter (
    10uF and 2.5mH ) and a 6 inch speaker. When the tones arrive, you can hear
    them easily and also see them on a CRO - the first discovery is that the
    tones here in the inner west of Sydney are at 744 Hz and not 1050Hz, as
    often quoted.

    The tones first arrived at 4:55 & 4:59pm on Sunday - then at 5:15, 5:16 &
    5:17pm - then 9:30, 9;31 & 9:32pm - then
    10:02 & 10:03pm and finally at 10:25 & 10:28pm. Long (ie 25 second) and
    short bursts of tone were involved.

    Very likely, 4:55pm on a Sunday corresponds with the beginning of the
    evening load peak, 9:30pm with the tapering off of that peak and the later
    times with ever lowering demand.

    The afternoon pattern is repeating itself, almost exactly, as I write.

    The level of the superimposed tone is about 12V rms. It is not a pure sine
    wave, but has several components at 100Hz intervals above and below 744Hz -
    as seen using the FFT function on a Rigol scope.

    Could these be AM modulation products, produced by non-linearity in the
    distribution system ?

    This link says that such tones are found in NSW and south eastern QLD plus

    ..... Phil
  2. swanny

    swanny Guest

    I was involved in the design of some equipment that decoded the ripple
    control signals a couple of decades ago. Here's a bit of what I remember.

    The tones were different frequencies depending on your local County
    Council distributor. They were generated at the substations by the CC. I
    think they did this to avoid interference with each other. From memory
    Propect was 740Hz and Sydney was 1050Hz at the time.

    The signals decoded to binary, 10 bits. The bits had significance, I
    think some were allocated to groups and command types, with a certain
    pattern being the 'on' pattern for the device and the inverse being the
    'off' pattern.

    I think there was a sync start pulse that synchronised the receivers to
    the start.

    The bits were simple on-off keying superimposed on the mains.

    There's a bit of stuff here:
  3. Phil Allison

    Phil Allison Guest

    "Phil Allison"
    ** The 744Hz tone is in fact heavily amplitude modulated at 100Hz -
    producing the upper and lower sidebands previously mentioned.

    Close examination of a CRO trace show that during the "flat topped " parts
    of the AC wave, the level of the tone drops to about 25% of the level during
    other parts of the wave. So there are two, distinct dips in level per cycle.

    Why does the level dip at all ?

    The impedance of the network ( at 744Hz) must drop during the "flat topped"
    periods to a fraction of the usual value and the only obvious reason is the
    combined effect of hundred of thousands of electrolytic capacitors that are
    effectively " switched " into circuit by diode bridges during these times.
    Think of every PC, TV set and CFL on the network.

    744Hz is a tad under 15 times the mains frequency of 50Hz - so the (
    incremental ) impedance exhibited by all those electros in parallel is very

    ..... Phil
  4. Thanks for that, very interesting.
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