Off Peak Control Tones

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:

"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 low.

.... Phil

imple 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 wi= th a series tuned LC filter ( 10uF and 2.5mH ) and a 6 inch speaker. When t= he 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 7=

44 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 - t= hen 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 corres= ponds 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 aftern= oon pattern is repeating itself, almost exactly, as I write. The level of t= he 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 NZ. .... Phil

Thanks for that, very interesting.