I am trying to design the Powerline Communication (PLC) module JUST for the 5-socket power strip (for demo purpose). So, I only care about the powerline network for those 5-sockets only. However, I need to know the transfer characteristics of the power strip lines so that I can choose the optimum frequency band for modulation. What is the best way to conduct this experiment so I can have a transfer characteristics of the copper plates (Line to Neutral) in the power strip? What I want is the impulse response in frequency domain up to 500 MHz. How do I do that in lab? I have a frequency generator (square, pulse, sine, sawtooth) etc and 5Ghz BW agilent oscilloscope.
Electronic – How to get the transfer characteristics of power strip lines
power line communicationtransfer function
Related Solutions
You are right, the real question is what does the impedance of your 12 V power line look like at high frequencies.
I don't know, but this is something you can measure yourself. Try to feed a signal onto the 12 V line thru a resistor (with cap to block DC but large enough to not add significant impedance) and measure the attenuation.
133 kHz sounds very low to me. I would try around 1-2 MHz for starters. With a good filter, you can get by with very little signal at the receivers. After all, think of how little the signal is in the antenna of a AM radio.
It would probably help a lot if you can add even a little inductance to each connection of the 12 V line. At 150 A, that would be big and expensive to actually buy inductors, but maybe just wrapping the feed cable around a few loops would help. Three turns around a broomstick might make a difference. The reason I said broomstick is because then it will essentially be a air core inductor, so you don't have to worry about saturation current. 1 µH at 2 MHz has a impedance magnitude of 13 Ω. 1 µH is going to be hard to achieve with a few loops of cable, but it is easy to mentally work from there. 100 nH, which might be possible, will be 1.3 Ω
Yes, the standard bias-T looks something like this:
DC power source >>---L1--+ +--L2->> "unregulated" DC power to regulator
| |
bidirectional RF ----C1--+- mixed RF + DC -+--C2---- bidirectional RF
GND--------------------------------------------------------------------GND
where L1 and L2 are equal-size power inductors and C1 and C2 are equal-size data-coupling capacitors.
I would avoid connecting anything else to the "mixed RF + DC" line other than those two inductors and two capacitors. (Or perhaps four capacitors, if I had a separate "transmit capacitor" and a "receive capacitor" at both ends).
Since you likely have some sort of connectors between the two devices that typically have 0.1 Ohm of resistance each, coupling capacitors that give an impedance of less than 0.1 Ohm (across the entire bandwidth) will be more than adequate (and perhaps overkill).
So a capacitor with capacitance at least 1/(2*pi*90 kHz * 0.1 Ohm) =~= 18 uF is more than adequate (and perhaps overkill). You'll want a cap with low parasitic resistance (ESR), so mica, film, or ceramic -- rather than tantalum or electrolytic. You'll want a cap with low parasitic inductance, so surface-mount -- rather than through-hole.
Standard off-the-shelf capacitors and inductors are more than adequate up to 10 MHz or so. People that work with higher frequencies use striplines and resonating stubs that may appear to be black magic. Although there are a few people who claim it isn't. a b c d e
EDIT:
capacitor sizing
Inevitably, not all the energy sent out by the transmitter will make it to the receiver. If I cut the cable between the transmitter and the receiver and add "a few more connectors" in the path between them, a little energy is lost each time the signal crosses a connector.
Practically all digital communication systems can tolerate a lot more loss than that caused by "a few more connectors". So keeping the distortion to something less than the loss of "a few more connectors" is overkill. (I prefer to get my first prototype working with oversized components, rather than pick stuff that is right on the verge of not working).
inductor sizing
Alas, I don't have a rule of thumb for figuring out how much inductance to specify. Perhaps whatever you have that is generating or receiving your G3-PLC data might have some sort of datasheet with some recommendations?
Ed Mullins and Anass Mrabet in "Analog Front-End Design for a Narrowband Power-Line Communications Modem Using the AFE031" have many tips you might find useful. In particular, their figure 27 seems to indicate that, with PRIME or G3-PLC, a standard off-the-shelf voltage regulator will work, one where the only inductance between the power line and the large bulk power storage capacitor that powers all the electronics is a standard EMI filter.
(via http://www.ti.com/solution/power_line_communication_modem , via http://www.ti.com/plc ).
The datasheet for one particular such voltage regulator has a detailed list of materials; its EMI filter consists of a
- 1 L1 Inductor, AC line, common choke, 27 mH, 54P512-276 Vitec Electronics Corp.
- 1 L2 Inductor, high current choke, 3.3 μH, HCP0703-3R3-R Coiltronics/Cooper
(nearby datasheets also mention a "54PR515-146" and a "AF5169-146" AC-Line Common Mode Choke).
It is getting easier to search popular electronic suppliers for "common mode chokes". Alas, while I see many with "at least" 27 mH, and many "at least" 3 A, finding one that meets both specs is difficult. Perhaps the Bourns Inc. 7122-RC (4 A, 25 mH) might be adequate?
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Best Answer
Look for something called a Line Impedance Stabilisation Network, or LISN. You'll find a circuit diagram for it in MIL-STD-461.
It's essentially a diplexer, a high pass filter for signals above a few kHz, and a low pass for the mains current at 50/60 Hz.
The Stabilisation part of it is that it isolates the circuit under test from the (presumably very low, and unknown) impedance of the mains supply.
In your case you'd turn it around and use it to isolate the load, and feed the RF into the mains.
I don't think the LISN goes as high as 500 MHz, but up there the mains impedance is irrelevant. You can just disconnect the power and test it directly with your equipment. To be thorough, try a few different power extension leads on the power strip, to get an idea of the range of values you can expect in practice.