# How to couple a signal on to a DC line

I am using DPSK modulation on my signal and want to couple this onto a DC-signal. How will the hardware be set up for this? Is it enough to just couple my signal through a capacitor on to the DC-line?

EDIT:

• The AC impedance between my DC and ground should be quite high, having no decoupling capacitors or the like at the moment. This may be altered at a later stage.

• My carrier frequency is ~110kHz, available bandwidth between 90-120kHz, using OFDM to utilize the band as much as possible and to avoid noise. This is done using the G3-PLC standard.

• I do not know the impedance of my load at carrier frequency, since I will try to just communicate directly between two modems which I will design for impedance matching later with no extra load on the line.

• I found that putting a series inductance in front of my DC power supply should be used as a choke for the high frequency signal.

EDIT2:

Will this circuit make more sense?

The sensor will also be able to couple its signal back on to the DC line, and both sides of the line should be able to send and receive data. The receiver part is not drawn in because I don't know how I would do it.

• What is the AC impedance between your "DC line" and ground? If it's too low (e.g., lots of bypass capacitors), your signal will be shorted out. – Dave Tweed Dec 4 '12 at 12:25
• In most cases an inductor will be placed between the DC source and the capacitor, to prevent the AC signal from being grounded by the low impedance of the DC source. – Wouter van Ooijen Dec 4 '12 at 13:41
• source and load impedance could be defined so you can impedance match RF at both ends and have neither loading each other. DC might be defined 0 ~10MHz RF is a~b? Coupling could be LC low Q if not overlapping spectrum or -RF transformer coupling if overlapping spectrum or directional coupler stripline if high RF. – Tony Stewart Sunnyskyguy EE75 Dec 4 '12 at 16:43
• LC low Q? Directional coupler stripline? My knowledge as a student hasn't covered those topics yet, please provide schematics or examples. Link to theory is also great – chwi Dec 4 '12 at 18:24
• You have not given enough information. What is the carrier frequency of your DPSK signal? What is the impedance of your load at carrier frequency? What have you got to prevent noise from your load being in the same band as your signal? – Jason Morgan Dec 11 '12 at 15:00

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.

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?

• Thank you for an extremely good answer! Could you elaborate a bit of why the impedance of my coupling capacitor should be less than that of the connectors resistance? Also, do you have any suggestions of inductor sizing, keeping in mind that I will work with ~20V, ~3A (60W)? Again, thank you for a great explanation! – chwi Jan 6 '13 at 11:09
• @Wilhelmsen: please shoot me an email. I have an upcoming project where DC power line communication might come in handy. – davidcary Jan 11 '13 at 18:24
• christian(dot)<myusername>87@gmail.com – chwi Jan 12 '13 at 10:42