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Although I spent many years in the electrical field, I specialized in Electrical Metrology (a long time ago). Bearing this in mind, I really know very little about satellite television system theory.

As we recently lost our ability to acquire certain transponders used by our satellite television receiver, I began looking into the basics of this area of RF theory ... and, in short, I'm intrigued by the idea of sending a DC signal over the same transmission line use to transmit RF signals. I'm certainly not clear about it at this point, but, based on what I've read (thus far), this combination of DC and RF is commonly referred to as "Phantom Power" by the satellite people.

Having used many power splitters and directional couplers in my day -- mainly for calibration testing of Precision Measurement Equipment (PME) -- I'm especially puzzled by a component of this area of electronics known as "DC POWER PASS". Apparently, the idea is to pass a DC voltage over the same coax used to transmit down-converted RF satellite signals. As this seems to run contrary to the division of power I was [once] accustomed to, I would really love to read a lucid explanation of this method of controlling satellite television system circuitry.

I hope that I've been clear enough.

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    \$\begingroup\$ You pretty much nailed it -- phantom power sends DC over the same wires that signal is sent. Because the power is DC, and the signal is AC, it's easy to separate the two. It's commonly used with microphones, too (just about every computer mic is an electret, which needs around 5V of phantom power, and there are pro microphones that need 48V). \$\endgroup\$ – TimWescott May 16 at 0:48
  • \$\begingroup\$ the component "radio frequency choke",, a non-resonant inductor, separates the RF from the DC. \$\endgroup\$ – analogsystemsrf May 16 at 1:56
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Basically, it's AC superimposed on DC, with a filter to separate them out at the other end

Satellite TV line-power schemes (and other power-over-coax setups) work by simple superimposition; they add the AC signal atop the DC supply voltage on one end using a coupling capacitor and RF choke to create a summing node, and then use a filter (coupling capacitor and RF choke) to separate them out at the other end, as follows (although the values certainly aren't right, and OA1 is really a generic amplifier block):

schematic

simulate this circuit – Schematic created using CircuitLab

Note that this is not the same as true "phantom power" as applied to balanced lines such as pro-audio mics and PoE; these setups have the DC voltage set as a common-mode voltage from both signal lines to ground, or between two pairs of signal lines for that matter.

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  • \$\begingroup\$ This is a nice diagram of the overall process, but, considering that "power pass" type (multi-output) splitters are usually inserted into these systems, I was hoping to see more of the actual/practical symbols that apply to common satellite television systems. In other words, where would a multi-channel "power pass" type splitter be inserted into this diagram so that more than one Integrated Receiver & Decoder (IRD) can be employed? I was also wondering if, by "LNA," you mean Low Noise Amplifier? \$\endgroup\$ – Red Oak May 16 at 17:05
  • \$\begingroup\$ @RedOak -- yes, a LNA is a Low Noise Amplifier :) and the splitter goes in the middle -- one tap would be the "power" tap that passes DC power, and the others would be RF-only to keep the power supplies in the receivers from fighting each other \$\endgroup\$ – ThreePhaseEel May 16 at 22:09
  • \$\begingroup\$ Considering the "low noise" benefit of the Low Noise Amplifier depicted in the diagram, is this phrasing analogous to the same two words in another acronym: LNB (Low Noise Block Downconverter)? I think that I've allowed myself to get bogged down by the "Downconverter" part of this satellite television transmission device because I quickly realized that the IF used by the satellite receiver (IRD) had to be in a lower range (converted to a lower frequency) ... but I'm beginning to wonder if there's an amplification component to this process, as well. Anyone have any thoughts? \$\endgroup\$ – Red Oak May 17 at 21:37
  • \$\begingroup\$ @RedOak -- a LNB will amplify, yes (between having a front-end LNA stage, and having a mixer with conversion gain) \$\endgroup\$ – ThreePhaseEel May 18 at 0:31

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