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How LNB can transmitted data to receiver in dc voltage, I measure the voltage which is out of the receiver and a found it dc signal

My question is how the receiving signal gets to the receiver back in dc voltage?

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    \$\begingroup\$ The DC power is going to the LNB. The AC signal is returned from the LNB on the same wires. \$\endgroup\$
    – Transistor
    Commented Jul 27, 2016 at 12:22

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I'm sure it is not just DC but there is a (tiny) RF signal superimposed.

The DC is there for power supply of the LNA (power going to the LNA).
The RF is the received Signal (coming from the LNA).

Both components can easily be separated by a high pass filter (capacitor)/low pass filter (choke coil).

schematic

simulate this circuit – Schematic created using CircuitLab

A similar technique is used in the audio range for supplying power to preamplifiers that are integrated in microphones. It's called phantom power or phantom feed.

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  • \$\begingroup\$ how can the RF signal transmitted by DC \$\endgroup\$
    – m salim
    Commented Jul 27, 2016 at 12:09
  • \$\begingroup\$ @Curd: I agree. The output of the LNB will still be very low; low enough that ordinary meters would not detect it (the down-converted signal will still be in the GHz range at a very low power) \$\endgroup\$ Commented Jul 27, 2016 at 12:12
  • \$\begingroup\$ @m salim: The RF signal is not transmitted by DC, it is superimposed (i.e. added) to the DC. \$\endgroup\$
    – Curd
    Commented Jul 27, 2016 at 12:14
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Your voltmeter is not capable of measuring the signal from the LNB. You are only measuring the DC voltage that provides power to the LNB.

The DC you measured does not come from the LNB. The DC voltage is provided by the receiver.

The signal from the LNB is a radio frequency signal at around 2 giga Hertz. Your volt meter cannot measure that kind of thing.

The RF signal "floats" on the DC voltage that you measured. The inputs circuits on the voltmeter filter the RF signal out. There is no special filter in your meter that intentionally removes the RF. It is just that the meter is designed to work with DC or low frequency AC, and making a meter that also responds to RF would make your meter very complicated and very expensive.

Nothing that a typical hobbyist has at home would be capable of measuring or displaying the signal from a satellite TV LNB.

I suggest you look up low noise blocks on wikipedia, as well as looking up satellite TV on wikipedia. That will provide at least a basic background, and then you can ask more specific questions about the parts of that background that you don't understand.

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