Return Loss for LNA question, dB
I'm comparing two low-noise-amplifiers and I'm looking at Input Return Loss.
One says its 14dB. The other one says it's -8dB.
I do know that higher values are better. Lower values means more signal is reflected back.
Am I completely off-track if I think that 0dB means that 100% of the signal is reflected back?
3dB means 50% signal is reflected, 6dB means 25% reflected, and so forth.
So what's up with the negative number then? Does it reflect 100% of the signal and then add some more signal of its own to send back to the antenna? I know I'm wrong about this but can somebody explain this to me?
The amplifiers can be expected to have |S11| < 1, respectively positive dB value of return loss, respectively negative dB value of reflection factor. Surely not negative return loss dB value.
It's kind of what I figured, plus I saw another thread on this forum after my message that explained it.
This LNA (RF3858) definitely shows Input Return Loss as negative dB.
They also show the gain of the LNA in low-gain mode as a negative dB.
So in essence this wonderful LNA does not only reflect more signal than what's coming in, but it's making the signal weaker on the output. So you'd be better of replacing the LNA with a resistor and you'd still have more gain. Definitely one heck of a device :)
From theory, since the return loss is the Log of the ratio Pin/Preflected, it has to be a positive number, as FvM said. However for historical reasons, sometimes (not so rarely) it is represented as a negative number, that is the Log of Preflected/Pin.
Your datasheet uses the last definition even if not strictly correct. Howeverer the RF3858 has an internal LNA that can be by-passed. When it's working as LNA its gain is about 21 dB with input and output return loss of about 8 dB. When it is in by-pass (you can use this feature to receive high level signal in order not to saturate the LNA) the gain is negative (that is a loss). Nothing is said about return loss under this last condition.
Of course you can choose (if you have to receive the signal in a wide dynamic range) to use an LNA combined with two external switches to a fixed attenuator, so in case of low signal you can pass it through the LNA and in case of large signal you can pass it through the attenuator. But in this case the component count will be higher as well as the needed PCB area.
Thank you everyone for clarifying this!
I do understand now that it makes sense to have a negative gain option for an LNA in case of strong signals, as well as that return loss may at times be represented in negative dB albeit not technically correct.
Obviously, this is a case of the Marketing Department not knowing anything about their product. The product shown in your attachment is more an attenuator, not an LNA.
A "return loss" number should always be positive - the word "loss" is the negative indicator, since "return loss" is abs(dB(S11)).
Even the output spurious is listed incorrectly - it is either -70dB, or 70dBc (it is listed as -70dBc).
It's amazing how a marketing department can make the whole company look stupid.
Its all a bit tricky.......
Does a 10db attenuator actually have gain ?
Does a 10db amplifier have a loss or is that a gain ?
There are conventions, which have to do with the context, where positive or negative numbers are assumed without needing to be indicated.