Return loss should be positive or negative?
Wikipedia has a definition of return loss:
RL=Pin/Pr
In dBm, log10(RL) = log10(Pin)-log10(Pr)
It also says that it is desired to be large for less refection.
I agree the above statement. From those remarks, both RL and log10(RL) should be positive.
But some IC, such as this one from MaximIC MAX2830:
http://datasheets.maximintegrated.com/en/ds/MAX2830.pdf
On page 11, it is negative.
How to understand the difference between them?
Thanks,
RL is only negative for active devices
as per the definition RL=10 *log10 (Pin/Pr )
for a passive device Pin > Pr therefore Pin/Pr > 1 -> log10(Pin/Pr) > 0
simples :)
ps
Wikipedia also explains why is return loss often expressed as a negative number.
Well, I always assume it is negative for passive devices. That way it works with the math. Lets say I have a reflection coefficient of |ρ|=0.2. I would then say the return loss was 20*LOG(0.2)= - 14 dB. also if I look on a network analyzer, the return loss will be plotted as minus dBs...i.e. below the 0 dB line. A marker on an agilent network analyzer will read "- 14 dB".
I know the word "loss" gets some people, but the convention seems to be this way for that last 40 years or so.
The correct relation between return loss and reflection coefficient is RL = -20log10|Gamma|
so in your example Gamma=02 -> RL = 14
I always thought VNAs show Snm parameters instead of Return/insertion losses.
I agree with this explanation too.
Return Loss by definition is supposed to be a positive number. dB(S11) would be negative and is what is commonly displayed in reports.
You should check out this article written by the IEEE Trans. APS Editor-in-Chief. It is freely available online and was published in 2009.
http://ieeeaps.org/aps_trans/docs/Re...ssAPMag_09.pdf
Very good link to the PDF.
For clarification:
RL = - dB(S11)!
Return Loss is wrongly expressed as a negative quantity these days. The parameter S11 is negative while return loss is positive. Literally speaking, a negative loss will be a gain. One never says 'I suffered a loss of -S100'.
Interesting point of view, the VNA shows db(S11), not return loss. You guyz are right.
The "LOSS" in RL is how I justify this confusion.
If you send a signal in to a port, and you measure the reflection of that signal you should never
get a signal that is Greater then the original signal. so (Pin/Pr) will be > 1, or if you say (Pr/Pin)
you will get the VNA values of -RL.
i.e.
Pin = 4 ; Pr = 2
RL=10log(4/2) = 3.01 (Pin/Pr)
S11= 10log(2/4)= -3.01 (Pr/Pin)
Thanks for the answers. For active devices, what does the negative return loss mean? It means that the reflected wave is stronger than the incident wave?
For this IC chip: http://datasheets.maximintegrated.com/en/ds/MAX2830.pdf
on page 11, it gives a negative curve. The reflect wave is stronger?
Please clarify this for me.
Thanks.
For active devices, a negative return loss in dB is possible, but this indicates an unstable situation. ( link from Microwaves101 )
In this particular datasheet, I think that it might just be a typographical error that someone has missed. The specifications table on page 3 does indicate positive return loss values.
Read the link above for a good discussion.
Return loss is a positive value. S11 graph plots negative value. So for example Return loss is 10dB or S11 is -20dB.