Return loss should be positive or negative?

[ruwan2]

Hi,

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:
https://datasheets.maximintegrated.com/en/ds/MAX2830.pdf
On page 11, it is negative.

How to understand the difference between them?
Thanks,

 

[Inti Innovations]

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.

 

[biff44]

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.

 

[Inti Innovations]

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".

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.

 

[jeeudr]

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.

**broken link removed**

 

[flanello]

Very good link to the PDF.
For clarification:
RL = - dB(S11)!

 

[Twisha]

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 -$100'.

 

[biff44]

Interesting point of view, the VNA shows db(S11), not return loss. You guyz are right.

 

[Element_115]

Hi,

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:
https://datasheets.maximintegrated.com/en/ds/MAX2830.pdf
On page 11, it is negative.

How to understand the difference between them?
Thanks,

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)

 

[ruwan2]

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: https://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.

 

[jeeudr]

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: https://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. ( **broken link removed** )

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.

 

[ok1987]

Return loss is a positive value. S11 graph plots negative value. So for example Return loss is 10dB or S11 is -20dB.