Superhub : Inaccurate downstream power levels reported by SuperHub?

[ferretuk]

Originally Posted by qasdfdsaq

The context of the thread is explicitly about incorrect units. And yet you think it's helpful to leave them out completely?

??? The only post I can see where units have been missed off that introduces any confusion is:

Originally Posted by qasdfdsaq

Ermm 6dB is 4:1.

That wasn't posted by me...

---------- Post added at 20:13 ---------- Previous post was at 20:11 ----------

Originally Posted by Sephiroth

Qasi's right.

I uploaded a document here that explains it all. (Ignore in the post where I say 11 dBmbv - it should have read 22 dBmv).

If Qasi had quoted 'power' he would have indeed been correct but he was arguing about a post that referred to voltage.

The modem reports in dBmV - An increase of 6dB in this figure would mean that the signal amplitude had doubled and the received power would have gone up by 4.

[qasdfdsaq]

Originally Posted by heero_yuy

Remembering that 6db is 2:1 (Voltage)

Originally Posted by ferretuk

A gain of 2 is 3dB (power) and 6dB (amplitude).

---------- Post added at 20:15 ---------- Previous post was at 20:13 ----------

Originally Posted by ferretuk

If Qasi had quoted 'power' he would have indeed been correct but he was arguing about a post that referred to voltage.

No units means you're still taking rubbish.

Again, read the definition of dB. 6dB is 4:1

6dB (voltage) is 4:1
6dB (power) is 4:1

[ferretuk]

(Referring to the post above) Nothing incorrect in that... Those gains hold true for any power unit or amplitude unit.

---------- Post added at 20:20 ---------- Previous post was at 20:16 ----------

Originally Posted by qasdfdsaq

---------- Post added at 20:15 ---------- Previous post was at 20:13 ----------


No units means you're still taking rubbish.

Again, read the definition of dB. 6dB is 4:1

6dB (voltage) is 4:1
6dB (power) is 4:1

Wrong!!!!

[qasdfdsaq]

Right!!!

You still haven't grasped the concept of dB.

dB (anything) != dBmV

6dB = 3.998x

(6dB) (voltage) = (3.998x) (voltage)

[ferretuk]

Originally Posted by qasdfdsaq

Right!!!

You still haven't grasped the concept of dB.

dB (anything) != dBV

Wrong!!!

dB (Amplitude) != dB (Power)

http://www.rapidtables.com/electric/decibel.htm
http://crca.ucsd.edu/~msp/techniques...tml/node6.html
http://en.wikipedia.org/wiki/Decibel

[qasdfdsaq]

Funny how one person can be bigoted enough to argue "mbps" cannot mean "megabits per second" but "db (voltage)" can mean "dBmV"

mbps != Mbps
but
db (voltage) = dBmV?

?!?!



---------- Post added at 20:35 ---------- Previous post was at 20:35 ----------

Originally Posted by ferretuk

Wrong!!!

dB (Amplitude) != dB (Power)

Bloody hell. How thick can you be.

dB (asdfasdfasdf) != dBasdfasdfasdf

Power and amplitude have nothing to do with your ineptitude at grasping the concept of a space.

---------- Post added at 20:48 ---------- Previous post was at 20:35 ----------

Screw it, I give up.

Next time you complain about someone using "m" when they really mean "M" I'll just remind you how you use " " when you really mean "mV" (and by your logic "mbps" is wrong but "ps" is fine.)

[Sephiroth]

Amplitude ratio has no meaning in this context. The measurement on the SH is in dBmv. The reference power level at 0 dBmv (1.33 nano-Watts). Power is what matters in order to receive an intelligible signal. +3 dBmv doubles the power level.

[ferretuk]

Originally Posted by Sephiroth

Amplitude ratio has no meaning in this context. The measurement on the SH is in dBmv. The reference power level at 0 dBmv (1.33 nano-Watts). Power is what matters in order to receive an intelligible signal. +3 dBmv doubles the power level.

Amplitude is relevant as the SH gives a voltage reading not a power reading. It is, however, academic as no-one likely to put a 'scope across the feed

You've agreed with I said:

Originally Posted by ferretuk

The modem reports in dBmV - An increase of 6dB in this figure would mean that the signal amplitude had doubled and the received power would have gone up by 4.

The confusion, as I also said, is that the modem reports an amplitude but labels it as power. The reference level quoted is 0dBmV (i.e. 1mV) which equates to 1.33nW into 75R

[Sephiroth]

The SH doesn't give a voltage reading. It gives a power reading based on a conversion from a reference point. Voltage is a field value; dBmv is a ratio and thus not a field value.

Can you tell me what the voltage is at 0 dBmv? It can't be zero!

[ferretuk]

Originally Posted by Sephiroth

The SH doesn't give a voltage reading. It gives a power reading based on a conversion from a reference point. Voltage is a field value; dBmv is a ratio and thus not a field value.

Can you tell me what the voltage is at 0 dBmv? It can't be zero!

It gives a field value - dBmV

dBmV is the voltage ratio with respect to 1mV RMS across 75R i.e. 0dBmV is 1mV RMS (Interestingly the definition w.r.t. 1 volt, dBV, is independent of impedance)

No different to the concept of a power ratio. dBm (or dBmW) is the power ratio with respect to 1mW i.e. 0dBm is 1mW.

As the impedance is defined for dBmV then the power is also known - 0dBmV gives a power of −78.75 dBW (−48.75 dBm)

The confusion, as I've tried to explain, is that just quoting 'dB' without qualifying whether you're talking about amplitude or power is ambiguous.

Just saying '6dB is a ratio of 4' is true for power but not amplitude. Without context, both of these statements are true:

n increased by 6dB = n*2
n increased by 6dB = n*4

Saying the 'voltage has increased by 6dB' or 'the power has increased by 3dB' is non-ambiguous. In both case the parameter has gone up by two.