Inaccurate downstream power levels reported by SuperHub?
 

Hi everyone,

I had somebody from VM out to adjust my power levels the other day, when the SuperHub was reporting in the region of -6dBmV for each downstream channel. When the engineer connected the SuperHub's end of the cable to his handheld meter, it showed -9dBmV for three or four different downstream frequencies.

After adjusting the connection at the green box, his meter showed 0dBmV for the downstream channels he tested - but the SuperHub was still reading 3dBmV higher. Just curious if anyone else has ever seen anything like this (and if it's anything to be concerned about)? For what it's worth, my SuperHub has 6 downstream channels bonded.

SB

Re: Inaccurate downstream power levels reported by SuperHub?
 

Yes - entirely normal.

Re: Inaccurate downstream power levels reported by SuperHub?
 

i believe the tap point the engineer uses is for testing purposes only and doesnt always give the same results the customers' do.

Re: Inaccurate downstream power levels reported by SuperHub?
 

The difference is down to one reading coming from a piece of expensive measuring kit and the other coming from a piece of cheap domestic kit.

Re: Inaccurate downstream power levels reported by SuperHub?
 

From what little I know of cable modems, could it be that the measurement point for input power is after the AGC (assuming the SH has one)?

Re: Inaccurate downstream power levels reported by SuperHub?
 

Unlikely, given that the AGC is there to normalise the input to a known voltage.

<Speculation>

More likely a reading from the AGC/front end eq circuit that shows the adjustment needed and therefore allows the calculation of what the input level is.

</Speculation>

Re: Inaccurate downstream power levels reported by SuperHub?
 

Ermm 6dB is 4:1.

Re: Inaccurate downstream power levels reported by SuperHub?
 

Wrong - Read the post again...

Re: Inaccurate downstream power levels reported by SuperHub?
 

Correct. Read the definition of dB.

Re: Inaccurate downstream power levels reported by SuperHub?
 

Power != Voltage

Sorry to bang on about units but the 'power' is erroneously labelled as the level given by the SH (and VMNG300) is quoted in dBmV (voltage) not dbmW (power).

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

Re: Inaccurate downstream power levels reported by SuperHub?
 

dB has nothing to do with power or voltage. It is a unitless ratio.

We are talking about dB. Nobody said anything about dBmV or dBm.

For someone who likes to bang on about using correct units I'm surprised you don't seem to understand a unitless ratio is unitless.

Re: Inaccurate downstream power levels reported by SuperHub?
 

Agreed but the ratio described depends on whether it's power or amplitude.

The context of the thread is about the levels reported by the SH (and has dBmV figures quoted) and you have the audacity to call me a pedant...

Re: Inaccurate downstream power levels reported by SuperHub?
 

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

And yet you bitch about other people not being helpful by using "mb" when the context of the thread is clearly megabits? At least they put a unit in there instead of arguing about incorrect units and not even quoting a unit to begin with.

Re: Inaccurate downstream power levels reported by SuperHub?
 

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).

Re: Inaccurate downstream power levels reported by SuperHub?
 

Thanks for the replies everyone, I guess I just wanted to know whether the difference indicated a faulty SuperHub or not.

From what I'm hearing, it sounds like the SuperHub's either measuring from a different point, or is less accurate than the equipment the engineer used.

SB

PS - thanks for another informative read Seph.

Re: Inaccurate downstream power levels reported by SuperHub?
 

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

That wasn't posted by me... ;)

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

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.

Re: Inaccurate downstream power levels reported by SuperHub?
 

---------- 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

Re: Inaccurate downstream power levels reported by SuperHub?
 

(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 ----------

Wrong!!!!

Re: Inaccurate downstream power levels reported by SuperHub?
 

Right!!!

You still haven't grasped the concept of dB.

dB (anything) != dBmV

6dB = 3.998x

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

Re: Inaccurate downstream power levels reported by SuperHub?
 

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

Re: Inaccurate downstream power levels reported by SuperHub?
 

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 ----------

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.)

Re: Inaccurate downstream power levels reported by SuperHub?
 

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.

Re: Inaccurate downstream power levels reported by SuperHub?
 

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:

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

Re: Inaccurate downstream power levels reported by SuperHub?
 

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!

Re: Inaccurate downstream power levels reported by SuperHub?
 

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.