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.