Superhub : Inaccurate downstream power levels reported by SuperHub?

[SnareBuckle]

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

[ferretuk]

Yes - entirely normal.

[General Maximus]

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

[ferretuk]

Originally Posted by General Maximus

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

Originally Posted by SnareBuckle

When the engineer connected the SuperHub's end of the cable to his handheld meter, it showed....

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.

[Sephiroth]

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)?

[ferretuk]

Originally Posted by Sephiroth

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)?

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>

[qasdfdsaq]

Originally Posted by heero_yuy

There's also the tolerance on any bit of kit, I would have thought that +/-2db was quite realistic so the SHUB and the engineers device could easily disagree by upto say 4db?

Remembering that 6db is 2:1 (Voltage)

Ermm 6dB is 4:1.

[ferretuk]

Originally Posted by qasdfdsaq

Ermm 6dB is 4:1.

Wrong - Read the post again...

[qasdfdsaq]

Correct. Read the definition of dB.

[ferretuk]

Originally Posted by qasdfdsaq

Correct. Read the definition of dB.

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).

[qasdfdsaq]

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.

[ferretuk]

Originally Posted by qasdfdsaq

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

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

Originally Posted by qasdfdsaq

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

Originally Posted by heero_yuy

Remembering that 6db is 2:1 (Voltage)

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...

[qasdfdsaq]

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.

[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).

[SnareBuckle]

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.