Optimizing for the warm weather...

Optimizing for the warm weather...
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Optimizing for the warm weather... Eric 03-30-2005
Posted by Eric on March 30, 2005, 9:17 pm
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Hi,

Now that the weather is becoming warmer with each day, I have noticed that
the downstream power level to my cable modem is beginning to decrease. The
warm weather also is causing my cable modem to increase it's upstream power
level as well. The SNR is also slightly decreasing with warmer weather.
During the very cold nights last winter, I was seeing SNR at around 38 and.
With the warm days now (currenty 80 degrees), the SNR is between 35 and 36.
Downstream power has gone from -3.5 and -4 dBmV to between -7 and -8 dBmV.
Upstream power has gone from 42 dBmV to 45 dBmV. Everything still looks well
within specs, but I'm concerned that it may pass the thresholds once the 90+
degree temperature arrives.

The cable modem is located right where the cable comes into the house,
connected to a wireless router. There are three pieces of additional
hardware giving attenuation to the cable line once it comes in: a surge
protector, a splitter, and a male/male coupler. The splitter is very solid,
it's frequence range is rated up to 1Ghz, and it is spec'd at giving 3.5 dB
of attenuation. As a recent test (while 80 degrees outside), I ran the
cable line coming in directly to the modem to see what the downstream was
and it was a solid +1 dBmV. I then ran it through the splitter (plus a
three foot section of my own RG6) and got -2.5 dBmV. I then added the
coupler to the splitter, and found it to give an additional 2 dB of
attenuation. So, with the splitter, coupler, and a small section of my own
RG6, the downstream power level is at -4.5 dBmV -- attenuating the original
+1 dBmV by 5.5 dB.

I then ran it through the surge protector, which seems to introduce another
2.5 dB of attenuation. The attenuation of the surge protector, however,
doesn't seem constant. It seems to jump between 2 and 2.5 dB -- giving the
total output between -7 and -8 dBmV.

Unfortunetly, I never took at look at signal level coming in during the cold
weather. I suspect that it must have been hotter than +1 dBmV though, since
I was using the surge protector, splitter, and coupler at the time and
seeing between -3.5 and -4 dBmV.

This will be the first summer that I will have the cable modem. Should I
expect much more loss as the weather begins to climb from 80 to 90+ degrees?
As I have read, the range for the downstream power level should be
between -15 and +15 dBmV, but idealy you want to keep it within -10 and +10
dBmV -- with 0 dBmV being optimum. Should I look at removing one of the
three pieces of hardware before the summer months arrive? Perhaps locating
a good splitter that has a female connector for input and males for the two
outputs (to remove the coupler)? Is the surge protector absolutetly
necessary for coax -- or does the cable modem (plus the cable HDTV STB box)
provide their own surge protection for the coax inputs? All the electrical
inputs for the cable modem, router, HDTV, STB, Dolby Digital receiver, ect
are all going through the surge protector of course.

Or, perhaps, I really don't need to be too concerned about it. From
everything that I have, it all looks well "in the green".

Is this looking pretty good?

Currently, at 76 degrees outside, and cable uses QAM 256:

Downstream power level: -6 dBmV
SNR: 36 dB

Upstream power level: 45 dBmV
(This will fall back to around 42 dBmV by evening. Without the splitter,
coupler, surge protector, it only requires around 38 dBmV to talk to the
cable company.)

Thanks!
-ES





NMFall 20%
Posted by Rick Kunath on March 31, 2005, 7:48 am
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Thec wrote:

>
> Now that the weather ambientming warmer with each day, I have noticed that
> the downstream power level to my cable modem is beginning to decrease.
> The warm weather also is causing my cable modem to increase it's upstream
> power
> level as well. The SNR is also slightly decreasing with warmer weather.
> During the very cold nights last winter, I was seeing SNR at around 38
> and. With the warm days now (currenty 80 degrees), the SNR is between 35
> and 36. Downstream power has gone from -3.5 and -4 dBmV to between -7 and
> -8 dBmV. Upstream power has gone from 42 dBmV to 45 dBmV. Everything still
> looks well within specs, but I'm concerned that it may pass the thresholds
> once the 90+ degree temperature arrives.
>

The cable plant losses increase as the ambient temperature rises. Cable
plants work best cold. The active devices in the cable company's plant have
correction circuitry to compensate for this. ALC (automatic level control)
holds the output level of an active device constant and ASC (automatic
slope control) holds the pre-emphasis level constant. Cable plant active
devices pre-emphasize the higher frequencies of the baseband because of the
higher loss of the coaxial hardline as frequency increases. This "slope" as
it is called allows the downstream active device to see a flat (or nearly
so) frequency response on the baseband. Since the loss of hardline changes
with increasing temperature, the ALC and ASC circuitry automatically
increases the gain and slope as appropriate to maintain a constant level to
the next downstream active device. This is adjustable, and may be operating
incorrectly in your case.

> As a recent test (while 80 degrees outside), I ran the
> cable line coming in directly to the modem to see what the downstream was
> and it was a solid +1 dBmV.

This is pretty low, though legal. I'd expect to see +15 dBm in a properly
set up and designed system at your demarcation point.

> I then ran it through the splitter (plus a
> three foot section of my own RG6) and got -2.5 dBmV.

Good so far.

> I then added the
> coupler to the splitter,

Describe the coupler? What kind of device is this?

>
> I then ran it through the surge protector, which seems to introduce
> another
> 2.5 dB of attenuation.

Dump the surge supressor. Get yourself a Regal gas-discharge ground block,
and place this at the demarcation point.

As to cable plant signal level variations, expect some change, but the
changes you are reporting would indicate that your plant is incorrectly set
up or has incorrectly operating ASC/ALC circuitry.

Call your cable company and have them send out a line tech to take a look at
their plant.

Rick Kunath


Posted by Ed Nielsen on April 1, 2005, 8:19 am
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Those numbers are well within the operating range of a cable modem.
Cable modems are designed to wrok with an inout window of -15 - +15dBmV,
and an output of 8 - 55dBmV. SNR is excellent -- supposed to be <35dB.

(More below)


Rick Kunath wrote:

> Thec wrote:
>
>
>>Now that the weather ambientming warmer with each day, I have noticed that
>>the downstream power level to my cable modem is beginning to decrease.
>>The warm weather also is causing my cable modem to increase it's upstream
>>power
>>level as well. The SNR is also slightly decreasing with warmer weather.
>>During the very cold nights last winter, I was seeing SNR at around 38
>>and. With the warm days now (currenty 80 degrees), the SNR is between 35
>>and 36. Downstream power has gone from -3.5 and -4 dBmV to between -7 and
>>-8 dBmV. Upstream power has gone from 42 dBmV to 45 dBmV. Everything still
>>looks well within specs, but I'm concerned that it may pass the thresholds
>>once the 90+ degree temperature arrives.
>>
>
>
> The cable plant losses increase as the ambient temperature rises. Cable
> plants work best cold. The active devices in the cable company's plant have
> correction circuitry to compensate for this. ALC (automatic level control)
> holds the output level of an active device constant and ASC (automatic
> slope control) holds the pre-emphasis level constant. Cable plant active
> devices pre-emphasize the higher frequencies of the baseband because of the
> higher loss of the coaxial hardline as frequency increases. This "slope" as
> it is called allows the downstream active device to see a flat (or nearly
> so) frequency response on the baseband. Since the loss of hardline changes
> with increasing temperature, the ALC and ASC circuitry automatically
> increases the gain and slope as appropriate to maintain a constant level to
> the next downstream active device. This is adjustable, and may be operating
> incorrectly in your case.
>
>
>>As a recent test (while 80 degrees outside), I ran the
>>cable line coming in directly to the modem to see what the downstream was
>>and it was a solid +1 dBmV.
>
>
> This is pretty low, though legal. I'd expect to see +15 dBm in a properly
> set up and designed system at your demarcation point.

Not low -- perfect. Cable modems operate within a window of -15 -
+15dBmV. Much above or below that and sync goes away. With +15dBmV
(15dBm would be ~+63dBmV) at the groundblock, as the levels fluctuate
connectivity would be intermittent.

Most CATV systems in the US design their systems to have 15-20dBmV at
the tap, which would typically be ~10-17dBmV at the groundblock. 64
QAM carriers run at 10dB down fron the nearest analog carrier, which
means that at a groundblock that has +11dBmV as read by a regular Signal
level meter, the cable modem would read +1dBmV. 256 QAM is 6dB down
from analog.

Even with +15dBmV at the groundblock, a 2-way splitter for the cable
modem connection, and the outlet 50' from the splitter, the cable modem
would read 2.1dBmV (256 QAM) or -2.1dBmV (64 QAM).
>
>
>>I then ran it through the splitter (plus a
>>three foot section of my own RG6) and got -2.5 dBmV.
>
>
> Good so far.
>
>
>>I then added the
>>coupler to the splitter,
>
>
> Describe the coupler? What kind of device is this?
>
>
>>I then ran it through the surge protector, which seems to introduce
>>another
>>2.5 dB of attenuation.
>
>
> Dump the surge supressor. Get yourself a Regal gas-discharge ground block,
> and place this at the demarcation point.
>
> As to cable plant signal level variations, expect some change, but the
> changes you are reporting would indicate that your plant is incorrectly set
> up or has incorrectly operating ASC/ALC circuitry.
>
> Call your cable company and have them send out a line tech to take a look at
> their plant.
>
> Rick Kunath


CIAO!

Ed N.


Posted by Dave on April 1, 2005, 2:03 pm
If you were  Registered and logged in, you could reply and use other advanced thread options


I think you meant to say that the SNR should be >35 (greater than 35).


> Those numbers are well within the operating range of a cable modem. Cable
> modems are designed to wrok with an inout window of -15 - +15dBmV, and an
> output of 8 - 55dBmV. SNR is excellent -- supposed to be <35dB.
>
> (More below)
>
>
> Rick Kunath wrote:
>
>> Thec wrote:
>>
>>
>>>Now that the weather ambientming warmer with each day, I have noticed
>>>that
>>>the downstream power level to my cable modem is beginning to decrease.
>>>The warm weather also is causing my cable modem to increase it's upstream
>>>power
>>>level as well. The SNR is also slightly decreasing with warmer weather.
>>>During the very cold nights last winter, I was seeing SNR at around 38
>>>and. With the warm days now (currenty 80 degrees), the SNR is between 35
>>>and 36. Downstream power has gone from -3.5 and -4 dBmV to between -7 and
>>>-8 dBmV. Upstream power has gone from 42 dBmV to 45 dBmV. Everything
>>>still
>>>looks well within specs, but I'm concerned that it may pass the
>>>thresholds
>>>once the 90+ degree temperature arrives.
>>>
>>
>>
>> The cable plant losses increase as the ambient temperature rises. Cable
>> plants work best cold. The active devices in the cable company's plant
>> have
>> correction circuitry to compensate for this. ALC (automatic level
>> control)
>> holds the output level of an active device constant and ASC (automatic
>> slope control) holds the pre-emphasis level constant. Cable plant active
>> devices pre-emphasize the higher frequencies of the baseband because of
>> the
>> higher loss of the coaxial hardline as frequency increases. This "slope"
>> as
>> it is called allows the downstream active device to see a flat (or nearly
>> so) frequency response on the baseband. Since the loss of hardline
>> changes
>> with increasing temperature, the ALC and ASC circuitry automatically
>> increases the gain and slope as appropriate to maintain a constant level
>> to
>> the next downstream active device. This is adjustable, and may be
>> operating
>> incorrectly in your case.
>>
>>
>>>As a recent test (while 80 degrees outside), I ran the
>>>cable line coming in directly to the modem to see what the downstream was
>>>and it was a solid +1 dBmV.
>>
>>
>> This is pretty low, though legal. I'd expect to see +15 dBm in a properly
>> set up and designed system at your demarcation point.
>
> Not low -- perfect. Cable modems operate within a window of -15 -
> +15dBmV. Much above or below that and sync goes away. With +15dBmV
> (15dBm would be ~+63dBmV) at the groundblock, as the levels fluctuate
> connectivity would be intermittent.
>
> Most CATV systems in the US design their systems to have 15-20dBmV at the
> tap, which would typically be ~10-17dBmV at the groundblock. 64 QAM
> carriers run at 10dB down fron the nearest analog carrier, which means
> that at a groundblock that has +11dBmV as read by a regular Signal level
> meter, the cable modem would read +1dBmV. 256 QAM is 6dB down from
> analog.
>
> Even with +15dBmV at the groundblock, a 2-way splitter for the cable modem
> connection, and the outlet 50' from the splitter, the cable modem would
> read 2.1dBmV (256 QAM) or -2.1dBmV (64 QAM).
>>
>>
>>>I then ran it through the splitter (plus a three foot section of my own
>>>RG6) and got -2.5 dBmV.
>>
>>
>> Good so far.
>>
>>
>>>I then added the coupler to the splitter,
>>
>>
>> Describe the coupler? What kind of device is this?
>>
>>
>>>I then ran it through the surge protector, which seems to introduce
>>>another
>>>2.5 dB of attenuation.
>>
>>
>> Dump the surge supressor. Get yourself a Regal gas-discharge ground
>> block,
>> and place this at the demarcation point.
>>
>> As to cable plant signal level variations, expect some change, but the
>> changes you are reporting would indicate that your plant is incorrectly
>> set
>> up or has incorrectly operating ASC/ALC circuitry.
>>
>> Call your cable company and have them send out a line tech to take a look
>> at
>> their plant.
>>
>> Rick Kunath
>
>
> CIAO!
>
> Ed N.




Posted by Ed Nielsen on April 1, 2005, 6:36 pm
If you were  Registered and logged in, you could reply and use other advanced thread options


My bad! Thanks for catching that.


CIAO!

Ed N.


Dave wrote:
> I think you meant to say that the SNR should be >35 (greater than 35).
>
>
>
>>Those numbers are well within the operating range of a cable modem. Cable
>>modems are designed to wrok with an inout window of -15 - +15dBmV, and an
>>output of 8 - 55dBmV. SNR is excellent -- supposed to be <35dB.
>>


Similar ThreadsPosted
Cold weather = modem dropout January 18, 2007, 12:10 pm

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