Splitting cable for internet access

In my Motorola SB5120 modem that connects to Comcast in S. California, the downstream signal is around 739.8 MHz, and the upstream signal is around 25.34 MHz. So a passband that includes the low range enables the reverse signal going upstream.

The quality features in splitters includes a high return loss, low intermodulation distortion, and good electrical sealing between the 2 halves of the splitter case. This website will give you an idea what good hardware is:

*TimDaniels*

Does the modem work without the splitter? If so, check the signal levels. Most will be accessible somewhere starting at http://192.168.100.1/ If you are already marginal, the splitter will make it worse. You might need an amplifier before the splitter. It's also possible the splitter is cheap garbage with a fake label.

Yes, the 5MHz is really critical. As was written by a previous poster, his return path is 25.35MHz. Mine is 22MHz. If you go to the address posted by another , you can see what your actual return path frequency is. But bandwidth is not the only critical factor. Just as important is the integrity of the shielding. The back needs to be soldered to the case, not just epoxied (stuck on with glue). If it is epoxied, signals floating around in the air will disrupt your cable modem's signals, in addition to possibly causing problems with your TV.

Specs of a splitter should be 5-1000MHz, -110dB RFI/EMI immunity, Printed circuit board design, Return loss should be upper teens or above, Isolation should be greater than 25dB. Don't forget solder-backed.

CIAO!

Ed N.

Seguros Catatumbo wrote:

Cool, i knew of the non-routable ip of the cable modem but never bothered to test if it had a layer 7 interface... So it appears my cable provider disabled the signaling features, crap. Here is the only info i can see:

WebSTAR DPX100

Modem Serial Number 0000000000000000

Cable Modem MAC Address

Hardware Version 1.0

Software Version 1.1.2 r1.1.3.1-0716 (dpx100-v112r1131-0716a.bin)

Receive Power Level 10.25 dBmV

Transmit Power Level 49.00 dBmV

Cable Modem Status operational

Of course the modem works fine without the splitter, just wanted to know what features should the splitter have so i can view tv on my tv tuner card and surf the net at the same time.

Thanks all

Another very important spec, in addition to those I listed previously, is blocking capacitors on ALL ports.

CIAO!

Ed N.

Seguros Catatumbo wrote:

Damn, i bought one with these specs:

• Model: TSB-21G * Insertion Loss (In-Out) : 3.5dB Typical * Return Loss (Output): 28dB Typical * Isolation (Out-Out): 34dB Typical * Dimensions: 2.3=CB=9D (L) x 1.9=CB=9D (W) x 0.6=CB=9D(H)

Which exceed the specs you mentioned. But it still doesn't work. The cable led doesn't light up. How would the cable company give me tv access if i asked them to?

Can they cripple the signal intentionallly if i only have internet access or something so i cannot split it without an amplifier?

Typically, if a subscriber has internet only, a filter is installed in the line (usually at the tap (or the lockbox in an MDU environment)) which blocks the frequencies from ~50MHz up to the downstream frequency of the cable modem. Depending on the downstream frequency, there may also be another filter that blocks frequencies above the datastream. Amplifiers do no good in that scenario.

Earlier, you had posted levels. Was that without the splitter installed? If so, what are the numbers with the splitter installed. What is the quality of the cable you are using to go from the wall to the splitter? Avoid cables that have push-on connectors like the plague.

That is a very good splitter you bought.

CIAO!

Ed N.

Seguros Catatumbo wrote:

In 2 western states where I've lived, the cable companies *remove* a highpass filter if the service is to include Internet in order to allow the upstream frequencies below 50MHz to pass. That is, they try to keep the junk that is generated below 50MHz by 20-year old TV sets and other regional noise out of their system by blocking it with the filter. It's only when they *have* to allow passage of such frequencies for Internet service that they remove the filter. Interestingly, a cable company manager told me that the filter's cutoff frequency varies with geographical region due to the variance of the ambient noise with region.

*TimDaniels*

High-pass filters are not present on a line that has HSI (they block everything below ~50MHz, and cable modems need that return path of

5-42MHz to be able to talk to the CMTS), but it was not high-pass filters to which I was referring. The filters used block from 50MHz up to the downstream used by the cable modem, which can be anywhere from 50MHz to 860MHz. If there are no analog channels above that datastream (downstream frequency), then only 1 filter is used. If there are analog channels above that frequency, a second filter is typically installed which blocks those channels. Around here, the downstream frequency is 519MHz and there are only a couple of analog channels above that, so a 2nd filter is not used.

If a subscriber does not have HSI but does have digital cable, they sometimes use a high-pass filter that passes 5-13MHz as most DCTs use a frequency within that window.

CIAO!

Ed N.

Timothy Daniels wrote: