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Recommended downstream RF poweer values

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derytelecom
Recommended downstream RF poweer values

Hi,

What is the Recommended downstream RF power values pour a 32 channels ?
How do we calculate ? is there a formula to do it ?

Thnaks

pascal

mbowe
Are you wanting to know what

Are you wanting to know what power to hit the forward transmitters? (lasers)

derytelecom
Hi mbowe,

Hi mbowe,

Yes just like attachment, but for 32 channels

File attachments: 
kwesibrunee
RF Power Primer

there are two different types of RF-Power at play:

total power - the combined power of all the channels, no matter how many channels you have combined together the total of their power remains the same

per-channel power - the power level of an individual channel, which changes depending on how many channels you have combined together. (this sounds like what you are asking)

The RF rule of thumb is that every time you double the channels your per-channel power decreases by 3-3.5 dB. The total power stays the same, you are just splitting the total power over more channels.

Most modern CMTS have 58 dB (+/- 1-2 dB) as there maximum total power output (more importantly the default setting)

To get per-channel max power output use following table:

1 Channel 58 dB
2 Channel 55 dB
4 Channel 52 dB
8 Channel 49 dB
16 Channel 46 dB
32 Channel 43 dB
64 Channel 40 dB

If you are using HFC (fiber and nodes) what you really want to know is what input RF level, per Channel, your transmitter wants. You will have to check your transmitter documentation, typically it is 15-30 dB per channel.

say for example your transmitter wants 15 dB per channel, and you are using 32 channels, you need to add 28 dB attenuation from output of CMTS (with a total output power of 58 dB) to input to transmitter. Typically this is done in a combining network, by adding inline pads, or if you are using modular combining adding pads to your splitters/combiners.

Some CMTS have the ability to limit the total output power of the CMTS and you can also use this to attenuate the signal, but if you change the total output power you will need to adjust the levels in the above table.

i.e. if you limited your total output power on your CMTS to 50 dB by adding attenuation on the CMTS your table would look like this

1 Channel 50 dB
2 Channel 47 dB
4 Channel 44 dB
8 Channel 41 dB
16 Channel 38 dB
32 Channel 35 dB
64 Channel 32 dB

so again using 15 dB input, per channel to your receiver, using this scenario, 32 channels, you would only need 20 dB of attenuation from CMTS -> input of transmitter.

Depending on the CMTS you use you may also be able to specify the per-channel output power, i.e. no math.

e.g. on a Casa CMTS (Disclaimer, I work for Casa)

interface qam 0/0
power 360 per-channel !--- This sets the per-channel output power, it is in tenth dB so 360 == 36.0 dB

versus

interface qam 0/0
power 580 !-- this is the total power for all channels

regardless of how you configure it, on a Casa CMTS you can see what the setting is by:

show interface qam 0/0 power

Calculated Total Power: 567
Configured Per-Channel Power: 399
Send to FPGA Power: 567
Power adjusted: 567
Spectrum-tilt: 0

CHAN_ID FREQUENCY ATTNU B_POWER TILT T_POWER CH_POWER
0 357000000 0 399 0 399 399
1 363000000 0 399 0 399 399
2 369000000 0 399 0 399 399
3 375000000 0 399 0 399 399
4 381000000 0 399 0 399 399
5 387000000 0 399 0 399 399
6 393000000 0 399 0 399 399
7 399000000 0 399 0 399 399
8 405000000 0 399 0 399 399
9 411000000 0 399 0 399 399
10 417000000 0 399 0 399 399
11 423000000 0 399 0 399 399
12 429000000 0 399 0 399 399
13 435000000 0 399 0 399 399
14 441000000 0 399 0 399 399
15 447000000 0 399 0 399 399
16 453000000 0 399 0 399 399
17 459000000 0 399 0 399 399
18 465000000 0 399 0 399 399
19 471000000 0 399 0 399 399
20 477000000 0 399 0 399 399
21 483000000 0 399 0 399 399
22 489000000 0 399 0 399 399
23 495000000 0 399 0 399 399
24 501000000 0 399 0 399 399
25 507000000 0 399 0 399 399
26 513000000 0 399 0 399 399
27 519000000 0 399 0 399 399
28 525000000 0 399 0 399 399
29 531000000 0 399 0 399 399
30 537000000 0 399 0 399 399
31 543000000 0 399 0 399 399
0w (L) 649000000 0 399 0 399 399
0w (U) 743000000 0 399 0 399 399

derytelecom
Thanks kwesibrunee,

Thanks kwesibrunee,

This answers my question.

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