Lineman Forum: pilot channel question. - Lineman Forum

Jst wondering how important it is to use the pilot ch in ur system to balance amps,because i got a feelin some people dnt do it on all the balancing in this system. What could this actually cause in the long run? The pilot is differ from other areas so changing this in the meter to balance this area is not always done,if the pilot from the other area is flat with the differ pilot would this be cool?

Hey Hw77,

That is a good question seeing the pilot channels are not something extra they are real channels and are very important to CATV. The Pilot Channels help the AGC's in you amp compensate for temperature. There are 2 pilot channels one on the low end usually ch 2 or 3 and one on the high end usually ch 61 or 62. they help the slope and gain of the amp as it get hot or cold. every time you balance an amp you should look at the pilot and they should be the same at every amp.

When I set up my meter I look at 4 channels 2, 30, 61, 117 but when I look at tap levels I look at 2, 30, 78, 117 Because I don't need to see the pilot channel outside of the amps.

All AGC's should be setup to the Hi pilot. For the SA you take the level of the Hi polit subtract 29 (that is the spec of mine) and that is the pad you put in the AGC then adjuast for the temp at 68 degrees and use the temp chart to turn up or down to compensate

(Levels 41 at hi polit ..... 41-29=12.... 12 pad for the AGC if the temp is 68 no adjustment needed)
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There is an exception I have 2 SA III AGC's. 1 has just a gain controll and the other has a slope and gain controll then I look at both polits
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Hope this helps
olo131

It's extremely important to use the pilot channel.. That's the home of your AGC setup... If someone there isn't using it, correct them..

If nothing has changed since the last time the amp was set up, and the cable is still good, then setting up the amp using high and low channels is acceptable, but the AGC in those amps that have it need to be shut off while the adjustment is made. Then, as others have stated, use the procedure for setting AGC levels to arrive back at the proper output.

As others have also stated, if your meter supports it, have multiple channels in your tilt plan. You can then choose which channel you want as your high channel depending on the bandwidth of the system you are in. The AGC channel should always be measured with the LEVEL function of the meter, not the TILT function. The LEVEL portion of the meter is more accurate. If the cable is good, you should still have the correct system tilt after the AGC alignment.

Dak

Old c-cor gear has ALC and they seem to stick all the time, i really dont trust them, anybody else have a problem with the E72 line extender.

This post has been edited by memphis10: 23 December 2009 - 02:17 PM

Just curious ALC (Automatic Level Control)? So what is the difference to an AGC?

olo131

Never used the gear.. Can't help you there.. I would assume it's the same as AGC, but uses a different name..

You mean...The same but different..... Right?

ROFL.. Exactly!!

Well thanx for the responses here is my delima,i have on system with pilot ch being ch76,and all the other areas use 499.25 i think. Well the amps are antec and these suckers just always are wiggin out,so is it me or are these amps not hold their level?? If anybody can come up with documentation on the right way to set these amps up would be apperciated i could not find any no where. The antec are 870mhz gear and the pilot is @ 535.25mhz. Its gets tiresome hitting every amp in every run all the time due to these damn asg not holdin.

i would say automatic gain control for AGC. The antec are ASG control,now what does that mean,lol.

Broke out some books, (I have to many) because I had to know

AGC.....Automatic Gain Control
A more sophisticated method of controlling the amplifier gain involves sensing a signal level and adjusting the gain of the amplifier ahead of the sensing point to bring the level back to a standard, or expected, level. This is called an automatic gain control (AGC) system

Yet another aspect of the present invention, in this exemplary embodiment, allows use of dual slope AGC systems at low cost. In the days of long amplifier cascades (the 1970s and 80s), it was common practice in the cable industry to use dual slope AGC circuits rather than the single-pilot ASG systems used today. These dual slope systems employed two pilot carriers, one near the low end of the spectrum and one near the high end. Two AGC detectors measured the amplitude of both pilots and adjusted not only the gain, but also the tilt of the amplifier station to compensate for variations due to temperature. This was done to ensure that at the output of a long cascade of amplifiers, the signal was still tilted according to the designed tilt. Tilt is desirable because coaxial cable has more loss at high frequencies than at low.

In modern practice, in which the length of an amplifier cascade is limited to typically six amplifiers or less, dual slope AGC systems are not used. The shorter cascade is less sensitive to errors in tilt. Since a conventional dual slope system requires two AGC detectors, cost is higher. Nonetheless it would be desirable to use dual slope techniques if the cost can be made lower.

ALS......Automatic level control (ALC) with high/ low switch for easy setup
I searched google and found: In TV amplifier, ALC means AGC+ASC(Automatic Slope Control).
AGC: Use power level at one frequency point as a reference to control the gain of amplifier;
ASC: Need to detect power level information at two different frequency points to adjust the slope of gain to frequency. So In this situation, ALC is much more complex than AGC.

ASG.....Automatic slope and gain (ASG) detector technique including a pilot signal
In ASG systems, it is recognized that the variation of attenuation with temperature exhibited by the coaxial cable is also a function of frequency. Accordingly, it is the practice to not only adjust the gain of the amplifier station according to the sensed signal level, but to adjust the gain vs. frequency characteristic ("slope") of the station according to the sensed signal level.

Because the entire spectrum of signals on the cable is very wide (downstream transmissions in North America presently use frequencies from 54 to about 870 MHz.), it is common practice to sense only the level of one signal, called the "pilot" signal. Previous systems employed two pilot signals, one at the low end of the spectrum and one at the high end. Such systems were called dual pilot systems, but they are not common today due to reduced cascade lengths which resulted from employment of fiber optic transmission.

olo131

What I can remember about the Antec gear is that you absolutely must set up the amp correctly in manual first, and make sure that the interstage equipment is correct. It really helps to set this stuff up when the temp is a perfect 60 - 70 degrees. I have seen a few instances where techs would screw around with the interstage pad and eq to get output. That is a big no no. The ISEQ and pad should be identical in every like kind of amplifier, within +/-1 dB.

I am including three Antec manuals. Hopefully these are what you need.

Good Luck,

Dak

This post has been edited by Dak: 24 December 2009 - 06:33 AM

We use Ch 61 for our AGC Pilot on SA SYSTEM II and SYSTEM III gear. One question though... were going to remove just about all the analog early next year. If they replace Ch 61 with a 256QAM carrier, how will that affect amp setup? QAM is 6 dB lower aint? Or will they just keep some kind of analog black screen on Ch 61?

There will always be analog carriers out there. Your 2 pilots and leakage tag will for one will always be there... The all digital world is just for customers.

olo131

I love the good 'ol pilot on 61.. It will never go away. Customers may see all digital and HD, but in tech world analog will remain forever..

We don't have it, but I've seen newer Motorola gear that has a "QADU" module that allows the MSO to run a QAM carrier as the amplifier AGC control. They have it as an option. Although what a job it would be to swap out all the ADU modules with QADU modules. I would think it would be better for a new rebuild. What about a hybrid setup during the rebuild, what a mess to keep track of , some amps on ch 61 adu, some on ch 70 qadu.

http://www.motorola....pdf?localeId=33

That's a cool Idea but not a cheap one....certainly not for my cheap ass company, but what will you do for leakage. I think you will still need some analog bandwidth in a HFC system. I see new or full upgrade systems getting that cool stuff!

I would stick with analog for AGC..

Tried and true