Pre-Amp Noise Testing

Small Engine

I was wondering why all the experts on this forum were giving advise on getting a low noise pre-amp but not really explaining why...It was if this was a car web site and all the experts were suggesting that you get a blue car but not offering any reasons why, now if they were saying that studies show that blue cars don't break down as much as other colored cars that would make sense. anyway back to pre-amps and noise, sense April 2009 I have been using a RS 30 db gain pre-amp with a db noise of 7-8 with marginal success. so I decide that I would try a low noise pre-amp with as close to as much gain as I am currently using. I ended up with a (nothing fancy) Winegard 4800 28 db gain and 2.7 db noise, the best amp on the market was a CM that offered 2.0 db noise, so 2.7 was close enough. What ever noise did or didn't do I now had a amp that at least cut out 2/3 of the noise. Testing this amp over the last several days I have concluded that a lower noise pre-amp has a better chance of holding a signal without pixilating, if I have at least 17% of the signal I can watch tv with out any cut-outs/pixilation just pure tv just like I was hooked up to a dish, once the signal gets below 17% it starts breaking up etc..doing this test using the RS pre-amp I never was able to get a solid signal below 20+%. So I have concluded that a lower noise pre-amp will give the owner a better chance of reciving and veiwing tv stations that are not coming in very strongly.

Of course I could be totally way off base


Mike

__________________
Antennas are like Real Estate.. its all about location.. location.
My TvFool
XG-91 DB8 Wg-4800 Ac-10G212

DoctorCAD

No, a low noise pre-amp only allows a cleaner signal to come through the amp without affecting it. It won't make your signal stronger at all.

Back to your car analogy. A 0.5 micron oil filter vs. a 3 micron oil filter. Nobody would tell you that the 0.5 lets less dirt through, they would assume that you could figure that much out.

IDRick

A pre-amp sets the noise floor for the distribution system. NF on tv tuners range from 5 to 10 dB while NF on quality pre-amps range from 1 to 3 dB. A low noise pre-amp (assuming no overload conditions) will improve your signal at the tv by two ways: a) increase signal due to lower NF for the pre-amp than for tv tuner and b) amplification covers distribution losses through splitters and cable runs.

Let's use a hypothetical example: assume 3 dB NF on pre amp, 5 dB NF on tv tuner and 100 ft of rg6. The distribution losses in this case would be ~6 dB and the difference between NF is 2 dB. Adding a pre-amp in this case would increase the signal to the tv by 8 dB (6dB distribution loss + 2dB difference in noise factor).

The net effect of adding 8 dB will vary depending on the situation. With moderate to strong signals, all that changes is the signal strength reading from the tv tuner. With weak signals, the user may go from dropouts to consistent, stable reception.

aka.Hooper

EXACTLY!
Think of it as Quantity -vs- Quality...

You're trying to get a signal to the tuner. You're antenna has collected enough signal to put it, let's say 10dB above the noise floor. Now say you have 10dB loss in the rest of your system, including cable, splitters, etc. Obviously you need an amp.

If you add an amp which has for instance, 30dB of gain but 8dB of noise you're not in too good a shape - you have Quantity but no Quality, in that your final signal will have much power when overcoming only 10dB of system loss but will be only 2dB above the noise floor. Throw in some atmospheric anomalies and your signal has fallen below that noise floor...

Replace with an amp which has only 12dB of gain and 3dB of noise and you're in much better shape - 12dB more than overcomes your system losses of 10dB and your signal is still 7dB above the noise floor.
[EDIT: And also able to overcome the noise introduced by your tuner - Hi Rick!]

Make sense?

__________________
Some details on my DIY antenna builds:
http://mysite.verizon.net/res11d41p/
Some tips for newbies posting to the forum for the first time:
http://www.highdefforum.com/local-hd...ng-advice.html

IDRick

To further illustrate Hooper's example (hi Hooper!), assume 20 dB signal at antenna, 8 dB NF for pre-amp one, 3 dB NF for pre-amp two, 10 dB distribution loss, 5 dB tuner loss.

What is received signal with no pre-amp?

Signal after balun +20 dB
Distribution loss -10 dB
Tuner NF -5 dB
Signal = +5 dB

What is the signal level with a high noise pre-amp?

Signal after balun +20 dB
Pre-amp NF -8 dB
Signal = +12 dB


What is the signal level with a low noise pre-amp?

Signal after balun +20 dB
Pre amp NF -3 dB
Signal = +17 dB

Using a low noise pre-amp (versus no amp) increased the tv signal by 12 dB through covering the distribution losses and lower NF for the pre-amp. Furthermore, using a low noise pre-amp provided an additional 5 dB of signal to the tv compared to the high noise pre-amp. A low noise pre-amp clearly gives you more bang for the buck than the high noise pre-amps!

JB Antennaman

When you start out with a crappy signal and then you butt it into a crappy wire and then you run it through a crappy amplifier, nothing but crap is going to come out of the other end.

Actually, if you had a signal that was ok and you needed 10 db of gain to over come the loss in the wire and you had a pre amp with 12 db of gain and 3 db of noise, you will still have a loss of 1 db because 12 - 3 = 9 and you needed 10 to over come the loss in the wire from the antenna to the television by way of say maybe a splitter that had 3 db of loss and 100 ft of wire that had 7 db of loss at 700 Mhz.

Your signal analysis is not correct in the fact that there is no set amount of signal for your signal strength meter. Not all tuners are the same and so the number you give us for your signal strength has no meaning to us. Just as two tuners of the same manufacturer could have the same amount of signal coming into them, yet shows a different reading for the amount of signal that it see's.

That is one reason why I like the Channel Master CM 7000 box.

I have quite a few of them and they do not loose the signal until they get down around 15% and usually when a signal gets that low, there is so much flutter that it doesn't matter anyways.

With a digital signal, you can loose 80% of the signal and still have a good signal - in terms of picture and sound quality. In the old days, with analog, when the signal degraded, you still had some sound and a snowy picture, but you could still watch it as long as it came in.

With the digital signal, it is all or nothing. Either you have a cinema quality picture and sound, or you have pixelation or you have nothing at all.

__________________
There is no exception for good reception!

JB Antennaman

When you start out with a crappy signal and then you butt it into a crappy wire and then you run it through a crappy amplifier, nothing but crap is going to come out of the other end.

Actually, if you had a signal that was ok and you needed 10 db of gain to over come the loss in the wire and you had a pre amp with 12 db of gain and 3 db of noise, you will still have a loss of 1 db because 12 - 3 = 9 and you needed 10 to over come the loss in the wire from the antenna to the television by way of say maybe a splitter that had 3 db of loss and 100 ft of wire that had 7 db of loss at 700 Mhz.

Your signal analysis is not correct in the fact that there is no set amount of signal for your signal strength meter. Not all tuners are the same and so the number you give us for your signal strength has no meaning to us. Just as two tuners of the same manufacturer could have the same amount of signal coming into them, yet shows a different reading for the amount of signal that it see's.

That is one reason why I like the Channel Master CM 7000 box.

I have quite a few of them and they do not loose the signal until they get down around 15% and usually when a signal gets that low, there is so much flutter that it doesn't matter anyways.

With a digital signal, you can loose 80% of the signal and still have a good signal - in terms of picture and sound quality. In the old days, with analog, when the signal degraded, you still had some sound and a snowy picture, but you could still watch it as long as it came in.

With the digital signal, it is all or nothing. Either you have a cinema quality picture and sound, or you have pixelation or you have nothing at all.

Anytime you amplify noise, you raise the signal noise floor.

If you have a amplifier that has 7 db of noise, it is a pretty crappy amplifier. Might I suggest that you tape a dollar bill on it and give it to the garbageman and tell him - here I just gave you something.

Truthfully I don't know which would be worth more, the dollar bill or the crappy amplifier!

__________________
There is no exception for good reception!

NonMcTubber

I have a slightly different take here.

We are simply testing side by side, a RS shack preamp with 30 DB gain and 7.5 db noise vs a winegard with 28 DB gain and only 2.7 DB gain.
( all other variables such as antenna choice, coax, splitters, and all else drop out as factors because they are held as constants. )

Maybe over simplistically, we can do the math as:
real radio shack gain =30-7.5=22.5 DB
real winegard gain= 28-2.7=25.3 DB

And then say the winegard offers the better gain by 2.8 DB. And sure enough, our OP reports his signal meter also calibrated in DB's shows a basic 3 DB improvement in what is a usable signal.

But to say the radio shack is junk and the winegard is gold is a mistake. Because our OP, small engine has seen a resultant improvement because he is at the deep fringe of reception due to his location.

And I am fairly confident that I could use either amp and not see any difference at all in terms of real world results. Its all very situational and depends on location, location, and location.

__________________
High definition is not the definition of my life.
But knowledge is power and HD does have some potential to increase content variety and choices.

IDRick

NMT,

The gain difference between your example pre-amps is not important (as long as both cover distribution losses). Received signal at the tv increases by almost 5 dB with the Winegard over the Rat Shack. This large difference in received signal will show a nice increase in % signal strength from the tv tuner. Since costs are almost the same, it certainly makes sense to purchase a low noise pre-amp.

jdemaris

Anecdotal events don't always work as good predictors.

I did many amp swaps at several locations. Used amps from as high as a 4 dB noise rating down to .6 dB noise rating. In my case, there was hardly any difference. Fringe channels that pixelated with high-noise amps did the same with low-noise. The one common denominator I found was that higher gain amps sometimes worked better - even with a higher noise rating. In some cases, it made the difference of being able to view a certain channel, or not.

Does it always work that way? I doubt it. Too many variables, including some amps out-of-the-box might not perform as advertised.

Tower Guy

The best way to calculate system noise figure is to use the Friis equation. http://www.microwaves101.com/encyclo...deanalysis.cfm

The problem with the observations is that noise figure specs and actual noise figure may be off by several db, especially in the case of the Radio Shack preamp. The loose spec means that an amp that works well for Small Engine may not work as well for anyone else.

jdemaris

I'm curious about what model amp you were using? I didn't know that Radio Shack sold one with that poor a rating anymore.

Some older models from Radio Shack were known for testing much worse then adverstised.

On the other hand, my son just tested a new Winegard AP-8275 and it tested dead-on, as adverstised.

As I understand it, once the noise going to your TV exceeds the noise-rating of the TV tuner itself, you get trouble. Many modern digital TV tuners operate with a noise factor of around 10 dB. So, if your amp really was making 7 dB of noise or more, along with any other problems - seems it might do considerably worse than an amp with a few Dbs lower noise rating.

aka.Hooper

This is gonna be off the wall.

Lets say your signal is like flying a hot air balloon.
And the ground represents the noise floor.
You start out at some given height - the signal's NM at the antenna.
The distance you need to travel is the losses in the system. i.e. cable, splitters, etc.
Without fuel as you travel along you naturally lose altitude.
If you touch down early you are done, and did not make your destination... the tuner.

So you add a tank of gas (the amp) which doesn't actually lift you any higher, and in fact even drops your altitude a little. (the extra weight pulling you down is the noise the amp introduces) But this fuel enables you to travel further, at a slower rate of descent.

And so you can now reach your destination without crashing...

That would depend on how high above the noise floor your original "OK" signal was...
If you started with a signal at +10dB above the noise floor then you would have no problem, and still have a signal at +7dB above the noise floor, with 2dB of power to spare at the tuner. (Which is essentially superfluous)
If OTOH you started with a signal of only +3dB you would be in trouble, and would need yet a lower noise amp to receive the signal. (But not necessarily one with any more gain.)

And I agree with NMT here. Obviously the deeper fringe you are, the lower your signals are above the noise floor, and the more important the NF of an amp becomes. IOW, if you're close in & have strong signals & a lossy distribution system - a cheap high gain, high noise amp may well work fine for you.

What enables you to receive a signal is that it remain above the noise floor.
When you add an amp it is going to amplify the noise it receives along with the incident signal, and also add it's own noise contribution, thus effectively lowering the noise floor margin. But the signal now has more power to overcome further losses in the system - as the newly raised noise is attenuated along with the incident signal, and the relationship is maintained - unless more noise is introduced.

EXACTLY!
I thought you were always in the right camp on this one JB!

__________________
Some details on my DIY antenna builds:
http://mysite.verizon.net/res11d41p/
Some tips for newbies posting to the forum for the first time:
http://www.highdefforum.com/local-hd...ng-advice.html

IDRick

Thanks for the link Tower Guy! Here's another calculator based on the Friis equation" http://www.digitalhome.ca/forum/show...68&postcount=1

jdemaris

I can stick a Wade VIP-307, or a Winegard HD8200 up 30' in the air, point in any direction, and not get a single channel without a preamp.

That is my working definition of a deep-fringe area and I can hook up just about any amp rated with a noise factor of 4 Db down to .6 db. Makes no difference in actual reception. Problematic channels remain, as do the good ones. I haven't tried any amps with higher noise ratings then 4 Db though.

Subsequently, at least in my deep fringe situation, by deductive reasoning, I deduce that 4 Db or lower makes little difference here.
That is, unless the amps are doing something totally different then as advertised, which I doubt. For that reason, as a very loose general rule-of-thumb, I'll adhere to the idea that incoming noise can't exceed tuner noise - until I hear something better that seems to work in the majority of situations.