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Old 7-Oct-2015, 2:40 PM   #21
NthrnNYker
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Quote:
Originally Posted by rabbit73 View Post
This is what the 7777 specs looked like from the CM 2006 catalog:



I was able to verify those specs when I did tests using some old model 7777s connected in cascade (series) to simulate strong signal overload.
http://www.digitalhome.ca/forum/1597002-post3490.html

Now, all we have that we can trust are the tests done by ADTech and others with the proper test equipment.
Yup, that verifies what I had heard reputed elsewhere -- the Channel master 7777 has higher gain and slightly lower noise than the 7778 (2.8 db vs. 3.0 -VHF & 2.0 db vs. 2.2 -UHF).....all things being the same, if the 7777 doesn't overdrive my tuners with the signals available in my location -- I think I will go with the 7777

Thank you !

Karl
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Old 7-Oct-2015, 3:29 PM   #22
rabbit73
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Careful, those specs are for the OLD preamps, not the new ones which say for both
Quote:
Noise figure typically less than 2dB
http://www.channelmaster.com/TV_Ante..._p/cm-7777.htm
http://www.channelmaster.com/TV_Ante..._p/cm-7778.htm

and I have not see any independent verification of those figures

New 7777
Gain 30dB
Max input level 15 dBmV, which is equal to -34 dBm using a conversion factor of 49
Max output level 40 dBmV, -9 dBm

New 7778
Gain 16dB
Max input level 34 dBmV, -15 dBm
Max output level 50 dBmV, +1 dBm

Notice that the specs for the OLD 7777 said:
"Maximum input is output capability minus gain."

They fudged the figures for the new 7777:
40 dBmV minus 30 is 10 dBmV, not 15 dBmV, which is equal to -39 dBm.

The calculations go like this using Noise Margin numbers:

Your strongest signal is WRGB on real channel 6 with a NM of 26.7 dB. If you add the antenna gain of 6 dB, you are at 32.7 dB. If you then add the preamp gain of 30 dB and subtract the preamp noise figure plus distribution loss of 5 dB, you are at NM 57.7 dB. That looks OK.



Interpreting Noise Margin in the TV Fool Report
http://www.aa6g.org/DTV/Reception/tvfool_nm.html

Using signal strength from the Pwr (dBm) column of your tvfool report, WRGB 6 is -64.2 dBm. If you add the antenna gain of 6 dB, you are at -58.2 dBm, so that's OK for your preamp input.

However, there might be a problem if you want to use your HD8200XL TV antenna for FM, because your strongest FM signal is -36.9 dBm, which is 27.3 dB stronger than your strongest TV signal. I don't see any gain figures for the HD8200XL on FM, so plus 7 dB giving you -29.9 dBm, which is more than the rated max input of the new 7777. You would need to use the FM trap in the 7777 and a separate antenna for FM.
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Last edited by rabbit73; 7-Oct-2015 at 7:00 PM.
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Old 7-Oct-2015, 7:07 PM   #23
rabbit73
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THREE TYPES OF OVERLOAD

There are three types of preamp or tuner overload, in order of increasing signal strength:
1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.

As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.

Intermodulation Distortion

The IMD creates new spurious signals within the preamp (or tuner) itself that can interfere with the reception of your weakest desired signals if the spurious signals are stronger than the noise floor of the weakest desired signals. The spurious signals are caused by the interaction between two or more of your strongest signals.

IMD is not the only distortion that can be created within the preamp; you can also have distortion caused by signals so strong that the top of the strong signals are clipped, which causes compression of the signals. This can be seen if you increase the input to the preamp by, for example, 10 dB and the output increases by less than 10 dB.

Spurious Free Dynamic Range

The Spurious Free Dynamic Range needed is the difference in strength between your strongest signal and your weakest desired signal, plus 16 dB for the SNR of the weakest signal, using the dBm Pwr scale on your tvfool report. This difference is expressed in terms of dB, not dBm, because the original units are the same. The difference between the strongest signal and the weakest signal is the Signal Dynamic Range/Dynamic Range, which is 16 db less than the SFDR, because it doesn't include the SNR of the weakest signal.



Another way to think of SFDR is from the top of the strongest signal down to the bottom (noise floor) of the weakest desired signal. The top of the spurious signals must be at or below the noise floor of the weak signals if they are not to cause interference. The Signal Dynamic Range is from the top of the strongest signal down to the top of the weakest desired signal.

You can also use the NM scale for your calculations, but I prefer to use the Pwr scale because most tuners drop out around -85 dBm.
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Old 15-Oct-2015, 10:53 PM   #24
NthrnNYker
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Rabbit, ADTech, Tower Guy --- I would like to thank you all for your assistance.

Based upon your advice, I have made my purchases -- and for the level of system I ended up getting, I think I ended up with a bargain. I believe the system I purchased is about the most powerful one out there available to the ordinary consumer.

I ended up with the WineGard HD8200U clone -- the Solid Signal HD 8200XL -- for 1/2 the price @ $69

Also the Channel Master 7777 pre-amp for $54 - new from solid signal.

Believe it or not, Radio Shack is running a clearance special on their basic model of Antenna Rotor and control box for $29 (evidentially, they are getting out of the antenna business) - plus another $8 for the 100' of control wire.

A set of the 12" antenna mast wall mounts for $13 -- also through radio shack.

2 - 4' sections of mast @ $8 apiece.

Another $15 in coax stand-offs for the antenna mast, grounding block for the coax cable, wall pass-through for the coax, ect ect.

Total investment -- less than $175 for a complete antenna system.

An investment that will take about a year to pay for itself, given the $12 /mo cost for my basic-basic cable service which only provides the local channels that my antenna will now provide.

I will have to wait until after the week-end to mount this system --- snow event due here over the weekend up here in the wilds of northern NY I will let you all know after the installation how it turned out

A couple final questions if I may......

Firstly -- I intend to re-use the Coax cable that DirecTV left behind -- it's really high quality stuff -- but my question is....that Coax cable has a tiny, separate ground wire molded into the protective jacket...what would I connect that ground wire to, and should it be grounded anyways, given the grounding block at the antenna entrance ?

Second -- at a later time, if I choose to add more televisions and decide that I need to upgrade my system with a distribution amplifier....is there one that is particularly suited to work with the Channel Master 7777 (given the high-gain output of the CM 7777) -- or will any radio-shack version work ?

Again, thank you all --- you assisted in my decision-making process tremendously.

Karl

Last edited by NthrnNYker; 16-Oct-2015 at 2:54 PM.
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Old 17-Oct-2015, 1:24 AM   #25
Tower Guy
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The 7777 preamp has sufficient gain to allow the use of a splitter without any distribution amplifier.
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Old 17-Oct-2015, 8:19 AM   #26
NthrnNYker
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Quote:
Originally Posted by Tower Guy View Post
The 7777 preamp has sufficient gain to allow the use of a splitter without any distribution amplifier.
Thank you, Tower guy...

Thank you for ALL your assistance !

Karl

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