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-   -   Intermodulation rating on Preamp (http://forum.tvfool.com/showthread.php?t=7748)

homey 13-Apr-2012 1:24 AM

Intermodulation rating on Preamp
 
What is a good rating? I have a RCA product which does not list Noise Margin but list the intermodulation as greater than 60db?

mtownsend 13-Apr-2012 5:49 PM

Intermodulation ratings determine how the amp deals with multiple strong incoming signals. If you have two or more strong signals coming into the amp, they produce signal byproducts (undesirable internally-generated signal artifacts) that potentially act as interference on neighboring channels.

If your situation has a combination of very strong and very weak signals that you are trying to get, the intermodulation byproducts from the strong signals might clobber the weaker channels. The intermodulation rating on your amp gives you an estimate of how strong those intermodulation byproducts will be relative to your strongest signal.

Look at the signal power difference between the strongest and weakest channel you are trying to get. If that difference is more than 44.8 dB (that's 60 dB - 15.2 dB since you need a minimum of 15.2 dB of signal margin to lock on to a digital TV signal), then there is a chance that your amp may be the limiting factor in picking up those weaker stations. Things may or may not work depending on your particular situation (intermod products will vary by frequency and strength depending on the frequency and strength of your strongest signals).

If all of your desired signals are close in power, then this is not much of an issue.

A high intermodulation rating on an amp is generally a good thing, but it doesn't tell you anything about the noise figure of the amp.

If you need to pick up distant / weak signals (no strong signals present), a high antenna gain and low pre-amp noise figure are the most important parameters to target.

homey 13-Apr-2012 5:56 PM

Thank you for the information! I am suspect of the amp. It claims to use low noise margin circuitry but does not give a rating. The amp does help in that there are channels which completely drop off with it however the fact it does not list a noise margin scares me. I have some channels that drop in and out and was thinking i could gain some strength if I go to an amp with a lower noise margin? If I understand correctly if i go from an amp with a noise margin of 8DB to 3DB that is the same as picking up 5 DB in antenna gain?

Dave Loudin 13-Apr-2012 6:07 PM

Well, yes, you could look at it that way. However, you are still sacrificing 3 dB of signal-to-noise ratio that the antenna provided. Do you have that much loss in the cable between antenna and TV? If so, you are making a good trade-off.

Read the Signal Analysis FAQ, especially the part about noise margin, to understand what I mean.

homey 13-Apr-2012 6:36 PM

Thanks for the link. That clears it up well. I am currently running a 100 foot piece of RG6 directly to one TV. I have a three way splitter I will add later. Interesting that I have channels which I lose completely without the amp. One has a TV fool db rating of around 15DB. My net antenna gain on that channel according to the excellent HDTV primer charts is 12DB. What is that telling me? What is the relationship to net Db received at the tuner and signal strength?


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