Testing for FM interference of DTV

EscapeVelocity · 07-31-2009, 08:53 AM

Testing for DTV Interference

by Charles W. Rhodes, 06.22.2009

TVTechnology.com

EscapeVelocity · 07-31-2009, 08:55 AM

Great stuff here on FM interference.

I didnt know whether to post in this section, or the Converter Box section, or the Technology section.

Yes616 · 07-31-2009, 09:11 AM

Good stuff. This looks like as good a place than any to post.

Piggie · 07-31-2009, 10:31 AM

Quote:

Originally Posted by EscapeVelocity

Great stuff here on FM interference.

I didnt know whether to post in this section, or the Converter Box section, or the Technology section.

I saw this article somewhere because I have it bookmarked.
Could reference from the advanced thread.

EscapeVelocity · 07-31-2009, 10:33 AM

Quote and conclusion of the article...

Quote:

What Fig. 3 also shows, in my opinion, is a large range of robustness between different units tested. For example, at D = –80 dBm the worst unit will not function where the total FM power is greater than –31 dBm, while the best unit will work at –14 dBm.

Near the noise-limited desired signal power (–84 dBm according the FCC) these curves turn downwards because of the combination of receiver-generated noise plus the FMI. At –68 dBm, receiver-generated noise is negligible compared to the FMI so these curves have become remarkably straight. Some, but not all, curve upwards at higher desired signal powers. This is probably due to their RF AGC coming into play.

The good news is that FMI can be eliminated by installing a 75 ohm FM band stop filter at the input to the afflicted DTV receiving appliance. A much lower-cost alternative is a 75 ohm high pass filter (HPF) which attenuates FM signals and any low band VHF DTV signals. In many communities, there will be no DTV signals on Channels 2 through 6, so a high pass filter will work fine. However there are some communities with one or more DTV stations on low band channels. A way around this would be an A/B switch to bypass the HPF when a low VHF band channel is desired.

That is so awkward that most viewers would pay the higher price for an FM band stop filter (if available). However some FM band stop filters may affect DTV reception of Channel 6. Clearly, experimentation is in order.

Stay tuned.

The Winegard CA 8800 will do what the author is talking about, and its relatively cheap.

The other device he is talking about is commonly referred to as an HLSJ (or [VHF] High Low Singal Joiner [Combiner/Separator with Filters])

EscapeVelocity · 07-31-2009, 10:40 AM

Too bad he doesnt name names on the tuners and CECBs used in the test....so we can see which ones are better and the dregs as well.

EscapeVelocity · 07-31-2009, 10:43 AM

Oops.

Piggie · 07-31-2009, 12:22 PM

Quote:

Originally Posted by Yes616

Good stuff. This looks like as good a place than any to post.

Yes616, it's also referenced here. Advanced DTV Reception Thread If I find a good article like this I book mark in one of the table contents we make sticky. This one I put in the advanced thread.

If anyone sees anything that needs bookmarking like this write me or Jay. This keeps a "catalog or table of contents" so as the posts slips pages down we can still find the good ones.

Piggie · 07-31-2009, 01:53 PM

Quote:

Originally Posted by EscapeVelocity

The other device he is talking about is commonly referred to as an HLSJ

I am wondering now. The same days I could not watch highband were the same days Trip saw a lot of low band skip. I know even on my high band stack when the E's get intense I can see them pretty good. Plus I know that stack receives low band best off the back and side from when Channel 2, 4 and 6 where still on the air. The side of the stack faces west, where most of my E's come from.

Wondering now is it possible the strong E days are overloading my amp? Skip comes in pretty darn strong at times, like you are a couple miles from a station. I know on 6 meters and even 2 meters I have seen E-skip pin the meters or almost. In the order of being 10 go 30 db above 0dbm. If the TV E's are doing that, then I would have to guess I might be overloaded.

I think I will order a HLSJ and try adding it between my high band and the VHF input to the CM7777.

EscapeVelocity · 07-31-2009, 05:09 PM

I always forget about FM Interference when trying to help people assess their difficulties. Seems like it can be quite the problem, especially with some tuners/CECBs.


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