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antenna distribution amps and indeed shileded loop amps




 
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Author Topic: antenna distribution amps and indeed shileded loop amps  (Read 286 times)
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Opcom
Patrick J. / KD5OEI
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« on: March 23, 2021, 09:32:53 PM »

At this time there are many professional-grade high bandwidth analog video distribution amps on the usual e-marketplace and for very cheap cost.

The higher end RGB distribution amps made in the late 1990s, designed for analog HD in some cases have bandwidth of 300MHz, and they have 3 analog channels.

These things are not bothered operationally by the load being 50 Ohms for RF or 75 Ohms for video or whatever, even open, If you can figure out where the gain setting resistors are, you can set the gain up for what the ICs or devices will do.

What got me excitedabout his is a pair of Extron "DA6 RGBHV" video dist. amps. Tis specs are here:
https://www.extron.com/product/da6rgbhv

These use the OPA3691 Triple Wideband Current-Feedback op amp.
https://www.ti.com/lit/ds/symlink/opa3691.pdf
There are plenty of application circuits in the data sheet. From what I understand it looks like a really good amp.

The caveat is I don't understand the noise calculations in the data sheet -how to know the noise factor on these. Has anyone experimented with the later wideband distribution amps?
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« Reply #1 on: March 24, 2021, 10:18:11 PM »

It's going to be insignificant, imo. WAY under the ambient RFI noise/QRN/QRM at ur QTH.
Probably under the ambient noise even at a very quiet QTH.

These distribution amps, afaik, do not have gain, they're buffers.
Should work gangbusters, especially if they can be had for low $$.

How much are they going for?

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Opcom
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« Reply #2 on: April 01, 2021, 03:04:17 AM »

You are right they are just buffers. Some have a slight adjustment for gain but not like what we'd see on a radio.

Used, guaranteed professional ones cost $15-30. Those also have all metal cabinets which is a plus. LOL so is "free shipping"!

That's good news about the noise!

Some things come to mind, before I try them out. I already bought a 3 pro units just to experiment with. An Extron, a Kramer, and a Sgma. But the comments below are general to video DAs.

Some have a jumper for DC coupling option, so check that it's set for AC.

Terminate inputs and outputs with 75-1000 Ohms. It's usually not critical.

Video DAs have large electrolytic input and output coupling caps. Consider inserting a small capacitor like 0.01uF in series with the antenna input lead to limit the response to RF-only. 0.01uF is 76 ohms @ 210KC and only 9 Ohms at 1700KC

Some modern solid state gear might not like the audio-range signals these boxes will pass, or the low frequency transients of 3-5V from some of them when the DA is turned on and off. It's like an audio amp 'thump'.
If that is happening at the output going to the RX, it might be cured by a 0.01uF series cap, or maybe an RF transformer, or maybe a pair of reversed parallel diodes. I'm going to check with a scope and see what the ones I bought are doing in the transient department.

 Smiley
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« Reply #3 on: April 01, 2021, 05:38:03 PM »

Neat idea!  I've worked with some of these in the video world.  FYI, I would rank the brand names you mentioned from best to worst: Extron, Sigma, Kramer.  It probably won't mean much for RF, but just my 2 cents from experience.
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