FM Composite isolation

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davek
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FM Composite isolation

Post by davek » Fri Jan 20, 2017 9:39 pm

I have a question for the brains trust:

I may be facing a situation where composite program needs to be fed into a FM exciter. However the input equipment and transmitter are on separate floors, and are on separate power feeds. I don't have a lot of control over how this situation came to be, but I am anticipating that there will be a problem running unbalanced composite between the input equipment and exciter.

The program is delivered to the TX site on a composite link, and the exciter is a Digit-CD (ie: no balanced composite input).

In this situation, would a video isolation transformer work to break the ground loop? Or is there something else I might be able to use?

Thanks

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Re: FM Composite isolation

Post by ChuckG » Fri Jan 20, 2017 10:14 pm

We used 111-C repeat coils for many years to isolate those newfangled Optimods from old Collins exciters. Worked fine.
The larger issue in my mind is the capacitance of the cable between your composite source and exciter input. It will roll off high frequencies first, starting with your stereo separation. I generally keep that cable as short as possible- on the order of 2-3 feet.
Can you move the composite source closer to the exciter? What exactly is the composite source and what feeds it?
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Deep Thought
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Re: FM Composite isolation

Post by Deep Thought » Fri Jan 20, 2017 10:25 pm

As Chuck said your biggest issue is not going to be a ground loop...it'll be the capacitance of the coax which will rob you of stereo separation.

You would be much better off moving the exciter to the composite STL receiver location and running RF and control lines from the transmitter to the exciter if the distance is more than a few dozen feet. If you can keep it under 50 feet something like LMR198 should work for the composite run. A video isolation transformer will work to break any ground loops.
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davek
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Re: FM Composite isolation

Post by davek » Sat Jan 21, 2017 12:09 am

Thanks for your thoughts.

Chuck: The composite source is a STL receiver. Cable length is about 50 feet (which is not ideal) but this will be a temporary situation while the main transmitter is worked on.

DT: Good to know the video transformer stands a chance of breaking the ground loop. I'll take some measurements to determine the level of effect the longer cable will have on performance and report back.

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Dale H. Cook
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Re: FM Composite isolation

Post by Dale H. Cook » Sat Jan 21, 2017 8:35 am

davek wrote:
Sat Jan 21, 2017 12:09 am
DT: Good to know the video transformer stands a chance of breaking the ground loop.
I have used them for the same purpose, and was ready to suggest it before I saw that Deep had already done so. He is, of course, also correct about the role of coax in affecting separation. The Optimod 8100 manual specified no more than 15 feet of RG-58A/U between the composite output and the exciter input. That was in part because the 8100 had a fairly high output impedance of 470 Ohms. It suggested using RG-62A/U for runs up to 15 feet. It had instructions on a modification to reduce the output impedance, if needed. Above all, though, it recommended moving the exciter close to the Optimod.

The same holds true in your case. I have done something similar in the past, mounting the exciter in the rack holding the STL receiver, although I have never had a run anywhere near 50 feet between exciter and transmitter. If the exciter has sufficient RF output available to overcome the loss in 50 feet of coax and still supply sufficient drive to the transmitter that would be the best solution.
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Re: FM Composite isolation

Post by BigRed » Sat Jan 21, 2017 3:31 pm

A couple of thoughts . . .

The 111C repeater transformers have no problem passing composite audio. More than one Gates TE-3 had one on the composite input, in addition to those afore mentioned Collins rigs. But . . . no guarantees on the group delay across 100-kHz. And then there's the problem of even finding a pair these days. It's doubtful that a lesser transformer (less "iron") will cut it. If you did have a pair you might be able to make a point-to-point composite transmission system but you're going to have to pay close attention to the Z of the system between the input transformer and the output transformer. And then there's the loss . . .

A far better option would be a Composite Line Driver system. At one time Modulation Sciences had one good for a couple of miles. I think up to 10-miles with modification. It worked on computer cable (the old "Twinax", not Cat 5). Dumped composite in at the STL receiver end and got composite out of the end at the transmitter. If Mod Sciences can't supply one there may be a used one kicking around somewhere. If not, these guys appear to have one available now :

https://www.profmbroadcast.com/product/ ... tchLang=en

(That Mod Sciences box worked very well, used one once in prep for analog AM stereo. STL receiver and the composite transmit end of the driver was at the base of one of the towers and fed a piece of Twinax to the transmitter building.)

The other thing to do is to use a low capacity coaxial cable for the composite runs. At 93-ohms RG-62 is the preferred choice. If I remember correctly Orban recommended RG-62 in the Oh!My!God! 8000 manual. (Typically the higher the cable impedance the lower the capacity/foot.) Pay no mind to the impedance We're NOT transferring power so that's not a concern in this case. Back in the days of that box it didn't take much capacity to upset the performance of an IC output. They're much better these days but keeping the capacity to a minimum is still a good idea. .

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Re: FM Composite isolation

Post by Kelly » Sat Jan 21, 2017 7:23 pm

I've used an analog video DA to split composite audio feeds to more than one exciter with good results. Just use a professional one, not a consumer version with RCA connectors. I've had good luck with the ESE brand DA's. That will give you pretty good isolation and will help make up losses over long composite runs.
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Re: FM Composite isolation

Post by dbuckley » Sun Jan 22, 2017 2:33 pm

I'm shooting from the hip here, but howzabout a pair of video baluns to convert the signal to balanced Cat5? Composite video goes to about 6MHz, so should be able to handle the MPX composite.

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Re: FM Composite isolation

Post by NECRAT » Mon Jan 23, 2017 10:32 am

Kelly wrote:
Sat Jan 21, 2017 7:23 pm
I've used an analog video DA to split composite audio feeds to more than one exciter with good results. Just use a professional one, not a consumer version with RCA connectors. I've had good luck with the ESE brand DA's. That will give you pretty good isolation and will help make up losses over long composite runs.
And honestly, you could easily find a B Stock video DA without much effort. Stations all over the country have perfectly good composite DAs sitting on shelves waiting for the recyclers, or collecting dust.
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davek
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Re: FM Composite isolation

Post by davek » Mon Jan 23, 2017 11:13 pm

Well curiosity got the better of me. I made a test measurement of stereo cross-talk using a 3FT length of Belden 1694A between stereo generator and exciter. Then repeated the test with an additional 100FT of Belden 1694A. I was quite surprised actually how little difference the extra long piece of cable made to the stereo cross-talk figures.

L -> R degraded by 3.7dB, R -> L degraded by 3.2dB.

Given this is only for a temporary transmitter setup, this result seems acceptable.

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Re: FM Composite isolation

Post by BigRed » Mon Jan 23, 2017 11:29 pm

That 75-ohm hose has similar capacitance/foot to RG-62. RG-62 weighs in at around 13.5 pF/foot, 1694A has a capacitance of about 16.2 pF/foot. Now try it with about 100-feet of RG-58 which is about double RG-62 at 26.4 pF/foot.

I'd be curious to see how a 100-foot run of RG-58 degrades the composite signal though. Might not have as much effect as feared if the STL's composite output is of sufficiently low Z and can drive capacitive reactants without issue.

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Re: FM Composite isolation

Post by TPT » Tue Jan 24, 2017 7:40 am

Long runs for composite audio can be a problem with modern transmitters with integrated exciters. Because of the layout of my transmitter building I have 25 feet of cable between the Omnia 3T and the exciter in my Nautel NV10lt. There's no-where to mount the Omnia in the transmitter--and the exciter can't be pulled out of the transmitter.

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Re: FM Composite isolation

Post by KPJL FM » Tue Jan 24, 2017 8:46 am

davek wrote:
Fri Jan 20, 2017 9:39 pm
I have a question for the brains trust:

I may be facing a situation where composite program needs to be fed into a FM exciter. However the input equipment and transmitter are on separate floors, and are on separate power feeds. I don't have a lot of control over how this situation came to be, but I am anticipating that there will be a problem running unbalanced composite between the input equipment and exciter.

The program is delivered to the TX site on a composite link, and the exciter is a Digit-CD (ie: no balanced composite input).

In this situation, would a video isolation transformer work to break the ground loop? Or is there something else I might be able to use?

Thanks
That doesn't make sense to me. Input equipment? You mean Stereo processor/generator with composite (only) output? Feeding a Digit on a separate floor? But there is another site? Being fed "on a composite link"? What kind of link? Sorry, but that just doesn't compute with me.
If you really mean the processor has to feed an STL xmtr on another floor, could you move the STL xmtr next to the processor, just extend the STL RF output?
Or if you mean the STL receiver is on a different floor than the Digit exciter in the FM transmitter, the STL rcvr only has composite out while the Digit is set for no analog in? Digits can be configured for temporary or permanent composite input. Many have done that just because the composite can be driven more and cleaner that the digital input.
If this is all for a temporary setup, you may be over-thinking it. As long as the commercials can be heard, anything you do is good enough. Just ask the SM/GM. :roll:
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