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PostPosted: Wed Jan 04, 2017 6:49 pm 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 72
Hmmm, not sure why you would introduce a third transmitter to keep the other 2 in sync.

In terms of a perfect overlap, there is no such thing.

Have you configured SFN in production before ?


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PostPosted: Wed Jan 04, 2017 7:43 pm 
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Joined: Mon Mar 17, 2008 1:40 am
Posts: 8609
There already is such a clock: GPS!


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PostPosted: Sun Jan 22, 2017 1:42 pm 

Joined: Mon Sep 23, 2013 11:17 am
Posts: 5
PLLs in transmitters all need a reference frequency to keep the transmitter on the correct frequency this is almost always a Crystal Oscillator or it can be an external reference of the same source if you're synchronizing two transmitters on the same frequency.

As an example the NRG PLL Pro4 uses a 6.4Mhz crystal which is divided down to 12.5khz. The RF is also sampled and divided down according to the frequency set on the 4059 divider IC, the result should also be 12.5khz for the RF and the two are fed into an ExOR gate for comparison and to generate a correction voltage by taking the average by using a loop filter, this voltage drives the varicaps which keep the carrier on the correct frequency.

To synchronize two transmitters, both carriers *must* have the same phase angle and offset or you'll get fm capture affect and or multipath distortion. This requires that the PLL reference on both transmitters also has the same phase angle/offset. GPS is one way to do it, however you can also use any other reliable reference that can be received at both transmitter locations. Local medium wave stations can be a reliable source for a reference, I've even heard of the 19khz fm pilot tone being used from analogue stl's using the tv bands.


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PostPosted: Tue Jan 24, 2017 8:03 pm 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 72
That's a lot of drama to synchronize signals.... I've looked at another broadcaster's configuration in an alternate location and even with all the best tech they could buy, the cross-over point was still rubbish.

I was able to successfully configure a cross-over point between 2 towns in between 2 mountains where normal reception was going to be pretty average anyway, so we masked the sync point that way and adjusted power at the remote site until it sounded right.

Now whenever you drive between the two locations it's pretty good.... it will never be perfect but blended stereo sound between to big mountains is better than the awful sound we previously had. At least now you can listen.

Our competitor has worse coverage through the mountain because their primary signal is about 200w EIRP lower and their remote site is further away so they don't have a cross-over point, instead opting for different frequencies. Initially I thought that approach would be easier, but what we discovered was when you are driving through mountains on winding roads, it's best to keep your eyes on the road rather than finding the alternate frequency for a radio station (AF is a challenge because even after switching to the new frequency coverage is again affected by the next mountain).

Running SFN between the two locations works well. If the config was between 2 sites out in the open, I would probably not run a SFN as I think it would be too difficult to obtain a decent cross-over.

/My2c.

BeDazzler.


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PostPosted: Wed Jan 25, 2017 2:04 am 

Joined: Mon Sep 23, 2013 11:17 am
Posts: 5
It all comes down to having both carriers perfectly synchronized, and it can be done but is very difficult.

Each transmitter antenna must be phase coherent with the others also or you'll get offset resulting in multipath, each feed line to each antenna in the SFN must be the same length because through coaxial cable you've got a velocity factor to deal with, i.e. RF doesn't travel at the speed of light over the cable and you'll get delay on one if its longer than the other etc...


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PostPosted: Wed Jan 25, 2017 3:21 am 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 72
Yep, it ultimately does work better with perfect synchronization, however in a real world scenario it highly unlikely that will be achieved.

The StereoTool SFN feature worked pretty well in our case and I wouldn't over-complicate a SFN setup as there will always be an element you can't control, so just do your best.


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PostPosted: Thu Jan 26, 2017 1:17 pm 
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Joined: Thu Jul 01, 2010 12:42 pm
Posts: 41
Location: Australia
Excellent thread, well done to all that contributed to the education of those needing filler SFN transmitters.

BeDazzler, it would be great if your able to provide a basic map of the transmission areas, with the goal of assisting those in design & implementation?


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