Portsdown MER and Constellation

Discussion about this major DATV Project. See also https://wiki.batc.tv/The_Portsdown_Transmitter
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N6GKJ
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Joined: Sun Dec 22, 2013 1:35 am

Portsdown MER and Constellation

Post by N6GKJ » Thu Nov 08, 2018 6:19 pm

Guys

I am uploading a screen capture from Mini-Tiouner. MER seems low and constelltion does not look right.

This is a later Filter Modulator board. No PLL filter board, no attenuator board. I have not gone in and set any level from the PLL.
The output of this board is about +5dbm.

Any hints?

Image
Last edited by N6GKJ on Thu Nov 08, 2018 6:28 pm, edited 3 times in total.
73,

Ron....
N6GKJ CM98ic

M0YDH
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Re: Portsdown MER and Constellation

Post by M0YDH » Fri Nov 09, 2018 11:03 pm

Hello Ronald
The constellation in the picture is distorted because there was no LO filter board in the Portsdown in question. FM board serial #33.
PhotoPictureResizer_180824_182707534-1008x756.jpg
PhotoPictureResizer_180824_182707534-1008x756.jpg (182.09 KiB) Viewed 188 times
I hope this clue is helpful.

73
David M0YDH

G8GKQ
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Re: Portsdown MER and Constellation

Post by G8GKQ » Fri Nov 09, 2018 11:20 pm

Hi Ron

I'm not sure that an MER of 21 is too bad from a filter-modulator board. It's never going to be as good as DATV Express, especially as at a symbol rate of 2222 KS, where you are using the 4000 KS filter. I would estimate that the weak signal performance of your TX is no more than 1 or 2 dB worse than a TX with a much higher MER.

Displayed MER is not consistent across MiniTioune software versions, so do not read too much into it.

Dave

G8GKQ
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Re: Portsdown MER and Constellation

Post by G8GKQ » Sat Nov 10, 2018 9:11 am

Hi Ron

I've just had time to check the BATC Demo Portsdown with exactly the same parameters as your transmission (with the same version of MiniTioune and same RF level) and I get an MER of 21 dB. The constellation looks exactly the same as yours - I don't see anything wrong there either. This is the unit that has achieved multiple contacts over 100 km.

So, I would suggest that you are doing everything right.

You need to be aware that running a symbol rate of 2222 KS will switch in the 4000 KS filter, so the transmission will be wider than the symbol rate suggests. The 2000 KS filter switches in below 2200 KS. You could change this by changing line 117 of rpidatv/scripts/ctlfilter.sh, but you would reduce the transmitted MER slightly.

Hope that reassures you

Dave

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