Poor constellations.

Discussion about this major DATV Project. See https://wiki.batc.org.uk/The_Portsdown_Transmitter
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M5TXJ
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Joined: Wed Jul 05, 2017 8:34 pm

Poor constellations.

Post by M5TXJ » Wed Aug 15, 2018 2:17 pm

Gents,
Do you think that radiated signals from the Pi could cause the problem I have? Solutions tried so far;
Added extra filtering to PSU's.
Tried signal direct to Minitioune from FM board via attenuator.
Powered FM board from battery.
Powered LO board from battery.
Fed LO filter board from sig gen instead of LO board.
Fed FM board direct from sig gen instead of LO board.
Any suggestions gratefully received.
73 Dave
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...are you sure I can't use a pair of 813's?... :shock:

G8GKQ
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Re: Poor constellations.

Post by G8GKQ » Wed Aug 15, 2018 3:19 pm

Dave

These constellations are nothing to worry about. They will reduce your weak-signal readability by less than a dB.

There is a long story as to why you have ended up with a system that has constellations like this. The short story is that the IQ filtering on the Portsdown F-M board is slightly too sharp, and the Serit tuner is very fussy. All the development and testing on the V2 board (that I believe you have) was conducted using the older Sharp Tuner, which shows a perfect constellation for that filter setting.

The modifications to widen the IQ filter bandwidths (only required at 500KS and below) just involves 4 parallel resistors or replacements:

680K in parallel with R24A (98.0K) [or replace R24A with 86.52K Digi-Key 311-86.6KFRTR-ND]
390K in parallel with R24B (49.0K) [or replace R24B with 43.26K Digi-Key 311-43.2KFRCT-ND]
270K in parallel with R24C (36.8K) [or replace R24C with 32.5K Digi-Key 311-32.4KFRCT-ND]
180K in parallel with R24D (24.5K) [or replace R24D with 21.6K Digi-Key 311-21.5KFRCT-ND]

R24A through D are on the underside of the board and you can use resistors with leads across them in parallel. These modifications are not required on the latest (400 series) BATC Shop boards.

Hope that helps. I'm still running with the same settings as you have.

Dave

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M5TXJ
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Re: Poor constellations.

Post by M5TXJ » Wed Aug 15, 2018 3:47 pm

Cheers Dave.

I should have posted a week ago, I've been driving myself nuts trying to find the cause!

73 T'other Dave.
...are you sure I can't use a pair of 813's?... :shock:

G8GKQ
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Re: Poor constellations.

Post by G8GKQ » Wed Aug 15, 2018 5:01 pm

Sorry Dave

Now added to the Wiki: https://wiki.batc.org.uk/Filter_Modulat ... Serial_400

Dave

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M5TXJ
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Re: Poor constellations.

Post by M5TXJ » Wed Aug 15, 2018 5:33 pm

No problem Dave, at least it looks a bit better now, indicated mer varying between 23-25db where it never got above 18db before.

73 Dave.
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...are you sure I can't use a pair of 813's?... :shock:

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