50 MHz DATV - plans and ideas...

The place to discuss reduced bandwidth fast scan amateur television on 70cms and below
G8PEF
Posts: 84
Joined: Mon Oct 30, 2017 6:42 pm

Re: 50 MHz DATV - plans and ideas...

Post by G8PEF » Tue Feb 13, 2018 7:34 am

Last night at a club meeting, I was asked if Portsdown could do RBTV on 50MHz …

I was sure I'd seen some discussion on the Portsdown thread(s), but I can't (despite a lot of s arching) find anything.

So, is anyone using Portsdown on 6m? Any particular frequency/SR? Any tips for setting up - filters, PAs etc?

73
John

g8gtz
Posts: 973
Joined: Wed Apr 07, 2010 6:26 pm

Re: 50 MHz DATV - plans and ideas...

Post by g8gtz » Tue Feb 13, 2018 8:49 am

Hi John,

The specs for Portsdown are here https://wiki.batc.org.uk/Portsdown_Output_Levels

As you can see it does not cover 50 MHz natively but an external; converter can be used - here's one Mike is proposing for 71MHz rxr and could be used on 50 MHz tx and Rx. viewtopic.php?f=76&t=4108

One or two people are "thinking" about 50 MHz so maybe it's time for some experimentation...

73's Noel - G8GTZ

g0mjw
Posts: 571
Joined: Sat Sep 20, 2014 9:15 am

Re: 50 MHz DATV - plans and ideas...

Post by g0mjw » Tue Feb 13, 2018 6:34 pm

Why can't the Portsdown be made to TX on 50 MHz? The LO goes down to 35MHz. Is it the modulator chip?

My receiver PCB (Noel has one now and is patiently waiting for me to send him the schematic) requires a 400MHz LO.
Converter prototype.jpg
Converter prototype.jpg (318.87 KiB) Viewed 641 times
Plan is to use the spare output from the Portsdown in CW mode. It filters 50-70MHz, amplifies and converts it to 450 - 470MHz for input to the Minitiouner. There are many different ways you could use the board - no requirement to stick with it's intended use. For a TX converter for example. It can't make tea.

Mike

G8GKQ
Site Admin
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Re: 50 MHz DATV - plans and ideas...

Post by G8GKQ » Tue Feb 13, 2018 11:32 pm

Hi Mike

The problem with the Portsdown is that the QPSK modulator chip is only in-spec from 140 - 1000 MHz. It’s just about OK at 71 MHz, but gets worse quickly below that. The constellation becomes trapezoidal.

Dave

g0mjw
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Joined: Sat Sep 20, 2014 9:15 am

Re: 50 MHz DATV - plans and ideas...

Post by g0mjw » Wed Feb 14, 2018 7:49 am

G8GKQ wrote:
Tue Feb 13, 2018 11:32 pm
Hi Mike

The problem with the Portsdown is that the QPSK modulator chip is only in-spec from 140 - 1000 MHz. It’s just about OK at 71 MHz, but gets worse quickly below that. The constellation becomes trapezoidal.

Dave
I see - so we either need a RX/TX frequency converter or another modulator. My current board will do that but only if the active device is bypassed to make it bi-directional. Then there would be a need for additional gain to overcome the ~10dB losses mostly important for the RX side. Maybe easier to use two boards one passive for TX, one active for RX. Or I could re-design I suppose but based on the level of interest so far I doubt there is any demand. The Pluto and Lime can do 50MHz and both are about to re breadily available again.

Mike

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