Digithin as a starter project

The place for discussion about the RPi add-on RB-TV modulator card developed by G4EWJ and supported by the BATC Shop
g8gtz
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Digithin as a starter project

Post by g8gtz » Tue Jul 14, 2015 6:59 pm

As Steve mentioned in his QRP post, there is real potential with the Digithin / Minituner projects to make a great starter project that we can use to get people on air easily and even push in to local radio clubs as an ideal club project.

For transmit you have the Rpi (everyone has one in cupboard!), plus Evariste’s software and the BATC Digithin kit which could be upgraded to include the 200 Mhz Si570 to produce an output on both 146 and/or 437 MHz. This can be used barefoot, as Steve and Arthur have proven, to work across town and get that first QSO.

Rob M0DTS published the design for the RD06 and RD15 146 MHz PA which is inexpensive and easy to build but will need a PCB layout (including an input filter) that works as it is quite prone to hoot due to the overall gain. Alternatively someone like Minikits may do something suitable.

For Rx we have Minitune USB kit with PCB and Minitioune software which will now run on any PC and to rx 437MHz we have the SUP L band upconverter.

However the missing link is an upconverter for 146MHz. We either need to find a consumer device or a design for a simple upconverter to L band with an F type output powered from the Nim.... Or maybe we can tweak the SUP oscillator down by 200 MHz???

Put these together with standard 2mt beam and pre-amp that people tend to have and you have a system with a total cost between £100 - £150 that produces excellent video and is good enough for people to work across town and get a real taste of ATV. Once they get the bug, the equipment they have, can be used to work better DX on 146MHz (100 KMs under flat band condx) & 437MHz and used as the basis for other bands.

We could do a CQ-TV style system book on how to put it all together, sell the components at cost through the BATC shop and we have something we can push out as a starters kit and try to get some radio clubs to adopt as a club project....

Thoughts / ideas particularly on the missing link of a 146MHz to L band u/converter.

73's

Noel - G8GTZ

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G1LPS Terry
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Re: Digithin as a starter project

Post by G1LPS Terry » Wed Jul 15, 2015 12:40 am

Hi Noel
Some good ideas there to promote and encourage others to join the ATV community.
If we could find a source of the sharp or samsung nim to stock then the SUP2400 would be ok for 2 meters rx also.
I often use one here for that purpose as its easy to implement. Works very well at 333ks. under performs at the lowest symbol rates though (phase noise?). It is a good match for the digithin at 333ks. Give it a go on a tt1600 card or sharp nim if you have one. I use a 5 pound satellite line amp between the sup2400 and rx to liven it up.

cheers Terry G1LPS

g8vpg
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Re: Digithin as a starter project

Post by g8vpg » Wed Jul 15, 2015 8:39 am

Noel,
Lots of good ideas there! For my 146.5MHz upconverter, I used a standard commercial mixer module (SRA-2, which I bought cheap at a rally, but there are lots of others available) and a crystal controlled 1152MHz source. This used a 96MHz crystal oscillator which is multiplied up twelve times. The design is quite an old one from the RSGB Microwave Handbook, but there are quite a few designs for these about because they are a standard component of 144/1296 narrowband transverters. I also used the G4DDK 2m pre-amp kit based on the PGA103.
Shaun.

g8gtz
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Re: Digithin as a starter project

Post by g8gtz » Wed Jul 15, 2015 9:15 am

Couple of comments:

The problem with the SUP2400 on 146 MHz is that it puts the signal above the normal L band tuning range at 2254 MHz so the Eardatek won't rx it. We've looked everywhere for Sharp and Samsung tuners, which have extended tuning range, and only managed to get a very low quantity at double the price of the Eardatek.

Re converter - yes, essentially all it is is an LO, mixer, amp and filter and we need a reproducible design -I did wonder about the Ultram Digilite units we stock in the shop but at £39 they are too expensive for a rx LO.

73

Noel

G4EWJ
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Re: Digithin as a starter project

Post by G4EWJ » Wed Jul 15, 2015 2:50 pm

Has anyone tried changing the 9.375MHz crystal in the SUP-2400? The next lowest one I can find is 8.912MHz, which would bring 146 out on 2135.

Brian

G4EWJ
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Re: Digithin as a starter project

Post by G4EWJ » Sun Jul 19, 2015 3:31 pm

I changed the crystal in a SUP-2400 to 8.912MHz to give an LO of 2281.5MHz and a transmission from a handheld on 146MHz gave a solid black screen on the analogue satellite receiver tuned to 2135MHz. I'll have to set up my DigiThin on 146.5 MHz to try it for real.

I still haven't managed to take off one of these surface mount crystals without damaging a pad, but there are plenty of places to connect back on to.

Brian

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G1LPS Terry
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Re: Digithin as a starter project

Post by G1LPS Terry » Sun Jul 19, 2015 9:17 pm

That sounds promising Brian.

I tried a global adx plus from the junk box(Astra 1d days to shift frequency ON 1750MHZ satellite receivers )
There is a switch inside that changes it from a up-coverter to a down-converter. 500mhz
Some may have these in the scrap boxes.
I worked across the shack wi8th the SUP2400

Also tried it to extend a 2ghz sat rx to rx 2440mhz that worked also

G4EWJ
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Re: Digithin as a starter project

Post by G4EWJ » Sun Jul 19, 2015 9:34 pm

I also want to try that SUP-2400 on 13cm with an MMDS antenna. I did try with a standard frequency SUP-2400, but I think the rx frequency was near 2400 and the rx was getting swamped.

I remember the ADX. I was just starting with satellites when 1D came on.

Brian

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G1LPS Terry
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Re: Digithin as a starter project

Post by G1LPS Terry » Mon Jul 20, 2015 12:02 am

hI Brian
I have used the Sup2400 with an MMDS down-converter.
This one
http://www.ebay.co.uk/itm/MMDS-AERIAL-D ... 2a2e13e020

I found it an excellent RX setup on 13cm. Had great results using one out portable

There's a small re crystal mod to do to them see Rob M0DTS site for details

Terry

G4EWJ
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Re: Digithin as a starter project

Post by G4EWJ » Mon Jul 20, 2015 11:08 am

That's the one. I've done the mod to give it an LO of 2048MHz.

I think the problem was that I had the SUP-2400 very close to the antenna while trying to receive someone on 2395MHz. At least 2281MHz will be outside the passband of the receiver.

I wonder if anyone has had any success with the ISS with one of these? I suspect not.

Brian

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