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Re: LF: A question to the experts

To: [email protected]
Subject: Re: LF: A question to the experts
From: DK7FC <[email protected]>
Date: Tue, 08 Apr 2014 14:43:18 +0200
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Hi Tobias,

Am 07.04.2014 23:33, schrieb Tobias DG3LV:
[...]it would be best to first express your design-target in more detail:

What minimum audio/IF-bandwith is acceptable for the MF/LF-receiver?
Well, for MF and LF, 10 kHz would be sufficient. For VLF, 24 or even 48 kHz would be fine. If it is possible (data rate (meanwhile i think i learned to calculate the data rate ;-) )) to use a 1 MHz sample rate ADC, then one could directly sample VLF to MF !! That may be the best option, because then there is no need to use an extra converter. This reduces the complexity of the circuit and the power consumption, i assume. So the limiting factor would be the bandwidth of the 2.4 Ghz device. I found some with 2 Mbps, also ready to use modules (don't want to spend to much time with that GHz stuff). With a ADC covering VLF to MF, one just may need a few notch filters for stronger local out of band stations.
OK, the answer to your question: 500 kHz.

What minimum dynamic range (in dB) is acceptable?
This has directly to do with the bits of the ADC, i learned. Well, for a directy sampling of the 2200m band we have to deal with DCF39 and HGA22 here. And if the 'far end device' has to be compact and has to deal with high temperature differences, we can't use complex high Q coils to get an extreme rejection of DCF39... Thus, i think we need at least 100 dB of dynamic range. So, from what i saw from the data sheets, we would need a 24 bit device. I also learned that it is not possible just to connect the 2 SPI ports of the two devices. So it becomes more complex.
What is the distance to be covered by the link-connection ?
2km or 3 km. It all depends of what is possible! I will need high gain yagis on both sides. But these are quite cheap avaliable at amazon or ebay.
Optical sight ?
Yes.

Is it possible to erect a (small, DIN A4) solar power module at the MF/LF-receiver site?
Maybe, if necessary, yes. But it will be more expensive and more risky of someone discovers and damages the setup when hanging in a tree in the forest. Thus i would like to keep the costs below 50 EUR, the costs of the far end system. It is not my private property so it must be hidden in a high tree that i can climb.
Shall the MF/LF-receiver be operated 24/7 ?
Yes. Ideally VLF to MF. But there can be compromises. Actually MF was the main focus. So, 8 kHz of BW and 80 dB dynamic range and 16 bit would be ok too. You see, the specifications are not totally fixed. It depends on what is possible. The limiting factor will be the data rate and the costs and the power consumption of the far end device.
What program will be used at your site in the institut, spectrum lab or different?
SpecLab!
But i don't know, when i would use that 1 MHz sample rate ADC, it may be more useful to realise a direct digital link, USB or LAN. This may have advantages but then the system becomes not so flexible for others.
Knowing this (and probably more) could help to design a suitable
link-connection between the remote MF/LF-receiver and your windows-PC.
The technology to be used cannot be specified without knowing this.
Yes that is clear of course.
But as you see there is a wide range of wishes and possibilities and limiting factors. Generally it would be nice to play arround with this stuff and get a sloution as useful as possible. So what is possible? And are there other factors that i don't see yet? Maybe the stability of the GHz link over that distance? At least i like the idea to learn about this stuff but i am on the first small steps. It looks like a completely different world of electronics to me! And it looks like all the informations are available in the web. The twente web SDR shows what is possible! But i don't need a solution reaching up to 30 MHz. 0...500 kHz is fine!
73 de dg3lv Tobias
(you have received this text in german language a week ago)
Did i?

Is that right, 500 kHz BW and 24 bit = 12 Mbps ?

It may appear embrassing to demonstrate my limits of knowledge about that digital stuff but i decided to stay self-reliant here. These limits can be pushed in one evening i know. It just wasn't necessary so far...

73, Stefan/DK7FC


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