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RE: LF: Re: MiniWhip antenna, fiber optic

To: _RSGB_LF <[email protected]>
Subject: RE: LF: Re: MiniWhip antenna, fiber optic
From: Lubos OK2BVG <[email protected]>
Date: Sat, 27 Jul 2013 07:18:39 +0200
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References: <CAKvcm2dpuftdtK6OdQv8fOfiw+4yQrPBQBHJ-FN2gDbT4P_oMQ@mail.gmail.com> <[email protected]> <op.wzq2tgvnyzqh0k@pc-roelof> <005b01ce798a$b0476600$6d01a8c0@DELL4> <op.wzrfpjyyyzqh0k@pc-roelof> <[email protected]> <7D6B8CBB38964C9C82487F5D0F3648FC@FMVXD1232> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <001601ce7c95$f235b360$6401a8c0@JAYDELL> <[email protected]> <[email protected]> <[email protected]> <[email protected]> <op.wz4k19tayzqh0k@pc-roelof> <[email protected]> <op.wz5pxy1kyzqh0k@pc-roelof> <[email protected]>,<[email protected]>
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Hello Stefan and Roelof!
Very interesting. Thanks for all technical information!

Lubos, OK2BVG 


Date: Sat, 27 Jul 2013 02:12:54 +0200
From: [email protected]
To: [email protected]
Subject: Re: LF: Re: MiniWhip antenna, fiber optic

Hello all,

There is a real progress again this night with my fiber optic LF/MF/VLF RX system.

I am now using a SFH250V photo diode (fast switching) and a different optical/electrical converter circuit. The design is taken from the link below. Since there was no LT6200 available this night, i took a LT1028 which works too. I will use the LT6200 later.

This is the schematic and receiver: https://dl.dropboxusercontent.com/u/19882028/MF/20130727_014150.jpg  The 1 pF C across the 1 MOhm resistor is important. Otherwise there are self oscillations to the full supply voltage. It is the first time that i worked with a 1 pF capacitor :-)

Well, there was some lack of sensitivity in the last system, even though the antenna was resonated to 475 kHz. Hence the changes in the design.

Now, the antenna is not resonated and covers VLF/LF/MF, at least! This will make performance measurements in the field more accurate. It is preferred to have a wideband antenna anyway.

I took a spectrum graph of the old system 0...1MHz: https://dl.dropboxusercontent.com/u/19882028/MF/SFH350V%2BSFH750V.png
And this is the new one: https://dl.dropboxusercontent.com/u/19882028/MF/SFH250V%2BSFH750V.png  which shows a much better frequency response.

Compared to the old system the gain on 475 kHz is 24 dB, on 137 kHz it is 23 dB. The coupling of the signal generator to the floating antenna is the same, so there is always an attenuation.

All in all this looks much more promising now and i will do a new /p test on the band soon. BTW the DC working point of the OP at 12V supply voltage is (of course) 6V when the LED transmitter is OFF and it rises to 8V when it is ON. So there is some room for further system gain.
And BTW, the OP can be loaded with 50 Ohm directly...

73, Stefan/DK7FC


Am 26.07.2013 00:29, schrieb Stefan Schäfer:
Hello Roelof,

Is there some progress on your side? I may have some useful informations for you.

In the last test i found that i need more gain and in the end i want to have a fiber optic antenna that covers LF and MF, maybe even VLF. Furthermore a resonated antenna is not suitable when the antenna is sometimes grounded or floating in the tests.
In my circuit the problem is the low rise- and fall- time of the SFH350V. I saw the problem years ago and ordered a few SFH250V photo diodes which should work well. The LED transmitter is suitable i think, although the SFH757 is faster of course. But i'm always looking below 1 MHz.
Now i seareched for some additional ideas for a the optical/electrical converter. Some time ago i built a transimpedance converter but i was sceptic about OP AMPs at 475 kHz. But i think i learned something and got some new ideas on these pages:
http://circuits.linear.com/all--Photodiode_Interface_*_Transimpedance_Amps-53
Seems that these frequencies are no problem at all, even if a high gain is desired. The LT6200 is available at RS, 4.56 EUR here. Quite expensive but worth to play with :-) OK, some may say it is overkill and the device can cause VHF oscillations, but anyway...

I guess it will be no problem to get the needed gain to go on with the experiments, even if no resonated antenna is used... BTW the output current of the device is even 60 mA. So maybe it is even suitable to drive the LED...

What do you think?

73, Stefan/DK7FC



Am 13.07.2013 12:55, schrieb Roelof Bakker:
Hello Edgar,

I don't know how linear these devices are, but that will show up in actual use.

The SFH756V and SFH350V are housed in a moulded plastic casing with a cable clamp:

http://www.avagotech.com/pages/en/fiber_optics/general_purpose_industrial_control_data_link_650nm/sfh756v/

I also found that for a wider bandwidth the SFH 757V / SFH 250V are more suitable.
These are rated at 100 MBd.
I will order them in the near future.

73,
Roelof Bakker, pa0rdt
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