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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