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Message-ID: <4C25D51A6FE646D5B8D9D37C87EA6BBC@lindavideo>
From: "Ken" <ken.h.wright@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Mon, 1 Feb 2010 17:56:45 -0000
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Subject: Re: LF: 2nd try of sending the pic of my loop preamp with the BF862
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Hi Stefan.
150dB/km, is a very high loss for modern fibre, even thhe early 850nm=
=20
multimode fibre was only 1 or 2dB /km and modern single mode fibre at=
 1300nm=20
or 1550nm the attenuation is substantially  less than 1dB/km. What opt=
ical=20
wavelength is the data sheet quoting this attenuation figure at? If yo=
u are=20
using a red LED into a single mode fibre then the attenuation will be=
 high,=20
the launch power will also depend on how the  LED is coupled into the=
 fibre,=20
ideally the LED should be mounted in mating coupler so the so the fibr=
e may=20
be mated up to it, air gaps of fractions of a millimeter will introduc=
e=20
attenuation. The same applies when you mount your receive photodiode.=
 In the=20
1980's and 90,s I spent many years both specifying and commissioning=
 optical=20
line sysyems in the UK. I hope this info will be of use to you.

73s.
Ken
M0KHW



----- Original Message -----=20
From: "Stefan Sch=E4fer" <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, January 31, 2010 10:43 PM
Subject: AW: LF: 2nd try of sending the pic of my loop preamp with the=
 BF862


Hello Chris,

I am not a communication engineer (but a power electronic engineer ;-)=
 ), so=20
rather an amateur in this field. So my answer is kept simple ;-)

In the datasheet of this cable i just found that attenuation is less=
 than=20
150dB/km, so abt 3dB for my 20m cable. I have tried a cable of 2m (for=
=20
indoor tests) and changed the cable to 20m. The difference (e.g. of DC=
F39 in=20
the Argo monitor) is less than one S unit.

In the moment i use a SFH350V (photo transistor)as the receiver. But=
=20
recently i read in the web that a photodiode has (significant) less no=
ise.=20
So i want to try a SFH250V in a next step to reduce the overall noise,=
=20
probably in combination with a second BF862 in the input stage of the=
 RX.=20
The SFH250V has even much better switching times. I have already order=
ed=20
some of those. I will try this and send the circuit here...

The cable i use has a PE shielding which is not suitable to exist 10s=
 of=20
years in the summer sun. But the distributor told me, it will be no pr=
oblem=20
for some years. And since LF is a living project, this is enough for=
 me.

The losses of the cable must be compensated by a gain of an amplifier=
 within=20
the system. This gain is not reached without a noise (e.g. by a OPA)=
 but=20
this noise can be much smaller than the noise that you fed into a sens=
itive=20
RX antenna, i expect. But the main aspect is, that one doesn't have to=
 worry=20
any more if it could be possible that a noise is fed from the PC/RX/ho=
use to=20
the preamp by the cable.

73s, Stefan/DK7FC



________________________________

Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von Chris Trask
Gesendet: So 31.01.2010 14:57
An: rsgb_lf_group@blacksheep.org; rsgb_lf_group@blacksheep.org
Betreff: Re: LF: 2nd try of sending the pic of my loop preamp with the=
 BF862



>
> Some OMs informed me, that the picture of the loop preamp is
> in a strange design. That may be come, since i (have to) use
> outlook exchange (?)
>
> Now i try to do it again. Those who asked for the picture,
> please give my a short confirmation if the picture is in the
> correct format now.
>
> I do some modifications to the RX in the moment and will send
> the new and perhaps better circuit soon. But the setup is
> already working vy fine!
>

     That is a very interesting approach to minimizing induced noise=
 on the=20
RX cable.  What are the loss characteristics of the optical link, incl=
uding=20
the diode(s)?


Chris Trask
N7ZWY
WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/