Hey Ken,
Tnx for your statement. Well, i am not an expert in optic links and the parts i
choose are probably not the best, that are available for this application.
Theese parts were available for me and they are suitable in principle, although
the attenuation may be some dB less. Today i measured the gain of a single
BF862 in a preamp application for LF and i calculated a voltage gain of 26dB.
Well, in that case it is not so important for me if the cable has 3dB loss or
0,03dB loss. The main aspect is, that the local noise is decoupled. Because if
it is not decoupled, the SNR can quickly decrease by more than 20dB, as i saw.
And 3dB losses are not 3dB less SNR.
Perhaps the transmission system can be improved by using special parts. But are
they available for everyone (private, not commercial customers) and are they in
a range below 5EUR? Perhaps you can tell me/us a more suitable type of cable
that is perhaps even cheeper?
The parts i use in the moment have no special mounting. The fiber optic cable
is just cut with a gripper and stuck plugged in the housing of the LED, thats
all. And it can be done many times, thats important...
So, do you have an idea of better parts that are cheep and available, with
variable length of the cable, so not preassembled? :-)
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Ken
Gesendet: Mo 01.02.2010 18:56
An: [email protected]
Betreff: Re: LF: 2nd try of sending the pic of my loop preamp with the BF862
Hi Stefan.
150dB/km, is a very high loss for modern fibre, even thhe early 850nm
multimode fibre was only 1 or 2dB /km and modern single mode fibre at 1300nm
or 1550nm the attenuation is substantially less than 1dB/km. What optical
wavelength is the data sheet quoting this attenuation figure at? If you are
using a red LED into a single mode fibre then the attenuation will be high,
the launch power will also depend on how the LED is coupled into the fibre,
ideally the LED should be mounted in mating coupler so the so the fibre may
be mated up to it, air gaps of fractions of a millimeter will introduce
attenuation. The same applies when you mount your receive photodiode. In the
1980's and 90,s I spent many years both specifying and commissioning optical
line sysyems in the UK. I hope this info will be of use to you.
73s.
Ken
M0KHW
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
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
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
150dB/km, so abt 3dB for my 20m cable. I have tried a cable of 2m (for
indoor tests) and changed the cable to 20m. The difference (e.g. of DCF39 in
the Argo monitor) is less than one S unit.
In the moment i use a SFH350V (photo transistor)as the receiver. But
recently i read in the web that a photodiode has (significant) less noise.
So i want to try a SFH250V in a next step to reduce the overall noise,
probably in combination with a second BF862 in the input stage of the RX.
The SFH250V has even much better switching times. I have already ordered
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
years in the summer sun. But the distributor told me, it will be no problem
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
the system. This gain is not reached without a noise (e.g. by a OPA) but
this noise can be much smaller than the noise that you fed into a sensitive
RX antenna, i expect. But the main aspect is, that one doesn't have to worry
any more if it could be possible that a noise is fed from the PC/RX/house to
the preamp by the cable.
73s, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Chris Trask
Gesendet: So 31.01.2010 14:57
An: [email protected]; [email protected]
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
RX cable. What are the loss characteristics of the optical link, including
the diode(s)?
Chris Trask
N7ZWY
WDX3HLB
Senior Member IEEE
http://www.home.earthlink.net/~christrask/
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