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Message-ID: <02E3EF3F48994AB88A1489042CE7EA02@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <12077821.1264946261487.JavaMail.root@wamui-cynical.atl.sa.earthlink.net> <38A51B74B884D74083D7950AD0DD85E82A1ACF@File-Server-HST.hst.e-technik.tu-darmstadt.de> <4C25D51A6FE646D5B8D9D37C87EA6BBC@lindavideo>
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Date: Tue, 2 Feb 2010 11:57:13 -0000
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Subject: LF: Loop preamp with the BF862 / optical link
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Dear Ken, Stefan, LF Group,

If I remember correctly, the components Stefan is discussing use a polymer 
fibre and are intended for short-distance optical links of a few metres, 
rather than telecomms-type applications with miles of fibre, so fibre loss 
is not really an issue. The ones I encountered some years ago used visible 
red LEDs, although I think there were infra-red ones too. The plastic fibre 
could be simply cut with a sharp knife, and clamped into the 
emitter/detector components with reasonable results, rather than requiring 
precision optical connectors. There is an overview at 
http://www.avagotech.com/docs/AV00-0143EN

Of course, since the optical link is effectively a signal-frequency gain 
component in the receiver front end, its noise figure will contribute 
towards the receiver noise figure. Since the overall efficiency of the 
electrical/optical/electrical conversion is rather low, with the loss of the 
fibre in addition, there will be much less signal power coming out of the 
detector than going in to the emitter. Also the detector and emitter will 
contribute their own noise. So the noise figure of the optical link by 
itself is probably rather high. But the FET voltage-to-current input stage 
driving the emitter will have a high power gain due to the very high input 
impedance, which would greatly reduce the impact of noise in the optical 
link - clearly the overall noise level in Stefan's system is reasonably low.

Most optical links are used with digital signals, so this analogue optical 
link is quite unusual - as well as noise, distortion products will also be 
produced by the optical link. But DK7FC's antenna obviously works, so the 
principle is viable.

Cheers, Jim Moritz
73 de M0BMU


----- Original Message ----- 
From: "Ken" <ken.h.wright@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, February 01, 2010 5:56 PM
Subject: 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...