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" <[email protected]>
To: <[email protected]>
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...
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