Re: LF: Loop preamp with the BF862 / optical link

[Andy Talbot <andy.g4jnt@googlemail.com>]

An experimental system I tested for work a few years ago used the low cost optical components from RS, the HBR2404 and 1404 devices with polymer cable.  This was for a remote wide band antenna with full galvanic isolation.   Measurements suggested 60dB dynamic range was feasible and an anlogue bandwidth in excess of 200MHz from a discrete drive and the optiacl recevers own internal preamp.    Noise figure wasn't a problem, but I do recall it being the the 'sub 10dB' region, adequate for hight HF through VHF.

 

The 60dB dynamic range was not good enough for our HF bands requirements, so another route was taken , but when used with a high Q antenna over a limited band, should be quite adequate. 

 

The old analogue cable TV systems used fibre (to the street corner where it was turned directly into RF for the coax final run.  Based on a few back-of-envelope calculations, at the bandwidth needed of 2GHz or so, must have had a total effective dynamic range in the 80dB region to avoid visible degradation on nearly 100 analogue TV signals.

 

 

Andy
www.g4jnt.com

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On 2 February 2010 11:57, James Moritz <james.moritz@btopenworld.com> wrote:

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...