Hello Jim, Andy, Sam, Ken, Group ;-)
Excellent comments! Thank you for these explanations. I see, it is your
business.
And what i read tells me, that the optic way cannot be the wrong in principle.
Recently i even got a mail from a OM who was going qrt due to the (local) qrm
and waked up now with much interest and hope. Perhaps he will be reactivated by
the optic way? I any case, it is an interesting alternative.
As i started with the optical way i compared the TX LED with a normal
p-n-junction of a bipolar transistor (base-emitter). Since bipolar transistors
are also used in preamps (although with much less power gain, so with the need
for a pre-preamp) and there has to be find a operating point where the junction
behaves linear (with the strongest signal that can occur at the input) i
thought this is comparable to the LED. And a bipolar transistor also has its
noise figure. So i just tried and was surprised by the results.
Perhaps you could also give some practical ideas for an LF application. It's
always interesting to get some alternatives.
CU, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von SAM JEWELL
Gesendet: Di 02.02.2010 13:25
An: [email protected]
Betreff: Re: LF: Loop preamp with the BF862 / optical link
Folks,
I think it is worth pointing out that most cable TV systems use fibre links, at
least as far as the cabinet. The light (laser) is amplitude modulated with the
RF carrier(s) at the cable head end or regional Centre. These carriers are
usually locally generated but can also be from directly off-air if the signal
to noise ratio is adequate. The modulation index can be held quite high for a
single carrier, progressively reducing for multiple carriers in order to
minimize intermodulation between carriers. Ultimately, with many carriers, the
modulation spectrum will be noise-like. Those of you who have worked with the
old telecom carrier based systems will recognise the same characteristics.
The earliest cable (HFC) systems carried AM VSB signals in the VHF and UHF
frequency range in their native transmission format with minimal distorion and
added noise. Digital modulation (using QAM modulation, but still amplitude
modulation of the light) came later.
>From the laser (or LED) input to photodiode/PIN/APD output there will be a
>certain link loss and system noise figure. If the system parameters are
>known, this can be modelled using e.g. AppCAD, Noisecalc. In some systems
>signal to noise ratio is critical. For LF systems, where there is a fairly
>high noise level, it may be far less critical (famous last words).
Not all fibre systems use digital modulation of the light, although in most
cases it is still amplitude modulation.
I should also point out that fibre is often used to carry the RF and IF signals
from the SHF rooms (at the back of the big dishes) at satellite earth stations
across the site to the demodulation and signal processing buildings, in
preference to using waveguide, as was done many years ago. Whilst working at BT
Research Labs my team produced one of the first radio over fibre test systems
for BT's Goonhilly and Madley Earth stations. These early fibre links used
multimode 50/125u fibre and multimode lasers.
As I mentioned in an earlier posting, microwave and LF are just the adjacent
ends of the spectrum. Not opposite ends!
73 de Sam, G4DDK
________________________________
From: James Moritz <[email protected]>
To: [email protected]
Sent: Tuesday, 2 February, 2010 11:57:13
Subject: LF: Loop preamp with the BF862 / optical link
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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