EMRFD Message Archive 2076
Message Date From Subject 2076 2008-09-15 13:15:34 Alan Melia BFO and LO isolation ? Hi all, I have often considered this problem over the years. I wonder if
anyone has considered using opto coupling for the isolation?? I wondered
whether now there are sufficiently fast opto to run at RF whether this might
also solve some of the problems with DC receiver LO feedthough. One could
even use fibre to isoalte the oscillator from the amplifier board.
2079 2008-09-16 12:17:48 ha5rxz Re: BFO and LO isolation ? The easy bit is the fiber. Find an optical transmitter and receiver
that can handle 75 MHz and I'll be a happy man. Right now I am
experimenting with balanced signals in some places using
ADN4667/ADN4668 LVDS chips with the twisted pair heavily shielded but
the problem is finding decent two-pole plugs and sockets that are RF
tight. At one point I considered using miniature twinax connectors
until I saw the price.
2080 2008-09-16 17:46:32 Art Re: BFO and LO isolation ? Fiber coupling is interesting.
I have wanted to do an antenna mounted transceiver for QRP Field Day
for a long time. Photonics is interesting because the fiber is small
and extremely light weight. And, the dynamic range of a photodiode is
outstanding, much better than any HF receiver needs. By locating the
battery and the transceiver at the antenna, the only link needed is
the frequency control for the receiver, the frequency control for the
transmitter and some means to switch between transmit and receive. In
this manner, every single bit of that 5 watt QRP signal goes into the
antenna-no coax loss.
While some might argue that an extra 1.5db of coax loss is hardly
detectable, I beg to differ. If the signal you are trying to hear is
at the threshold of your hardwares ability to detect it, a one db
increase in erp now makes that signal audible (and workable). In weak
signal mode, every single db counts!
75 MHz is not a problem for an optical link-the small photodiodes
used today have bandwidths well into the GHz range.
If you only want to transmit the LO with a fiber optic system, I
think you can do so quite easily by using the hardware in an analog
mode rather than messing with digital signals at UHF and higher-which
is where the fiber optic systems are designed to operate.
I have experience with photodiode receivers and laser generating
transmitters, but have little hands
2081 2008-09-17 01:16:02 Lasse Re: BFO and LO isolation ? I remember Al Hefrick K2BLA(?) did use a opto coupler to isolate the
oscillator, i.e. reduce load sensitivity. But the question here is if
it's the oscillator that radiates or if it is leakage through the
detector... You still need to feed the mixer with 7 dBm!
BTW I have run RF over optical fiber over a GHz, so with slightly poor
dynamic range, ~60 dB, you can transfer broad band signals over long
Alan Melia skrev:
> Hi all, I have often considered this problem over the years. I wonder if
> anyone has considered using opto coupling for the isolation?? I wondered
> whether now there are sufficiently fast opto to run at RF whether this might
> also solve some of the problems with DC receiver LO feedthough. One could
> even use fibre to isoalte the oscillator from the amplifier board.
> Alan G3NYK
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