From Dave G3YMC
John Sexton wrote:
I decided to try another pre-amp, and built a design by Lloyd Butler,
(VK5BR) from the old LF source book using a single op-amp the
OPA111AM. I
chose this design because a couple of years ago
Thanks John for the interesting technical email on your receive loops.
I would confirm that the Burr Brown OPA111 is not necessarily a good
choice for 136kHz loop preamps. This device (I have the datasheet) is
really designed for precision data aquisition applications, and
although listed as a low noise op amp, it has a unity gain bandwidth
of typically 2MHz and an open loop gain at 136kHz of around 25dB. Yes
it will work, but there are proably better ways of doing the job we
want.
However I soon noticed a "new" RTTY station on 136.25 and realised
that it must be an Inter Modulation Product,
but where and from what?
Nearly all cases of reported intermodulation products in receivers can
be put down to inadequate front end selectivity. In my transverter
(which uses an E310 or similar FET input amp) I have two parallel
tuned circuits tuned at 136 - the first at the antenna front end, and
the second at the drain of the FET. One of these is fixed at 136,
which presents problems of course if I want to listen elsewhere, but
this gives me a bandwidth of a few kHz and I have had no problem
whatsoever with intermods. G3LNPs technique of using a second nulling
loop is probably unnecessary if proper attention is given to the front
end selectivity.
It is refreshing to note several ideas coming from the earlier issue
of the source book. Although it was suggested this could be thrown
away now we have the smarter looking new one, quite a few ideas are
only in that one, eg transmit loops, SWR bridges, GDOs etc. The
information in it certainly has not dated, even though newer
techniques and practices have come along since.
Cheers Dave G3YMC
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http://ourworld.compuserve.com/homepages/sergeantd
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