Re: LF: RE: Todmorden receiver

[Paul Nicholson <vlf0403@abelian.org>]

> What type of isolation transformer are you using?

All my receivers use Walters OEP transformers, A262 range
which are available from many places.

For isolation, A262A6E at the output of the rx's line driver
and A262A7E at the house end.

The input transformer between loop antenna and LT1028 is
an A262A1E.  This is most important to match the loop Z to
the LT1028 for best noise figure.

System noise of the new rx measures 0.088 fT in 1Hz near 8270
Hz, spice model predicted 0.074 fT.  The excess noise I saw
in the breadboard test has gone.

When the dust has settled and final tweaks done, I'll be
posting full details of E and H receivers.  They are quite
simple to build and no rare parts.  They are designed for
optimum performance with the particular E and H antennas
that I'm using - this is very important - you can't mix
and match pre-amps and antennas, they have to be designed
to work together.

Here's the response and noise predictions for the H rx,

 http://abelian.org/vlf/tmp/171217a.png

That's the end-to-end (magnetic flux to soundcard input) model
so it include the isolating transformers and cat5 cable.
Measurements on the bench are agreeing very well with this.

The system noise stays below 0.1 fT from 3kHz right up to
200kHz so I'm hoping it will work well also at 137 kHz.

--
Paul Nicholson
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