Pete KE9OA wrote:
Aperture is exactly the point I was going to get at.........this is
the
reason that the receivers I have built that use the Mini-Circuits
mixers as
the first stage have worked very well with an 8 foot diameter untuned
loop.
No IM products anywhere in the LW/MW bands.
wrote in message
oups.com...
Differential pairs have more noise than a single ended design,
which is
why the transformer is the way to go. An ideal transformer has no
noise. I real life transformer has the noise of the resistance in
the
windings. This is why the transformer choice isn't trivial.

I understand this, but take a look at the AD797, and other op-amps
that are
used at very low input signal levels. This device is designed for
ultrasound
transducer applications, strain gauge amplifier service, etc. There
are
quite a few very low noise op-amps that are suitable for RF service.

I'm more familiar with the LT1028, but I took at look at the 797, which
fortunately is an easier op-amp design to follow. It does have a
differential input, but that is because it is an op-amp, which requires
such an input. However, it is worse for input noise. It should be 41%
more noise than a single ended design.[Uncorrelate noise sources add in
a RMS fashion IIRC).

As far as transformers, I don't think there is such a thing as an
ideal
transformer. In my experience, the input Xl must be at least ten
times the
impedance of the expected driving source's impedance at the lowest
frequency
of interest.There is going to be a certain amount of DC resistance in
the
primary winding if we are going to be able to achieve the required
inductance to provide the proper load. If we are talking about a
single
turn loop that has a relatively low reactance in this application,
that
would be ok.
The ideal transformer was just thrown out there for discussion, since
the noise source in a model of the transformer would be the resistance
of the windings.


If you buffer (source follow) then use an amplifier, the noise of
the
source follower gets amplified. This is why you wouldn't normally
do
any buffering before amplification if low noise was the issue.
However,
in the case of a tuned loop, you need the high impedance to keep
the Q
high.

Exactly my point.....................in the LW/MW ranges, this is not
an
issue. When you are using a high Q loopstick, the output voltage out
of the
secondary winding is relatively high. This is observed from my own
experience. In direct comparison with the Palomar active loopstick,
my own
units have better performance in the areas of output voltage, and
small
signal pickup is slightly better because of the higher selectivity of
my
implementation.
For higher frequencies where the ambient noise is lower, a different

technique would be required.

Wellbrook gets around that problem by using a loop that is not
tuned. The assumption is the purchaser has a real radio and doesn't
need front end filtering. So cource having a high dynamic range
like
the 7030 is a plus.

That isn't such a good deal......................anybody can build
that type
of antenna for relatively cheap................the clincher is the
amplifier
stage that Wellbrook uses. I would like to take a look at a schematic
of
their circuit and see how they do it.

Ah, but that is the idea! You sell the amp and people come up with
their own loops. That is also the beauty of the tuned loop in that you
don't have to know exactly what the end user is going to use.




Loops using ferrite will not work as well as a Wellbrook IF the
Wellbrook uses a large enough loop. What you are gaining with the
Wellbrook is aperture. It soaks up lots of RF due to it's size