The old rx is down at the moment, being measured on the
work bench. The system noise floor is about 140nV/m in 1Hz.
That's very high but quite low enough for natural radio (for
which it is mainly used). The natural floor is about 10uV/m.
But after sferic blanking (necessary for weak amateur signals)
the natural floor may be down to 1uV/m and at that level the
system noise from the old rx becomes noticeable. One reason
for the new rx, which has about 5 to 10nV system noise depending
on the front-end bias resistance I eventually decide to use.
Unfortunately only the Alpha signals from the North East
provide a suitable reference. Other signals eg MSK are too
wide and amateur signals are too weak.
Bernd, DF9RB wrote:
> I plan to build this amplifier:
>
http://dg4rbf.lima-city.de/Ultra%20Low%20Noise%20AMP%20V1.7f%20_WebSeite.pdf
> Please your opinion about this concept?
Merging the currents from multiple front-end devices is a
conventional way to achieve ultra low noise and this looks
like a good implementation.
I tried this technique with two devices but the doubling of the
input capacitance reduced the signal by as much as the noise
was lowered, no overall benefit (in fact slightly worse).
This kind of thing shows up when you refer the system noise
to field strength V/m instead of just input voltage.
The system noise voltage is far lower than necessary and the
gain is very high. Perhaps it is intended to work with a very
small probe antenna. In that case the input capacitance is
likely to cause a significant division of the signal voltage.
I wonder how it performs in other respects. Dynamic range and
intermod.
Ultimately, a VLF pre-amp must be designed for a particular
antenna. Measure the noise when the input is loaded by
the equivalent antenna capacitance, and measure gain with
a low impedance test signal injected through that capacitance.
PS, this afternoon the old rx is back into operation.
--
Paul Nicholson
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