Dear Clemens, Markus,
Yes, what Markus wrote is exactly what I mean...
In general, the SNR does depend on the source impedance. Also, the overall
power gain of the preamp/receiver depends on the degree of mis-match at the
input, so the fact that the output noise level stays the same in the Perseus
does not imply that the SNR will also be the same. But unless the noise
figure of the preamp/receiver is quite low, this effect will often be masked
by internal noise in practice. As Markus points out, with large RX antennas
the source noise power at 9kHz is spectacularly high, and preamp noise is
not usually a factor anyway. But for small whips and loops, preamp noise is
important.
The figures Clemens quotes for the Perseus RX are curious... With 732Hz
bandwidth and T = 300K, kTB would be -145.2dBm, so the -118.4dBm noise floor
level indicates a noise figure of about 27dB, which seems quite high. The
QST review quotes -127dBm noise floor in 2.4kHz bandwidth, which would imply
about -132dBm noise floor in 732Hz bandwidth, so some discrepancy there.
On a crude practical level, if you listen to your 9kHz RX output and hear
mostly crackling QRN, that is fine... if you hear mostly 50Hz buzzing and
other man-made noises, you either have a problem with local noise being
coupled to the preamp or receiver input, or need a better antenna
location... If you hear mainly "white noise", your receive system is
probably deaf!
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Markus Vester" <[email protected]>
To: <[email protected]>
Sent: Sunday, June 06, 2010 8:38 AM
Subject: LF: Re: VLF 9 kcs: preamp and antenna noise
Clemens,
I believe what Jim was suggesting is a comparison of preamp noise output
with the antenna connected versus a passive load, rather than between
different passive load impedances. The intention is to demonstrate that the
receiver noise contribution is neglegible against external noise...
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