[hpsdr] Alex 6M preamp

[John Ackermann N8UR]

Hi All --

I recently found myself doing some noise figure testing, and thought I 
would see what the Alex 6M preamp looks like; we've measured its gain 
and bandwidth, but I don't know that anyone has looked at the NF.

I measured from the "RX1 In" to the "To RX" jack, with PowerSDR set to 
6M and the preamp/attenuator dropdown set to 0dB.

My measured noise figure was 4.9 dB and gain was 21.28 dB at 51 MHz. 
(Again, that's not the noise of the amplifier device alone, but

Note that this test includes losses in the 55 MHz anti-aliasing filter 
and relays ahead of the preamp, which directly increase the noise 
figure.  It also doesn't say anything at all about Mercury's performance.

This result indicates that for 6M weak signal work, an HPSDR system with 
Alex in line would still benefit from an external low noise preamp.

73,
John