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<DIV><FONT face=Arial size=2>In reply to Ahti and Frank,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Ahti, from memory the NF measurements you suggest
are a little high but in the right ball park. The problem of connecting the
Janus balanced inputs directly to the QSD caps is noise pick up since this is
done at a high impedance level. A little (low noise) gain after the QSD
capacitors would allow Janus to be driven from a low impedance source and help
with the noise pick up. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Alternatively, we placed a header on the Janus
board that would allow a little daugher board to be plugged directly in. The
board could contain a QSD and the capacitors feed the balanced input over very
short leads. </FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>It seems we still have a NF problem with the
QSD in that it rises with increasing frequency. The Flex5000 figures I
have seen indicate that if we measure the NF on 20m then it increases by 10db on
10m and a futher 10dB by the time we get to 6m.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I may be mistaken but I have yet to see a QSD that
does not have this problem. In which case we may want to investigate using
say an H mode mixer followed by a 1dB effective NF audio stage. This has
the potential of not requiring a pre-amp on any of the HF bands. I recall seeing
an H mode mixer that had I and Q outputs but can't seem to find the circuit. To
optimise the H mode we need to change the bias point of the switches as we
change frequency but an I2C driven DAC could do this easily.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Some experimentation required!</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>73's Phil...VK6APH </FONT></DIV></BODY></HTML>