Rob VK5RC has completed his SolderSmoke direct conversion receiver. Congratulations Rob. And it is good to know what we have a Medical Doctor among our builders -- this adds prestige and expertise to the project.
Rob measures a Minimum Discernible Signal at -95 dbm. I think that is kind of weak performance. Much depends, of course, on how you define "descernible." Some books say you need a 10 db increase in audio. Others say you just need to be able to tell there is a signal there. I have gone with the latter definition and have measured MDS of around -120 dbm. I wonder why there is such a difference in measured MDS.
In any case, it is great to see Rob's receiver inhaling all of that great Australian SSB (see video above). Thanks Rob, and congratulations.
Going to the MDS definition, it is
ReplyDeleteMDS=NF[dB] + 10 log (BW[Hz]) -174 [dBm](at room temp).
The first item to check is what Audio BW you and Rob are measuring the carrier in. That's a big factor. Then multiply the BW by 2, since this is double-sideband reception.
Second, is Rob's front end BPF tuned for min loss?
Third, always check diode and transformer polarities. That's a common oops.
So what to expect with all ducks in a row?
BPF~1dB NF , DBM ~ 6dB NF, Mismatch Loss~ 8dB, AF NF~3dB ; net NF~ 18dB
If we assume AF BW of ~2500Hz (pure SWAG)
then: MDS~18 + 10log (2500x2) -174
or about -119[dBm].
Beyond that, the easiest improvement is audio filtering, compatible with the mode (SSB or CW).
Another is dealing with mismatch loss. That's something W7ZOI solved with a passive match ( 88mH ) or KK7B with a lower-Z AF first stage.
73!