Liberals for Guns schrieb:
Can someone help me understand something about IF vs synch. I've read
that IF shifting can get rid of an interference that is occurring
slightly lower or higher than the desired frequency. So, if you've
got noise within the filter range and it's on the lower side of the
frequency than you simply shift everything up so the noise is out of
the "window". My question is that I'm not sure why you just couldn't
use sync in to get rid of the interference. By just listening to the
upper sideband the noise wouldn't be there. Right? Do I need to pay
for an advanced feature like IF shift when I could get away with
simply using a sync detector function?
Synch detectors aren't all created equal. The one in the AR7030, for
example, is not sideband selective and therefore needs to be used in
conjunction with IF shift and a sufficiently narrow IF filter in order
to isolate one sideband. On the other hand, this is also the most
flexible solution overall, only beaten by something like Icom's Twin PBT
(of which I don't know the effectiveness). Do keep in mind that a good
SSB filter will allow for far greater sideband selection than an
ordinary (sideband selective) synch detector - while the latter may
allow for something in the order of 30 dB, the SSB filter in the AOR is
already at 48 dB in 3 kHz distance from the carrier freq, which includes
the filter bandwidth of about 2.1 kHz @-6 dB (i.e. with one -6 dB border
on the carrier frequency, a 1 kHz tone on the other sideband will
already be suppressed in the order of 50 dB). Additionally, IF shift can
also be used for SSB and may prove useful in adapting the CW offset to
the listening habits when used in conjunction with a narrow crystal
filter (or imagine you want to use a 1.6 kHz filter for SSB DX).
Interestingly, sideband selective synch detectors are the exception
rather than the rule - Sony can do this thanks to its CXA1376 IC (and in
the older 2010/2001D, with AM Stereo ICs like it's also done in the
Grundig Satellit 500 and 700), sideband selectivity is also claimed for
the SE-3, I don't know what Drake uses in the R8B and later SW8s, but
all the rest seem to use both sidebands (AR3030, AR7030, IC-R75, R8/R8A)
with only part of the receivers being able to compensate for this via IF
shift (like the AR7030).
Stephan
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