> Jerry, The reason, I believe that made the 75A4, with 500hz Collins
> mechanical Filters, such a fine 160 meter receiver in its time was the
> fact that on that band the design used single conversion rather than the
> double conversion it used on the other bands. I think this was the key to
> lower IMD. There was one drawback. On 160, if I remember
Hi Herb,
There is no inherent disadvantage in multiple-conversion schemes if
gain distribution is proper, the filters are of good design, and mixers
are properly designed.
The shortfall with the Drake R4C isn't rooted in the triple conversion
scheme, but how they engineered things. Narrow filtering is placed
behind two poorly designed mixers, and shielding around the filters
is somewhat poor. They probably had a target cost and size to
work with, and did what they could do with the components they
had available.
That's the reason an R4C changes from a mediocre receiver to one
of the best receivers available when narrow filters are added after
the first mixer. (This excludes R4C's under S/N 18700 or so, which
are unrepairable without converting the MOSFET second mixer to a
better design.)
I have an R4C modified with balanced mixers and solid-state IF
amplifiers that overloads at 23 dBm (2-1/4 volts) input, yet has -145
dBm noise floor (12.6 nanovolts). It's still triple conversion. The
design was just converted using modern mixer and amplifier
technology.
73, Tom W8JI
w8ji@contesting.com
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