[TenTec] New Sherwood Presentation - Powerpoint & Audio

[Zivney, Terry L.]

The direct conversion radios, such as the Flex 5000, make tradeoffs, too.
 
For example, the Flex 5000, and all other direct conversion radios have
a problem with the audio image.  With image reject mixers (which is
what the (I-Q mixer technique basically is), rejection of this image 
depends upon perfect balance between the channels. Critical (i.e.,
knowledgeable)  reviewers of these type of radios have noted that
these images may be as little as 40-50 dB down, and in well-perfoming
radios may be as much as 70 dB down from the desired weak signal. 
 
These images are much more of a problem than third-order intermod.
Third-order intermod requires two interfering signals of great strength
being in the proper (or unlucky, depending upon your viewpoint) frequency
in relation to the desired weak signal.  The direct conversion image,
on the other hand, requires only ONE strong signal to produce a
problem.  Which do you think is more likely to happen in a crowded
band, say 40m cw:  TWO signals EACH spaced at a specific 
frequency and 90dB above your desired weak signal - or - ONE
signal only 40, 50, or 70 dB (see above), spaced at the critical 
frequency, often only 10 or 20 kHz away from the desired signal?
My experience tells me that the second (Flex) is more likely to
be a problem.There are many signals 40 to 70dB above the noise
floor of my radio on 40m but there are relatively few precisely spaced 
PAIRS of signals 90 dB above my noise floor, even with a full-size 
3 element yagi.
 
Roofing filters exist in part to reduce images.  The roofing filter 
approach requires a second conversion in the radio.  In principle, 
this shouldn't be any more difficult than providing  good first 
converter, but years of experience and observation show that 
is not child's play.
 
Terry Zivney, N4TZ/9