Engineering isn't science per se, it is really the art of trade-off
management in the real world. Success is not perfection of the design
in every aspect, but rather perfection of the trade-offs to best meet
the goals. I really like my KX1. I wish it had more audio, my hearing
sucks, but I have a little headfone amplifier (a potential Elecraft
kitlet for those of us that live in a quiet world?). It does a good job
and the batts last a long time (if I remember to turn it off).
I'm not real anxious to see the KX2 show up anytime soon. I'd have to
buy it ... like you wouldn't know that? ... and right now, I have some
other plans for my "Hobby Account." Andrea has been hitting the
needlepoint stores fairly hard these last few weeks, and my account is
thus moderately full.
GARY: I haven't forgotten my KPA100 and all your advice, just haven't
had time to get to it.
Fred K6DGW
Auburn CA CM98lw
"One should strive to become at least as good a person as your dog
already believes you are."
(seen on a T-shirt at OMSI in Portland OR)
wayne burdick wrote:
There are potential issues with using any type of mixer besides the
present SA612 in the KX1.
First is PCB space. The present mixer, which requires nearly no
additional parts, barely fits. I seriously doubt that there would be
enough room for any type of multi-transformer bus-switching mixer,
even if were all SMD and replaced the 30 or 30/80 meter module. The
problem with the transformers is vertical space, which there is also
very little of. The alternative would be an active mixer with a much
higher IP3 than the SA612, but those that I'm familiar with also draw
a lot of current and have a very poor noise figure.
Second is conversion loss. The present mixer has about 15 to 18 dB
gain. If you use a passive mixer that has 5 dB loss, you'll have to
make up 20 dB or so gain somewhere, or the rig's audio output would be
way down. Increasing gain at audio is not recommended because it would
likely lead to microphonics, so the gain would have to be added at RF.
This suggests an MMIC RF amp after the lossy mixer -- but that's
another current hog. If you put an LNA (low noise preamp) ahead of the
mixer instead, you lose a lot of your IP advantage gained.
Finally, there's the LO. The present LO injection is on the order of
400 mVpp, but is working into a high impedance on pin 6 of the mixer
(about 2K ohms). If you use a different mixer that requires LO power
of say 0 dBm into 50 ohms, the LO will need a buffer amplifier, and
you also risk increases in RX and TX spurs.
The present KX1 mixer is an optimal solution *except* in the case
where you need more dynamic range. But this is why there's an RF gain
control on the rig. On 40 meters at night, you can probably adjust the
RF gain control for 10 to 20 dB of attenuation ahead of the mixer and
still copy 90% of the signals you can hear with it set for maximum
gain. That's an instant increase in IP3 of approximately this amount.
For rigs like the KX1, what the world needs is a Gilbert cell
multiplier (mixer) IC with gain, NF, LO drive, and current
requirements similar to the SA612, but with a 20-dB higher IP3. If
there is such a device available, please let me know :) For
example, none of Analog Devices' mixers meet these requirements.
73,
Wayne
N6KR
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