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
---
http://www.elecraft.com
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