Don,
OK, I'll turn that around, and ask why the KX2 does NOT convert directly
to baseband, rather than to 8 kHz? You said something about AM signals
breaking in. What's that all about?
Call it what you will, but I still maintain that a receiver, like the
KX3, which converts directly to baseband is really a direct conversion
receiver, just using quadrature mixers so the opposite sideband can be
suppressed. I suppose you could call it a phasing direct conversion
receiver. I can see, however, why it might not be referred to as DC to
avoid the assumption that it receives both sidebands, but that's just
marketing.
73,
Scott K9MA
On 9/10/2017 06:54, Don Wilhelm wrote:
Scott,
The advantage of using the baseband IF is that one can use a baseband
filter to provide a roofing filter. Such filters get more complex if
used at 8kHz.
It also allows RIT/XIT excursion that is equal on each side of the
carrier frequency. In the KX3, it is +/- 15kHz, while with the 8kHz
shift, the RIT/XIT range is +7 to -23kHz.
What is termed "Direct conversion" is a detector/mixer that goes from
RF directly to a single audio stream - it hears equally well on both
the desired and undesired side of the carrier.
In a Phasing or DSP based receiver, there are 2 outputs from the mixer
- out of phase by 90 degrees to each other. That allows the opposite
sideband to be suppressed. The process is done mathematically whether
by DSP techniques or analog components as in the KK7B "R2" or the very
old phasing detector transmitters and receivers available in the
1960s. Central Electronics made some (including the "Signal Slicer"
for a receiver adapter and worked at the 455kHz IF common in receivers
of that day).
73,
Don W3FPR
On 9/9/2017 11:39 PM, K9MA wrote:
I didn't realize the KX2 used an 8 kHz IF, though I see it is
mentioned in the manual. I guess that means the image is 16 kHz away
from the desired signal, and is suppressed by the I/Q demodulation,
as in a phasing transmitter. It does explain why the post mixer
amplifier cutoff frequencies are so high.
However, is not the KX3, when it's using the zero frequency IF really
a direct conversion receiver, just with quadrature mixers so one side
of zero beat can be suppressed? Perhaps the term "Direct Conversion"
is avoided because of the lack of opposite sideband rejection in
conventional DC receivers.
Just out of curiosity, why does the KX3 not use the 8 kHz IF all the
time, as does the KX2?
--
Scott K9MA
[email protected]
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