Scott,
The fact that it produces I and Q outputs from the quadrature mixer puts
it in the class of SDR transceivers, which are a different breed than
Direct Conversion.
You are free to call it what you will, but the rest of the world refers
to that as an SDR.
The 8kHz shift allows the opposite sideband and other garbage to be 16
kHz away from the desired signal where it is easily dealt with - no
interfering results.
73,
Don W3FPR
On 9/10/2017 10:51 PM, K9MA wrote:
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?
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