Re: KX2/3 DSP AM Demodulation

Posted by Don Wilhelm on Sep 10, 2017; 11:54am
URL: http://elecraft.85.s1.nabble.com/KX2-3-DSP-AM-Demodulation-tp7634311p7634322.html

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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