Posted by
K9MA on
Sep 11, 2017; 2:51am
URL: http://elecraft.85.s1.nabble.com/KX2-3-DSP-AM-Demodulation-tp7634311p7634344.html
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
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