[Elecraft] KX2 Offset I.F.

[Don Wilhelm]

Mike,

Think of the 8kHz shift similar to a low frequency (8kHz) IF if your 
thinking is along the lines of a superhet receiver.

Of course, the KX2 is *not* a superhet, it is a full SDR.  If the "IF" 
were at a higher frequency, the VFO would be further away from the 
signal frequency - by the frequency of the IF.

The SDR DSP processing is at 8kHz in the KX2, or selectable between 
baseband (zero frequency) and 8kHz in the KX3.

Using baseband has some advantages (the roofing filters), but also some 
downsides such as increased exposure to strong radio broadcast signals 
leaking into the baseband stream.  If you have ever experimented with 
Direct Conversion receivers, you will know about that breakthrough.

An SDR in simplified form is two identical Direct Conversion receivers 
operated 90 degrees out of phase with each other.  That allows the 
signal processing to be done mathematically (whether that be analog or 
digital).  A study into the Phasing Receivers of Rick Campbell KK7B 
design along with op-amp mathematics will give you the analog version of 
that signal processing.
The problem with analog implementation is that real components have 
tolerances that must be dealt with - digital mathematics can be exact 
calculations.

73,
Don W3FPR

On 12/22/2017 12:07 PM, Michael Zabel wrote:
> Hi Don,
> 
> Thank you for the great explanation.
> 
> I must have had the ATT on when I did the original power measurement
> (rough)  on the SDR
> 
> I tested again and this time I get:
> 
> -77dBm - Preamp On
> -55dBm - Preamp Off
> -74dBm - ATT on
> 
> Just relative differences with the absolutes unique to my setup of course.
> 
> The VFO signal is in the noise with the antenna disconnected from both KX2
> and SDR.
> The VFO signal is lower when using separate antennas for each as would be
> expected.
> 
> I was just not used to seeing a signal like that when using the SDR to
> track superhet's before, so it had me scratching my head for a bit until I
> considered the Rx architecture of the KX2(3).
> 
> Thank you for helping me get everything organized in my thinking, sometimes
> that is a monumental task!