[Elecraft] KX2 Offset I.F.

[K9MA]

Don,

I'm not sure there IS an accepted definition of a "Full SDR" radio, but 
it's just a matter of semantics, anyway.  I just wanted to point out 
that the KX2 and KX3 use local oscillators and mixers to convert the RF 
to the IF, like any superhet.  From there, it's pure SDR.  (Perhaps 
aside from the analog "roofing" filters in the KX3.)

Now, if a radio used a high speed ADC to digitize the incoming RF 
directly at something greater than the Nyquist rate, I'd call that "Full 
SDR".  So far, high resolution ADC's really aren't fast enough for the 
high end of the HF spectrum.  Direct sampling is sort of a gray area, I 
suppose, as the undersampled ADC really acts like a mixer, and needs to 
be preceded by a bandpass filter to eliminate the undesired alias products.

73,
Scott K9MA


On 12/22/2017 14:01, Don Wilhelm wrote:
> Scott,
>
> By your definition of an SDR, that may be true.
> Some SDRs today are direct sampling at the antenna, but think back to 
> SDRs in the recent past when high frequency direct sampling was only 
> achieved at great cost - the simplest example is the SoftRock 40 which 
> used a quadrature mixer to baseband.
> I think most would say the Softrock is an SDR.
>
> 73,
> Don W3FPR
>
> On 12/22/2017 1:53 PM, K9MA wrote:
>> On 12/22/2017 11:37, Don Wilhelm wrote:
>>> 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. 
>>
>> I must disagree.  All the digital processing takes place after the 
>> quadrature mixers.  Up to that point, it IS a single conversion 
>> superhet, but with two mixers with quadrature outputs. The same is 
>> true of the KX3, though it can be operated as a direct conversion 
>> receiver, which is simply a superhet with a zero frequency IF.
>>
>> This is in no way meant to disparage the design of the KX2 or KX3:  
>> They are truly excellent radios.
>>
>> 73,
>>
>> Scott K9MA
>>

-- 
Scott  K9MA

k9ma at sdellington.us