[Elecraft] KX2 Offset I.F.

[Walter Underwood]

For me a software-defined receiver is one where the demodulators and filters are implemented (defined) in software. Adding a synchronous AM detector is 100% software.

We already have precise terms for receiver architectures: superhet, direct conversion, and TRF (no conversion). Direct sampling receivers are essentially TRF designs, sometimes with very wide bandwidth. The simplest TRF receiver is a crystal radio. Well, I guess “talking light switches” (corroded connections detecting strong AM broadcast) are simpler.

One could design a receiver that used direct sampling with dedicated hardware for demodulation. I would not be at all surprised to hear those had been built for the military or the space program. They built hardware Viterbi decoders, after all.

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Dec 23, 2017, at 8:01 AM, K9MA <k9ma at sdellington.us> wrote:
> 
> 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
> 
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