[Elecraft] I/Q details for the KX3.

[Julia Tuttle]

Mike,

I think you're right that SDRs that digitize at a zero IF (like the KX3)
need I and Q channels to distinguish the AF sidebands. That is, if the
radio mixes 7030.4 kHz down to a sidetone of 0.4 kHz, it also catches
signals at 7029.6 kHz.

And I think that SDRs that digitize at a non-zero and 'properly
intermediate' IF (that is, neither 0 nor the VFO frequency) also need I and
Q channels to distinguish the IF 'sidebands'. That is, if the radio mixes
7030 kHz down to an IF of 455 kHz, it also catches signals at 6120 kHz.

But I think that SDRs that digitize at the VFO frequency itself (I believe
this is generally called "direct sampling" on RX and "direct digital
synthesis" on TX?) can get away with a single channel, since there's no
mixer to cause the "you mix A and B and get both A + B and A - B even
though you only wanted one of them" problem.

Does this match your understanding?

Thanks,

Julie

On Wed, Jun 22, 2022 at 2:04 PM Mike Markowski <mike.ab3ap at gmail.com> wrote:

> Jerome,
>
> This answer is for generic SDRs, since I don't have a KX3.
>
> An SDR always must use both I and Q, even for CW.  I and Q are two
> streams of samples of the same signal.  One of many advantages to i/q
> sampling is the ADC's can run slower (cheaper).  In the extreme and
> ignoring the intrusion of real life components on sampling, a 10 kHz
> wide signal could be sampled with I and Q ADCs running at 10 kHz.
> Nyquist is satisfied because you have 2 streams, or 20 kHz sampling in
> this example.  Aliasing prevents you from using just I in that case.
>
> The fact that Q is 90 deg from I, means we can plot them on axes 90 deg
> apart.  That allows re-appropriating the entire field of complex
> mathematics to work with the samples.  I and Q are both physical -
> trying to avoid the word 'real'! - streams of samples, but I can be
> called real and Q can be called imaginary.  Using Euler's Formula, they
> can be bundled into a concise exp(j phi) formulation to work with.
>
> I/Q imbalance is always a concern.  Maybe others can quantify it better
> than me saying smaller is better.  :-)
>
> 73 es GL!
> Mike ab3ap
>
> On 6/21/22 11:08 PM, JEROME SODUS wrote:
> > Hello,
> >
> > Although I have asked some of these questions on another forum, here I
> have expanded on them a bit.
> >
> > When the KX3 transmits a CW-signal, are both I and Q used?
> > If so, why would that be necessary?
> > (My guess is only the I is needed.)
> >
> > Same questions too for a SSB-signal.
> >
> > But, for any digital transmission like RTTY, FT8, or Olivia, wouldn't
> both I and Q would be active?
> >
> > The LO is used for the "In Phase" and is delayed by 90 degrees for the
> "Quadrature"; how is that delay done?
> > How much can the delay deviate from 90 degrees and still be useful?
> >
> > My reason for asking is that I'm to give a talk to the Radio Club in
> August about how a KX3 works and want to mention these details.
> >
> > TIA for any replies.
> > 73 jerry km3k....KX3#6088
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