[Elecraft] I/Q details for the KX3.

[Alan Bloom]

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.


Yes, but...  You still need to reduce the sample rate to a lower value 
that the DSP can handle.  Either you need to mix down to a low-frequency 
IF or all the way down to an I/Q signal at a zero Hz center frequency.  
Either way, the mixer needs to be quadrature to avoid the image.

The output of the ADC goes to a quadrature digital downconverter (DDC), 
which consists of two digital mixers (that multiply the ADC signal times 
the digital local oscillator), with the local oscillator to each mixer 
90 degrees out of phase.  In this P3 this is done in an ASIC 
(application-specific IC) and in the K4 it is done in an FPGA.

Alan N1AL


On 6/22/22 20:45, Julia Tuttle wrote:
> 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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