[Elecraft] I/Q details for the KX3.

[Julia Tuttle]

On receive, if you only mix the RF signal with only the I phase of the VFO,
you receive both sidebands, and you can't observe them separately. On
transmit, you have the same problem in reverse -- you transmit both
sidebands, and you can't control them separately.

For modes with a single sideband (SSB, DATA besides RTTY), the KX3 must
output the AF signal on both I and Q with the proper phase offset.

If it output the AF signal on I but left Q at 0 V, the TX mixer would
generate a mirror image (DSB-SC) signal.

(These are the "SSB and some data modes" covered by the TXSBNUL calibration
procedure on page 34.)

For modes that transmit a single frequency (CW, RTTY), the KX3 uses
*neither* I nor Q -- it leaves them at 0 V and controls the CW frequency
entirely with the VFO.

If it synthesized a constant sidetone frequency on I but left Q at 0 V, the
TX mixer would generate two CW signals, at the VFO frequency +/- the
sidetone frequency.

If it synthesized a constant sidetone frequency on I *and* Q with the
proper phase offset, it would also get a CW signal, but that would be
unnecessarily complex.

(These are the modes not covered by TXSBNUL.)

Digital modes like FT8 and Olivia (and RTTY if sent from a computer) are
handled just like SSB signals, just with some potentially distorting
features (compressor, equalizer, etc.) disabled.

The LO generates both the I and Q signals ("The synthesized, digitally
controlled local oscillator (LO) provides quadrature signals to the
transmit and receive mixers", page 49) -- there's no separate delay, so I
expect the phase difference would be quite accurate. I'm not sure how
accurate it actually is, nor how much it can deviate before it causes
problems.

If you haven't yet, I'd take a look at the Theory of Operation (pages
49-50) and KX3 Block Diagram (page 51) in the manual -- that's what I'm
basing my statements here on.

Related to how the KX3 handles RTTY internally, you might find the QRP Labs
QDX Digital Transceiver (https://qrp-labs.com/qdx.html) an interesting
architectural contrast. The KX3 handles CW and RTTY by simply leaving the
VFO output unmixed; the QDX handles *all* single-frequency digital modes
that way. It exposes an audio output over USB, but instead of running the
TX AF signal through a DAC and a mixer, it analyzes it in software to
determine which single frequency it contains, and adjusts the VFO to follow.

I hope this is helpful!

73,

Julie

On Wed, Jun 22, 2022 at 11:56 AM JEROME SODUS <jsodus at comcast.net> 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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