[Elecraft] KX3 direct conversion system

[Ron D'Eau Claire]

Us OTs recognize this system as the reverse of the phasing system of generating SSB that was popular in the 1950's. Phasing lost popularity as low-cost and high-quality crystal filters became available because, like the KX3, they depended upon highly accurate phase shift and gain control over the entire bandwidth of the audio channel to cancel the opposite sideband. (Remember, everything then was analog, meaning building wide-band analog phase shift networks that remained stable from discrete components.) Back in the "day" it wasn't easy to do and achieve more than 20 or perhaps 30 dB of attenuation. That was adequate in the 50's when almost all 'phone' stations were using double-sideband a.m. occupying 6 kHz of bandwidth. 

In the KX3 it appears to be returning with substantially greater phase and gain accuracy, and so opposite sideband suppression, than we could achieve back then.

73, Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Wayne Burdick
Sent: Wednesday, August 15, 2012 9:07 AM
To: ji1tll at jarl.com
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KX3 direct conversion system

須崎 純 (Jun) wrote:

> I built KX3 kit (S/N830) up last week after fixing some errors.
> As soon as trying some functions, I found it's really a direct 
> conversion system.  A weak leaked signal was observed with another 
> receiver just on the watching frequency.

You can reduce the KX3's VFO signal leakage to another receiver nearby by setting MENU:RX ISO to ON.

Yes, it is similar to direct conversion, except that there are two channels (I and Q, like other SDRs), allowing for image rejection. The
KX3 uses this architecture rather than superhet to provide the SDR feature, as well as to reduce the number of stages, allowing the unit to be very compact.

73,
Wayne
N6KR