[Elecraft] Re: KX1 mixer discussion

[Geoffrey Mackenzie-Kennedy]

Wayne Burdick <n6kr at elecraft.com> wrote:


> There are potential issues with using any type of mixer besides the 
> present SA612 in the KX1.
>
> First is PCB space. The present mixer, which requires nearly no additional 
> parts, barely fits. I seriously doubt that there would be enough room for 
> any type of multi-transformer bus-switching mixer, even if were all SMD 
> and replaced the 30 or 30/80 meter module.

<snip>

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Where I have gone astray is that I had thought that the discussion was about 
a new design of a rig similar to the KX1 but with higher IIP3, and not a 
retrofit modification. I agree that it would be difficult if not impossible 
to fit in a H-mode mixer in place of the KX1's existing mixer. In addition 
it probably would be difficult to find space for the IF and LO modifications 
required to take full advantage of the proven high performance of a H-mode 
mixer.

With regard to the front end mixer's conversion gain / loss, I would suggest 
that in high performance HF receivers a mixer exhibiting conversion loss 
rather than gain is more useful *provided* that the noise figure of the 
following IF system is held suitably low. Obviously the overall noise figure 
is that of the IF system increased by approximately the conversion loss of 
the mixer and insertion loss of the input filters and switching, but the 
overall IIP3 would be approximately that of the IF increased by these 
losses. While conversion gain improves noise figure, assuming that the noise 
figure of the mixer itself is of the right order, the IIP3 of the overall 
receiver will be less than that of the IF system by approximately the gain 
of the mixer minus the loss of the front end filters. Also the increased 
level of unwanted signals coming out of the mixer places greater stress on 
any IF stage that might be before the IF filters, and the filters 
themselves.

With a mixer exhibiting conversion loss, the place to make up the gain is, I 
believe, in the IF *after* the narrowband IF filters, not as you point out 
before the mixer nor at audio.

Forgive me for "old news" which is not relevant to a simple modification to 
the KX1, simply comment on one of your points.

The H-mode mixer requires approximately 0dbm LO power if properly driven, 
but because of the space problem in the KX1 not relevant. FWIW I have had 
fewer spur problems with this mixer than with a Level 7 or a Level 27 diode 
ring with proper LO injection, and properly terminated at all three ports.

To be fair to the KX1, I suspect that you did not design it to be able to 
cope with the BC stations on 40m as we hear them here, while still providing 
copy when the desired signal is riding the noise level at S1 or so, a 
situation where attenuators can be counter-productive. Depending on the time 
of day, the level of signal here from some BC stations above 7.100 MHz can 
reach -10 dbm  carrier or higher, with 'enthusiastic' modulation in some 
cases.

73,
Geoff
GM4ESD