Posted by
Karl Larsen on
Aug 31, 2006; 12:54pm
URL: http://elecraft.85.s1.nabble.com/KX1-improvements-tp393532p393536.html
Geoffrey Mackenzie-Kennedy wrote:
> Wayne Burdick <
[hidden email]> 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.
This is correct. The SA612A is a mixer designed to make life easy.
It has a oscillator you can add a crystal or L-C to and provide a LO. Or
this can be used as an amplifier for an external LO. It is a Gilbert
Cell design but not well done. It has gain.
>
> <snip>
>
> ------------------------------------------------------------------------------
>
>
> 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.
Well some have wondered if it could be a retrofit. It can't.
> 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.
Yes. The LO has to beefed up from 400 mV into 2k-ohms to something
else yet to be determined since the LO has to switch the FETs on and
off. The space problem is caused by the transformer, and the gain
problem is due to the H-mode having a loss of gain like -5DB.
>
>
> 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.
Actually the noise figure of the IF stages is reduced by the loss
through the mixer. This is also a problem.
> 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.
Yes this is all true. I have heard the H-mode mixer has a poor noise
figure but it is the loss of gain that causes this.
Nothing can be done without a lot of design work and breadboard
building. I don't see this happening.
73 Karl K5DI
>
>
> 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
>
>
>
>
>
>
>
>
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