[Elecraft] K2 RX audio output - again

[Geoffrey Mackenzie-Kennedy]

Ron AC7AC wrote:

>I believe that's why the K2 uses a fixed-gain wide-dynamic range I.F.
>preamplifier (Q22) ahead of the I.F. filter, and puts the variable gain
>MC1350 (U12) after the filter where it's reasonably well protected from
>off-frequency signals.

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A post mixer amp such as Q22 is in some cases required to reduce the IF 
system noise figure, and to protect the mixer from the large variations in 
input impedance vs frequency exhibited by many narrow bandwidth filters, in 
and close to the passband. Several types of mixer including diode rings 
generate additional intermodulation products when their load is allowed to 
vary, and the same is true of amplifiers unless the amplifier design 
addresses the problem.

While the filter can offer protection to later stages, the $64 question is 
what happens all along the receiver chain to the audio output when one or 
more big signals *do* get through the filter with little attenuation, 
alongside that weak signal which you are trying to copy?

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>It helps, too, that the K2 is a straightforward single-conversion design.
>More complex multiple-conversion receivers need more protection to do as
>well because of all the extra mixers and amplifiers they have. Of course,
>that complexity allows for some nice gadgets like front-panel "passband
>tuning" but it puts a huge load on the design to stay competitive with a
>more simple, straightforward single-conversion superhet like the K2.

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I won't be drawn into this discussion <g>

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>One of my homebrew designs from the 70's, which also used the MC1350 (that
>chip has been around a l-o-n-g time!), used pin diodes to control a 
>variable
>attenuator at the antenna input to the receiver in addition to varying the
>gain of the MC1350. After fiddling with it for a long time, I reverted to a
>simple step attenuator at the antenna input much like the K2 has.

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Understood. Sometimes variable attenuators using transformers wound on 
ferrite toroidal cores with a third 'DC' winding used to vary the core's 
permeability are useful, but draw too much current for use in a portable 
rig.

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>That choice speaks to a basic question in the design of anything: which is 
>the
>more eloquent and desirable, extreme complexity that does anything and
>everything as well enough and fully automatically, or a simple design that
>does specific things very well but which may require a more knowledgeable
>operator and greater operator intervention?

>I don't think there's any one answer to that question. If there were we'd
>not have both automatic and manual transmissions in cars.

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There is some middle ground between the extremes which you mention!

73,
Geoff
GM4ESD