[Elecraft] K3 Design

[n2ey at aol.com]

-----Original Message-----
From: Brett gazdzinski <brett.gazdzinski at verizonbusiness.com>

>I wonder if, with today's computer control and other
>fancy tricks like switched in band pass filters, wide range
>vfo gizmos, etc, if it would be possible to build a single conversion
>receiver with a 455 KHz IF frequency.

>With good filters and DSP, could it work?

You're looking at the wrong end of the receiver.

The main problem with a single-conversion HF receiver using a 455 kHz IF
is image response. Most of what you have described, such as VFO gizmos,
computer control and DSP, will not help that problem.

With really good front-end filtering, such a receiver will have
adequate image rejection up to about 40 meters. Maybe even as
high as 30 meters with lots of high Q tuned circuits. But that's about
the limit, if you want image rejection of more than 30-40 dB.

If you're willing to give up the higher HF bands, or put up with images,
there's no reason such a receiver couldn't be built.

Images are the main reason amateur HF receivers moved away from
the single-conversion-455-kHz-IF design more than a half century ago.
This was after receivers like the HRO and BC-342 had used two
tuned RF stages ahead of the first mixer.

W3FPR mentioned the 1957 article "What's Wrong With Our Present 
Receivers?"
by W1DX, and I highly recommend it today. It explains the classic 
problems of the
high performance HF superhet rx and the tradeoffs in design. The 
technology may
have changed but the basic problems remain.

All Elecraft rx designs are direct descendants of the ideas presented 
in that 1957
article. The two main advances in the intervening 50 years (besides the 
use
of sandstate instead of hollowstate) are the use of a lownoise lowdrift 
synthesized
LO, and improvements in the back end of the rx like IF DSP.

---

A much-overlooked article was in QST in 1971, called "An Experimental 
Receiver
for 75 Meter DX Work" by W1KLK. This rx used a high selectivity front 
end, lownoise/lowgain RF amplifier,
high dynamic range mixer, and single conversion to 455 kHz. But it only 
covered 80/75 meters.

All receiver designs are tradeoffs - no exceptions, including even the 
K3. The question is what you
trade off. For example, the K3 won't do a very good job receiving 
around its first IF (8.215 MHz). But there's
no ham band there. If it means better performance on other frequencies, 
lower cost and less complexity,
I'll take that tradeoff!

 73 de Jim, N2EY
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