[Elecraft] KX3 AM-breakthrough

[Bill K9YEQ]

Wayne,  Thank you for this very informative technical explanation.    When I
operated my HW-8 in the early 80's, I loved it because it was so simple to
build, easy to operate and required little power.  However, the KX3 is so
superior I really cannot compare what you have engineered the KX3 to be vs.
what the very challenged HW-8 was.  Incredible contrast; excellent design
using the modern techniques available, to produce an amazing piece of
equipment. 

73,
Bill
K9YEQ

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Wayne Burdick
Sent: Monday, July 29, 2013 11:28 PM
To: Igor Sokolov
Cc: Elecraft Reflector; Klaus Dittrich
Subject: Re: [Elecraft] KX3 AM-breakthrough

"Igor Sokolov" <ua9cdc at gmail.com> wrote:

> Every design has it's own limitations.

Yes, AM IP2 exists in all quadrature DC receivers to some degree because
perfect mixer balance is not possible. We have a goal to improve balance
using DSP techniques. 

But read on... 

Klaus wrote:

>> If I switch to RX SHIFT I will have a receiver with [reduced] narrow
spaced dynamic range.

True; according to Sherwood and our own lab tests, when the roofing filters
are turned off, the dynamic range at 2 kHz drops from 104 dB or so to about
96 dB, which still puts the KX3 into the top 6 of all radios Sherwood has
tested. His third listed value of 65 dB was a worst-case AF
opposite-sideband (OSB) image, not IMD. OSB images are related to just one
extremely strong nearby signal, and are less problematic than IMD, which
results from multiple combinations of signals. (The AF OSB image drops
considerably when you apply RX SHFT=8.0, because it is now well out of the
audio passband.)

Fortunately the RX SHFT setting is per-band, so you only need apply it on
affected bands. You also have the alternative of reducing preamp gain from
20 to 10 dB. PREAMP gain, too, is a per-band setting.

The KX3's direct-conversion architecture simultaneously satisfies the
requirements of small size and low current drain, optimizing for
portable/battery use. A superhet (like the K3) would have taken too many
parts (in particular, at least three crystal filters), and a
direct-digital-downconversion architecture would have required much greater
supply current, as well as being far higher in cost. 

73,
Wayne
N6KR




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