[Elecraft] KX3 AM-breakthrough

[Wayne Burdick]

"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