[Elecraft] K3 and AM RX [explanation]

[Wayne Burdick]

> When tuning across an AM station there are clicks and cracks, seems  
> to be on even kHz. On my radio it has been from day one. Is this  
> normal or is my radio faulty? I use latest FW. / Jim

Jim,

When tuning near a very strong continuous carrier, you may hear a  
small AF artifact at points where the PLL divider switches. This  
occurs at different intervals depending on the band. On 20 meters the  
interval averages about 15 kHz. We minimize the effect by introducing  
a small amount of DSP limiting during the switching window (~10 ms).

This is not avoidable with the present synth design, which was  
optimized for extremely low phase noise. There are two contributing  
factors. First, we follow the DDS with a very narrow crystal filter to  
dramatically clean up this signal; frequency shifts through the filter  
are subject to a small amount of group delay. (Other rigs using DDS  
use either no filtering, or very wide filtering, leaving them  
susceptible to DDS spurs.) Second, the VCO control bandwidth is low,  
improving close-in phase noise, but again causing a small re-lock  
delay when the PLL dividers are switched. (Most rigs use a wider VCO  
control bandwidth, and thus suffer from greater phase noise at small  
offsets that are important to ham operation, i.e. < 2 kHz from the  
carrier.)

We feel that this was the right design tradeoff, and I think most  
customers would agree. Dynamic range is greatly improved with a low- 
noise synthesizer, both on transmit and receive.

73,
Wayne
N6KR