[Elecraft] K3 synthesizer question

[Wayne Burdick]

The PLL provides coarse tuning steps only. The DDS provides fine tuning steps. The DDS crystal filter removes spurs from the DDS, which would otherwise be seen in the PLL's output. (Many radios use DDS directly, with no PLL, leading to unwanted spurious responses in both RX and TX.)

The PLL's step size is on the order of 20 kHz. Since the DDS filter is much narrower, we have to constantly recalculate the PLL divider values as you tune the VFO. A lot of magic was required in firmware.

73,
Wayne
N6KW



On Jan 14, 2015, at 3:40 PM, "James Lanier" <lanier.je at gmail.com> wrote:

> Just curious. The synthesizer uses a DDS which feeds the PLL circuit. The DDS output is filtered through a 4 pole crystal filter to remove spurs. So why use a DDS at all? The only thing I can figure is that a DDS always has lower phase noise than the DDS reference oscillator. Is this why?
> 
> Jim