[Elecraft] Re: Comparison: K3 and SDR 5000A

[wayne burdick]

Lyle Johnson wrote:

> 2b) In the typical QSD/ISD system, a quadrature signal is generated by 
> a DDS.  DDSes can have very low phase noise, but typically have 
> numerous spurs.  While mitigation techniques exist, the fact remains 
> that there are usually several spurs only 70 or 80 dB down inside a 
> given 200 kHz wide passband.  These spurs can mix with other signals.

Just a quick follow-up to Lyle's posting.

The K3's synthesizer uses a DDS, too, but it's almost completely 
isolated from the signal path, in two ways.

First, it is followed by a very narrow 4-pole crystal filter (about 2.5 
kHz wide), which removes harmonics as well as both narrowband spurs 
(over a carefully selected tuning range) and wideband Nyquist sampling 
spurs.

Second, the DDS drives a PLL and very low-noise VCO. The VCO has a high 
C-to-L ratio, and very little of the capacitance is represented by the 
varactor diodes. This is accomplished by breaking the VCO tuning range 
into 128 bands.

All of the above makes life difficult for your MCU firmware guy, but 
the result is a *very* clean signal. And this code was finished a long 
time ago    :)

73,
Wayne
N6KR


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