[hpsdr] synthesizers.

[pvharman at arach.net.au]

Phil N8VB and I have been testing a CORDIC DDS FPGA GPL core written by Darrell 
Harmon for a Spectrum Analyzer project he is working on - see

< http://dlharmon.com/ > 

Darrell's implementation is very good with a tiny number of spurs all ~120dB 
down over the range 0 - 55MHz (in practice these will be below the band noise 
of the receiver). At the moment we are both using this as the complex 
oscillator in the Mercury receiver. 

Since the CORDIC takes ADC samples directly and converts them into I and Q 
there is no need for a DAC to convert the DDS output into a sine wave. The fact 
we don't need a DAC eliminates the spurs that this would otherwise produce. 

By using a high quality reference oscillator for the CORDIC we can also get low 
phase noise. This now seems a very viable alternative to a synthesizer. 

73's Phil..VK6APH 



Quoting Naylor Jonathan <naylorjs at yahoo.com>:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hi Frank
> 
> > A string of
> > dividers off the VCO would allow 1 HZ tuning and low phase noise. I
> > bet this would be cleaner than any DDS.
> 
> You don't need particularly fine stepping of the LO. The DttSP core is
> quite capable of interpolating, so putting fine stepping requirements
> onto oscillators is unneeded. At this present time DDS's just aren't
> good enough for the most part, at least not when the quality of the
> rest of the hardware is so high, it becomes the weak link.
> 
> There are some new fractional-N synthesizers coming out from AD which
> run from a higher supply voltage which would provide a greater voltage
> swing.
> 
> >   frank 
> 
> Jonathan  ON/G4KLX
> 
> 
> 
>  
> 
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