[hpsdr] Horton LO

[pvharman at arach.net.au]

A few further thoughts about the local oscillator for Horton. 

We seem to be considering two options:

1.	A DDS based design
2.	Dividing down a GHz oscillator

Both would appear to give very good phase performance. The DDS may have spur 
issues and the divider method will be more complex. 

For those that have access to the latest DDS chips what short of performance 
are you seeing? Are the latest devices really suitable of use on  10/6m  
directly as a LO in terms of spur levels?

I had a look at Oleg  UR3IQ article on Fractional-N. His reported phase noise 
is quite good  (-120dBc/Hz at 2kHz) but looking at the reported performance of 
the Orion (which uses UHF oscillators divided down) they are getting -130dBc 
at 200Hz. I see that for some reason the Orion phase noise actually degrades 
at  40-50kHz, not quite sure why that should be. 

I also looked at the US patent (5,847,615) that Oleg mentions. More complex 
than Oleg’s circuit but should be capable of very good performance.

Returning to  option 2.  I can’t help thinking that if we use a GHz VCO and 
divide this down to  1-120MHz in say 40kHz steps and then divide by 4 to give 
us quadrature signals at 10kHz steps then we may be able to do this without 
the need to resort to Fractional-N techniques.  With 96/192k A/D sampling 
doing the fine frequency resolution in the DSP code will be fine.

Anyone with synthesizer design experience out there able to comment please?

73's Phil...VK6APH 


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