[hpsdr] Horton LO

[Leon Heller]

----- Original Message ----- 
From: <pvharman at arach.net.au>
To: <hpsdr at hpsdr.org>
Sent: Friday, June 16, 2006 6:17 AM
Subject: [hpsdr] Horton LO


***** High Performance Software Defined Radio Discussion List *****

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?


The Racal patents might be worth looking at, also. I remember my boss at 
Racal telling me how their fractional-N synthesiser worked, he mentioned 
that it was so good that Marconi copied it, and got sued.

73, Leon 


 1150440887.0