[hpsdr] Horton LO

[Robert McGwier]

Bob


pvharman at arach.net.au wrote:
> ***** 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. 
>   

This is almost surely a PLL design fault.

> 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.
>   

If we get the GHz VCO from multiplying some lower frequency oscillator 
up we will pay in an increased phase noise penalty and in fact, the 
phase noise will never be as good as the lower frequency oscillator 
inside. How does it work? If you have sent a candidate part or design, I 
have missed it. Can you repeat?


> Anyone with synthesizer design experience out there able to comment please?
>
> 73's Phil...VK6APH 
>
>
>   


-- 
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
Laziness is the number one inspiration for ingenuity.  Guilty as charged!


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