Re: LF: Alternative high resolution frequency source for LF

[Andy Talbot <andy.g4jnt@gmail.com>]

Fref	10	MHz
R div	20
VCO / Output Divider	50
External Divider	512
Final output freqeuncy	137000.5120	Hz
Step Size	10	uHz
DDS Chip Output Freq	70.14426	MHz
VCO Fundamental	3507.213108	MHz
Fcomp	0.5000	MHz
Step size at VCO	0.256	Hz
Denominator D	1953125
Fract  F	832452
N divider	7014
Actual Output Freq	137000.51202	Hz
Actual Output  step	10.0000	uHz
Output Error	0.00	uHz

On 25 November 2015 at 22:48, DK7FC <selberdenken@posteo.de> wrote:

Hmm,

Use Spectrum Lab with a GPS reference signal, compensating the soundcard drift, and a GPS-locked mixer and you're down in the 10 uHz range and also ready for nearly any mode used on the band! :-)

73, Stefan

Am 25.11.2015 23:00, schrieb Andy Talbot:

Although DDS sources seem to more-or-less dominate where high resolution / accuracy is needed, there is another route that could be worth investigating:   Fractional-N synthesis with addtional dividers.

For example, the LMX2541  with a  10MHz reference  phase comparator input will generate steps as low as 2.38Hz at its minimum internal VCO frequency of 2.2GHz.  A programmable output divider goes to divide-by 63, allowing steps of 0.038Hz at 35MHz .  Add a single chip binary divider of 256 (for the 137kHz band)  and you're down to 150uHz steps - which are completely deterministic.

Since phase noise and Fract-N sidebands are hardly an issue with this degree of output division, the phase comparator could be operated at a much lower frequency, say 500kHz, allowing steps of  lower than 10uHz.   Which is a significantly higher resolution than the 2.3mHz steps the traditional AD9850 32 bit DDS offers. 

I've used these Fract-N devices extensively at microwaves,  see http://www.g4jnt.com/LMX2541_Synth_Module.pdf ,    but haven't yet turned one into an LF exciter

Andy  G4JNT