[hpsdr] exact frequency determination

[k3it]

Sounds like Brian is getting ready to obliterate competition in the ARR FMT
test ;). A single digit millihertz accuracy will certainly place you at the
top of the charts for quite some time.

-- 
73 Vasiliy K3IT


On Wed, Nov 20, 2013 at 7:57 PM, John Marvin <jm-hpsdr at themarvins.org>wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
>  Brian,
>
> The formula that is used in theory is:
>
>     (Frequency * 2^57/122880000) >> 32
>
> If you reduce that it becomes:
>
>     (Frequency * 2^41/1875) >> 32
>
> So, theoretically any frequency that is evenly divided by 1875 would be  a
> "perfect" frequency. I use that rule when setting frequencies for the FMT.
>
> However, in practice, the fpga uses integer math. 2^41/1875 is:
>
>     1172812402.961066666....
>
> The fpga code rounds this to 1172812403 and uses that constant for the
> computation. So the actual formula used in the FPGA is:
>
>     (Frequency * 1172812403) >> 32
>
> Therefore there is always some error, however it is minimized at
> frequencies that are evenly divided by 1875. In those cases I believe the
> error is less than .1 mHz.
>
> This information should probably be put on the HPSDR wiki someplace.
>
> Regards,
>
> John
>
>
>
> On 11/20/2013 11:36 AM, Brian Lloyd wrote:
>
> ***** High Performance Software Defined Radio Discussion List *****
>
>
>
>
> Several of the things I like to do involve precise frequency measurement
> in the HF spectrum using a Hermes and/or Angelia board, and PowerSDRmrx.
> Since my frequency reference is either a GPSDO or an Rb reference, the
> potential accuracy is very high. The actual limit has become the error in
> radix conversion between the decimal frequency value and the binary
> frequency control word used by the NCO in the Hermes/Angelia FPGA
> "firmware" plus the error in PowerSDR's NCO frequency control word.
>
>  Now this error is not large, typically on the order of 20 or so mHz most
> of the time, but there are some combinations of control words that produce
> larger errors.
>
>  Now these errors are completely deterministic. When one enters a
> frequency in decimal that produces a binary control word for the nearest
> possible frequency, usually within a couple of handsful of mHz. But if you
> know the binary control word, it is possible to work that backwards to the
> ACTUAL frequency. So, yes, if I enter 7,150,000.000 Hz, I really DO want to
> know that the radio actually tuned to 7,150,000.037 Hz.
>
>  What would be nice would be to have PowerSDR actually include this
> calculation and display it, or, better yet, include it as the actual value
> represented when queried with CAT. Still, just knowing the calculation so I
> can do my own corrections by hand would be extremely useful.
>
>  Thanks in advance.
>
>  --
> Brian Lloyd, WB6RQN/J79BPL
> 706 Flightline Drive
> Spring Branch, TX 78070
> brian at lloyd.com
> +1.916.877.5067 (USA)
>
>
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at openhpsdr.org
> Subscription help: http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
> HPSDR web page: http://openhpsdr.org
> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/
>
>
>
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at openhpsdr.org
> Subscription help:
> http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
> HPSDR web page: http://openhpsdr.org
> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20131120/9d7df427/attachment-0004.htm>