[hpsdr] Question - phase noise and reflock

[Robert McGwier]

If the device is used as a frequency correction device, with no attempt 
made to do phase locking, then the phase noise performance hit will be 
almost nonexistent.  This would be accomplished by having the 
measurements made of the frequency offset (the TCXO steering voltage, 
WITHOUT actually steering the oscillator) and the results reported to 
the SDR software which would publish a corrected frequency to the rest 
of the environment.  There is no reason to pay a phase noise induced 
noise floor penalty UNLESS YOU NEED phase locking.  The phase noise 
under this scheme would still be determined by the stability of the 
final hardware crystal oscillator.  There would be infrequent phase 
discontinuities caused in the entire receive chain by correction the 
frequency in software.  I would set a "demanded accuracy"  in the 
software and if the TCXO correcting voltage change exceeded the demanded 
accuracy (threshold) then a change would be published in the software 
(and possibly DDS hardware).  This "dead band" would be the source of 
the continued phase stability.

Let me say that with a thermistor (50 C) soldered to my Volpey_Fisher 
oscillator in the SDR-1000,  it is the rock of Gibraltar (pun intended) 
and does not drift more than 1 Hz so these corrections to the software 
would be infrequent indeed if (say) between 0.1 and 1 part in 10^6 is 
enough accuracy.

Bob



Lyle Johnson wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>   
>> What is the effect on phase noise of the final LO  when you lock the 
>> clock standard?
>>     
>
> It will likely degrade it some, but that depends on how stable the LO is 
> in the first place and how much correction it requires.
>
> Neither the clock reference (Gibraltar) or an LO has yet been designed 
> for the HPSDR.  Or any other RF hardware for it -- yet :-)
>
> 73,
>
> Lyle KK7P
>
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at hpsdr.org
> Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
> HPSDR web page: http://hpsdr.org
> Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
>
>   


-- 
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!


 1147622068.0