RE: [linrad] frontend LO frequency stability ( was : audio channels )

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi Perer and All,

> End of all, such QRSSSSS.... dots with very narrow filtering ( 0.1-0.01 Hz
> or less ? ) demands for very stable LO, if the FFT bin bandwidth in Linrad
> may not be spoiled by frequency drift ( walking from the one bin into the
> neighbour bin which decreases the " readability " of Linrad's trace on the
> screen and " smears "  that trace.
There is no reason at all to go below 0.1 Hz bin width. The signal 
will be smeared out over about 10 bins if you use a sine squared 
window. 50% of the power will be in 5 bins, a bandwidth of about 0.25Hz.
You need a local oscillator with a short term stability that is better
than this.

> Thus, the crux of my question was : is a Linrad front end LO stability of
> better  than  1x10^-11 ( which I can get easily ) necessary for 
> my purpose ?
> Actually, it does cost me some effort to finish this type of LO 
> and if this
> is not worth the effort  I'll renounce and use a ordinary TC(V)XO 
> LO, being
> however then no better than 1 x 10^-7 ( if at all ).
I do not know how you define these numbers. The bandwidth of the LO should
be 0.1Hz or below. Frequency drift has to be in the order of 0.1Hz in 
10 seconds or below that. 0.01Hz/s or 0.5 Hz/min.

> Any commnets welcome ( should I use  my GPS disciplined LO for Linrads RF
> frontend yes or no ? ).
The TCXO inside the unit is surely good enough even if you do not 
connect any GPS antenna. I am using my Navstar GPS unit in this stupid way
to dicipline my HP8657A generators which are otherwise not good enough
to use as test signals when working with 144 MHz local oscillators.

The low noise local oscillators in the WSE high level converters that I 
currently work with are stable enough if they are kept at constant temperature.
An FFT that spans 40 seconds with a bin bandwidth of about 0.05Hz 
(50% interleaved transforms with a sine squared window) will have a bandwidth
below 0.1Hz most of the times when averaged over 2 minutes. It may go up
to 0.15 Hz occasionally but as I have them, lying on a table indoors, there
is nothing to gain with a better LO for 144 MHz QRSS except for absolute
frequency accuracy.  


73

Leif / SM5BSZ



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