[linrad] Re: Linrad, MAP65, and SDR-IQ

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hello Joe,

> So it seems that to provide a usable bandwidth of about 90 
> kHz with the SDR-IQ, MAP65 must provide an option to use 
> input data sampled at some new rate commensurate with 
> 66.6667 MHz, with an integer divisor somewhere around 700. 
> The divisor must be factorable into three integers 32 or 
> smaller.
> 
> Several good possibilities for the divisor, and their 
> factors, would seem to be:
> 
> 684: 2 2 3 3 19      ==> 19*6*6, 19*9*4
> 690: 2 3 5 23        ==> 23*5*6, 23*10*3
> 693: 3 3 7 11        ==> 11*9*7
> 696: 2 2 2 3 29      ==> 29*8*3, 29*6*4
> 700: 2 2 5 5 7       ==> 10*10*7
> 702: 2 3 3 3 13      ==> 13*9*6
> 704: 2 2 2 2 2 2 11  ==> 11*8*8
> 714: 2 3 7 17        ==> 17*7*6
> 715: 5 11 13         ==> 13*11*5
> 
> Am I thinking about this correctly?  Is the present Linrad 
> set up so that it can command the SDR-IQ to do these things?

YES:-)


Would it not be possible to allow any sampling frequency that
is a multiple of the sampling rate used for the final decode process?

I do not remember the details, but as I recall, 11025/2=5.5125 kHz
is used in WSJT. The ratio 12093.726 then becomes interesting.
12096=64*63*3 could then fit for using the SDR-14 at a sampling
rate of 347.2875 kHz (which is impossible) but if the decode
of MAP65 would be happy with say 5.5125/2 = 2.75625 kHz (or lower)
one could set the AD6620 to divide by 384 to supply 173.611111
kHz to Linrad and then the integer downsample by 63 times would
bring 2.755732 kHz, a factor 0.999812 from nominal which I guess 
is close enough with some margin.

73

Leif / SM5BSZ


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