[Linrad] Re: SDR-IQ USB Limits

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

Hi Thomas,

"Thomas D. Dean" <tomdean@xxxxxxxxxxxxx> wrote:

> I ran several tests, varying the decimation, keeping the FFT BW at 30Hz.
> I found a combination that fails.  See the bottom
> 
> CIC2, CIC5, RCF, RCF shift, sample clock shift, db
> 2,2,20,0,0,25 fft1 size 16K
> Spectrum Ok
> RX A/D Speed Error 252056 Hz (Nominal 833333 Hz)
This already failed. The data delivered by the SDR-IQ is only 252 kHz
so you are loosing 70% of the data. There should not be any errors
in /var/log/syslog because the errors are internal in the SDR-IQ which
is not capable of sending data faster than 252 kHz.

Presumably you get blocks of correct data but long sequences would
be missing. The spectrum might look nearly OK but if you look
at a single very strong signal you should see a broadening due
to the phase jumps that would happen for each block of missing data.

> 2,2,15,0,0,25 fft1 size 16K (Nominal 1111111 Hz)
> Spectrum Ok
> RX A/D Speed Error 253224 Hz (Nominal 1666666 Hz)
> 
> 2,2,10,0,0,25 fft1 size 32K
> Spectrum Wrong - NLK at 24.8 KHz is not visible, just noise
> RX A/D Speed Error 253224 Hz (Nominal 3333333 Hz)
> 
> 2,2,5, 0,0,25 fft1 size 64K
> Spectrum Wrong- NLK at 24.8 KHz is not visible, just noise
> 
> 2,2,2, 0,0,25
> Spectrum Wrong- NLK at 24.8 KHz is not visible, just noise
> 
> In all cases, I could press X and get to the mode help page and X again
> to get to the main menu.
> 
> The timer error message seems to be limited around 250KHz.  I started
> looking at this, but, 
> 
> This seems repeatable.
> 2,2,30,0,0,25
> FFT BW 30
> MEMORY ERROR mm=2[56]  data is0 (56)  Call no 198
Yes. This is the reason why there is a test to not allow far too
high sampling rates. 

I no longer remember the exact cause, but as I can recall it has
something to do with overflow in the computation of a buffer size.
The limit you see at 252 kHz is close to what I see on my SDR-14
so I will set the limit to 300 kHz for future Linrad versions.

73

Leif / SM5BSZ

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