Re: Linrad Block Diagram

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

Hi Guy,

> > Try to change "Parport Address" to 888. This is a bug,
> > a flag that says there is hardware on the system is not properly
> > initiated. Cheating this way should help to get freq and gain
> > controls for sdr-14 to work. (will be corrected for 02-41)
> 
> I tried that on v2.40. It didn't work for me.
> I am happy to continue with v2.39 until v2.41 is out.
Under Windows you would have to install a dll for the
parport also. I am waiting for some calibration data
which I will use to verify some new features in 2.41.
Hopefully it is not far away.

> > In the A/D D/A setup you have not specified
> > "Select direct input"
> 
> No, the SDR-IQ has no option for Direct Input, and that question
> does not appear. My stuff is directed only at the SDR-IQ.
Oooh! I forgot this difference...

> > The "RCF Output Shift" is very large....
> > The signal level required to turn on the red LED for
> > A/D overload is +5dBm. At +4 dBm, the amplitude
> > margin on the Linrad input is 9.55 dB which means that
> > the RCF Output shift is one bit too large. (At least.)
> 
> The SDR-IQ appears to be different!
> At maximum gain (0dB set in Linrad), A/D overload occurs at a SigGen
> input level of -23dBm, whereas -24dB is ok.
> (Measurements at the end of about 20m of RG223 at 28.1MHz).
OK.

> At -24dB in and max gain with RCF shift=6 bits,
> signal peak is at 114dB on  main spectrum,
> noise floor (SigGen still on) is 19dB,
> A/D margin is 9.15dB (as you noted above),
> and with SigGen off noise floor falls to 9dB, so there is SigGen
> noise!
I would not dare to say there is noise from the signal
generator on this experiment only. The 10 dB lower
noise floor could be due to sampling effects.

> At -24dBm in and max gain with RCF shift changed to 5 bits,
> signal peak is at 120dB on  main spectrum,
> noise floor (SigGen still on) is 25dB,
> A/D margin is down to 3.12dB,
> and with SigGen off noise floor falls to 14dB.
Now we can draw conclusions.
Signal is up by 6 dB. (A real worry if it was not)

Noise floor is up by 6 dB with generator on. Good.

Noise floor is up by 5 dB with generator off ?????
You do have a wideband noise floor in the generator,
the noise floor with generator off was not strongly
attenuated by being much smaller than the A/D resolution.
If you check this again, maybe it is actually 6 dB
rather than 5.

> With the whole receive system connected to the SDR-IQ,
> RCF output shift at the lower value of 5 bits,
> and gain set to my usual level of -9dB (attenuator not in),
> noise floor from antenna is at 29dB,
> preamp turned off reduces this to 13dB,
> and disconnecting transverter from SDR-IQ input drops this to 9dB.
OK. This looks fine. You might set "First FFT amplitude to" 500 to
move the noise floor a little lower. It would affect the level
required for a strong signal to become distorted when you force it
to be a "weak signal" by listening to it.

> Note that if I reduce the "gain" to -10dB,
> the -10dB attenuator kicks in and the internal preamp gain of the SDR-
> IQ rises by 9dB,
> the noise floors are then 27, 15, 14dB respectively.
> So the -10dB attenuator in the SDR-IQ lowers the noise drop from 16 to
> 12dB
> when the preamp is switched off.
OK. Not surprising. Maybe it would be better to control the amplifier
and the attenuator separately. It depends on the properties of the
variable gain amplifier.

You might change the function set_sdr14_att(void) in sdr14.c
which is like this for the SDR-IQ:
if(ui.rx_addev_no==SDRIQ_DEVICE_CODE)
  {
  sdr14cmd[4]=1;
  if(fg.gain > -10)
    {
// Gain is in the range 0 to 9 dB.
    t1=-0.05*fg.gain;
    t1=pow(10,t1);
    t1=127/t1;
    sdr14cmd[5]=t1;
    }
  else
    {
    t1=-0.05*(fg.gain+10);
    t1=pow(10,t1);
    t1=127/t1;
    sdr14cmd[5]=t1;
    sdr14cmd[5]|=0x80;
    }
  }
Try to change it like this:
if(ui.rx_addev_no==SDRIQ_DEVICE_CODE)
  {
  sdr14cmd[4]=1;
    t1=-0.05*fg.gain;
    t1=pow(10,t1);
    t1=127/t1;
    sdr14cmd[5]=t1;
    }
  }

> > Use "I=Soundcard test mode" to see the digital input
> > that Linrad receives. (use a normal mode to set
> > frequency and gain first) With the '+' button you can
> > increase the gain until you see the individual bits.
> 
> I tried this, but didn't really understand how to interpret the
> display,
> which said A/D Max ~4000, A/D Min ~-4000.
It works as an oscilloscope. Presumably you have a signal
generator connected and there should be sine-waves on the
screen. Press 'P' to freeze the track. Make the frequency
close to the center frequency to see the sinewaves well.

When you reduce the signal level (by 60 dB) you should see
a horizontal line only with  A/D Max ~4, A/D Min ~-4.

Press '+' 10 times for 60 dB gain and you will again see
the sine curves for I and Q. This time you will see
the 8 distinct levels belonging to the A/D converter.

Actually that is what I see with the SDR-14. The sdr-IQ 
has more noise, so maybe you will see many more than 8
levels of the A/D in use. Then try with lowest gain to
minimize the noise into the A/D. 

73

Leif / SM5BSZ


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