Re: Linrad Block Diagram

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

Hi Guy,

> BTW I have posted a list of the Linrad Setup Parameters I am currently
> using at
> http://www.vhfdx.radiocorner.net/LinradParameters.html

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)

In the A/D D/A setup you have not specified 
"Select direct input"

The "RCF Output Shift" is very large. On my SDR-14,
the signal received is only zeroes for input levels
up to -89 dBm. The signal level required for a single
sine-wave to be free of digitalization sideband noise
is about -65 dBm.

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.)

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.

With your parameters, an input signal of -76 dBm
produces +/- one bit. Your noise floor has to be
strong enough to give something like +/- 5 bits
on the average (I do not know the exact number, 
you can find it in text-books) for the quantization
noise to contribute less than 0.05 dB.

If you make a graph of noise floor level as measured
by Linrad vs gain you will find a straight line with
a knee where the noise floor goes to zero.

The knee is soft, it ranges over something like 20 dB.
The statement that your noise floor should increase
by 15 dB is valid only if it already is above the knee.
Otherwise it has to increase by at least 15 dB above
the level you would get by extrapolating the straight line
below the knee. Use the RCF output shift to set gain in 
6dB steps and check the noise floor you get.
Easiest if you change par_sdr14 with a text editor and
restart Linrad.



> I am waiting to add them to the Linrad public listing until I am more
> sure that they are
> reasonably free from obvious stupidities.
> I have also drafted some comment on using the SDR-IQ with Linrad at
> http://www.vhfdx.radiocorner.net/LinradSDRIQ.html
I have not really checked this. My memory is short and it would
take quite some time.

One thing though. The fft1 version numbers are for different
floating point implementations. In earlier versions there
was a slightly faster approximate algorithm (first transforming
the one-dimensional real input to a two-dimensional input,
I and Q at half the sampling rate in case the input was
from a single audio channel.) It was important long ago for
my own stereo receiver when I used 2*20 kHz from my XPOL
antenna on a Pentium MMX at 166 MHz. (If I remember correctly)

Now, the option to choose is not of much value on a single
channel system. Essentially they differ in how the cashe
is used. With a two channel system it is different because
modern computers have SIMD instructions (fft1 version 5)
and they run significantly faster. I have not written
a fast fft for single channel receivers since I do not
think there is any need. Maybe with the Perseus at 1MHz
I will have to reconsider;-) The optimum today should be
to use 32 bit integer multimedia instructions and to
put the work in parallel threads.

I have many things to attend to after having been away for
6 weeks so I will look at your sites later on.

73

Leif

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