[linrad] Re: Linrad new user

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

Hello Dave,

>   I don't see anything on the screen...no waterfall display or anything 
> that indicates the program is running. At one point I did get a "SDR-14 
> detected" message.
OK.

> I run a logging program and an email program most of the time while I am 
> at the radio. I don't think these two applications are too taxing on the 
> CPU.
No, total load on the CPU is no problem at all. The problem is latency.
The routine that reads the USB has to be serviced in time EVERY SINGLE
TIME when you run under Windows. Linux is forgiving, it will just
loose some data. This means that any software that you run in parallel
with Linrad has to be started before Linrad and that you must not resize
any window or do other things that lock up the CPU for a long time.
(If you run near the speed limit where the time window within which
Linrad must respond is VERY small.)

With two CPUs in the system it might be much better, but I really
do not know.

Just set a low frequency by setting big decimation numbers like CIC2=16, 
CIC5=10 and RCF=3 for a total decimation of 480 and a sampling frequency 
of 138.9 kHz. This should work on any computer with any operating system.
Once you see spectra and know Linrad is running you can step the sampling
speed upwards. (It will be easier to edit par_sdr14 than to go through
the Linrad setup when you try different speeds.)

> > Once you have decided what bandwidth to go for, you should find a pulse
> > generator to do calibration. Read here:
> > http://www.sm5bsz.com/linuxdsp/flat/ampcal.htm
> > Actually this should not be required with an SDR-14 because the
> > filter response is exactly known since it is determined by the
> > digital filters (except for the small slope on gain vs frequency
> > that your converters may give) but because of my laziness, Linrad
> > does not know what filter response the digital filters inside
> > the AD6620 will supply so you will have to calibrate to get
> > an absolutely flat frequency response.
> > 
> I have a test bench full of equipment good through 20 ghz, but I'm not 
> sure any of it pulses :-)
A low frequency square wave will be fine:-) Anything that emits a square
wave with fast rise times. You will need an attenuator if the output level
is TTL. Do the calibration somewhere between 7 and 14 MHz on the 0.1 to 30
MHz input.

> OK--Spectravue is the same...the larger the FFT sample size, the slower 
> the waterfall and the more sensitive it is.
I think you will find that Linrad differs. I will be interested in
your comments on that later on:-)

> I am leaving on a one week trip tomorrow and when I 
> get back am leaving one day later to go the MUD (Microwave Update) in 
> Pennsylvania. I will not have much time to experiment until after MUD. 
> If all else fails, I have a CD with Ubuntu 7.04 and a friend who is 
> really, really good with computers who will help me install it on my 
> machine.
There is no reason anything should fail. You can run under any
Linux distribution under X11 or with svgalib and you can run under
Windows 2000, XP and probably 98 and Vista as well. Finding the optimum
operating system could allow a slightly larger bandwidth, but if you
are happy with something like 160kHz the OS should not make any difference
at all to Linrad.

At the MUD you will have a chance to hear Roger, W3SZ. After that
I guess you will have no problem to get Linrad running. 

It is another thing to optimize parameters for a specific task.

As I understand it, first priority is finding every single station
that could perhaps be copied. Once this problem has an optimum 
solution for your environment, the next step is to find out how 
to filter out a weak CW signal as well as possible.

None of the two problems has been discussed in detail anywhere
as far as I know. It is easy in a specific situation, but a
general analyzis would be extremely complicated due to the
almost infinite number of combinations hardwares, interferences 
bandwidths of the desired signal(s) and timing issues on 
particular bands. 

73

Leif

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