[linrad] Re: Linrad new user

[Dave Sublette <arrl.net; k4to@xxxxxxxxxxxxxxxx>]

Hello Leif,

Well, Well! There is nothing like getting advice from the author of the 
software!! Thanks for answering.

Leif Asbrink wrote:

> > I have downloaded the software for LINRAD. I have scanned through the 
> > help file. I have attempted to run LINRAD and have entered some initial 
> > values in the setups. (They are probably wrong.)
> If you can see and hear signals they are not "wrong" - but they could
> be far from optimum for your purpose....
 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.


> > I would like to have someone help me through the initial learning curve, 
> > if only to direct me to which articles I should read to begin with. The 
> > learning curve appears to be very steep to me at this point.
> This seems to be a reasonable assumption;-)
> To start with, it would be a good idea to master the SDR-14,
> in other words to get an idea about what the AD6620 is doing.
 I have used the SDR14 for over a year with the SpectraVue software and 
I am disappointed in the results. That is what led me to try Linrad. I 
am a retired electronics engineer and so I so have some understanding of 
FFT processes. However, I have never worked with them professionally and 
would not claim to know much about it.

> In Linrad you have full freedom to select the decimation factors 
> and to get various compromises between spur rejection and visible
> bandwidth. For microwaves I guess spurs would not be any problöem at
> all, and then you should look for a small value for RCF decimation.
> Just play around with CIC2, CIC5 and RCF decimation to see what your 
> computer can handle. The A/(D clock is 66.66666 MHz so you need
> the product of all three decimation factors to be somewhere around
> 400 to 300 for a bandwidth between 166.66 kHz to 222.22 kHz.
> Maybe you will find that CIC2=16, CIC5=10 and RCF=2 for a total
> decimation of 320 and a sampling frequency of 208.33 kHz will be OK
> for your computer. Most likely under Linux, but under Windows you
> would probably have to use some more decimation.
This appears to be a good starting point for me. My computer is a dual 
CPU DELL XPS, about a year old.

> On my computer, a 2.66 GHz PIV, the limit under Linux is about 230 
> kHz, but under Windows it is about 180 kHz.
Ok on this. I will look for this.

> It will be a good idea to verify that Linrad does not fail when you
> do other things that you intend to do on the same computer in case you
> want to use it simultaneously for other purposes. This is more likely to
> be a problem under Windows than under X11.

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.
> 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 :-)


> Once you have a flat frequency response (after calibration) you
> will find that a slower waterfall will supply better sensitivity.
> The bin bandwidth as well as the averaging is important. I suggest
> you start by getting a calibrated system at the bandwidth of your 
> choice. The next step would be to decide whether to use the second 
> FFT or not and that will depend on what you can observe with a 
> calibrated system.

OK--Spectravue is the same...the larger the FFT sample size, the slower 
the waterfall and the more sensitive it is.
> This mailing list is far from overloaded and your problem is the
> same as many other operators face so I think it will be a good 
> idea to discuss it on the list.

Wonderful! Thanks. 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.
>
Thanks again, Leif. I will be continuing to work on Linrad as time 
allows. Being retired is very hard. There is so much to do and so little 
time.

73,

Dave, K4TO

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