[linrad] Re: Linrad new user

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

Hi Dave,

> I have just subscribed to this list. I would like to use LINRAD with my 
> SDR-14 to look for and demodulate weak CW signals on the microwave bands.
OK:-)

> 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 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.
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.

On my computer, a 2.66 GHz PIV, the limit under Linux is about 230 
kHz, but under Windows it is about 180 kHz.

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.

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.

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.

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.

73

Leif / SM5BSZ







> 
> I have instant messenger capability if you want to try a real time 
> session. I suspect we should email directly so as to keep the clutter 
> down on the reflector.
> 
> I appreciate any help.
> 
> Thanks,
> 
> Dave, K4TO
> 
> 
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