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Re: [Discuss-gnuradio] Mode S and ADS-B

From: Eric A. Cottrell
Subject: Re: [Discuss-gnuradio] Mode S and ADS-B
Date: Sat, 07 Jul 2007 09:47:24 -0400
User-agent: Thunderbird (X11/20060911)

Matt Ettus wrote:
> Eric A. Cottrell wrote
>> One thing to try is to use the scope program to set the threshold.  The
>> default is low for alot of front ends including the DBSRX.  The DBSRX
>> tends to be deaf.  Also I find a gain setting of 32 to 36 seems to work
>> best for the DBSRX.  
> The DBSRX typically has a noise figure around 3-5dB.  I wouldn't call
> that deaf.  You are connecting a discone to a wide open front end
> without a filter, so it is far more likely that you are experiencing
> desense from the thousands of other signals you are picking up.  Also,
> raising the gain above 32 would reduce the noise figure you see.
>> Getting the antenna up in the clear is important.  A discone will work
>> if you have a good receiver but I found building a simple 1/4 wave
>> ground plane on an N connector beats out the discone.

Well I got spoiled with my AOR 5000 as a front end. It uses a GAs MES
FET as a front end for 1.0 to 1.6 GHz.  Unfortunely the GAs MES FET is
not working but it only cuts the range in half.  I was using an external
Preamp but that got broken when I hooked it up in the car.  I assume
when they say 12 volts that they mean 12 volts and not over 13.6 volts.

I tended to use 36 as gain for the DBSRX.  I used my DBSRX board
yesterday and got different results from earlier tests.  It appears the
  Maxium chip is working but the LNA is gone.  Time to order another one.

> Probably because it is more frequency selective than the discone, thus
> reducing the strong interferers.
> When you set up a discone on the DBSRX you need to understand that you
> are picking up multiple cellphone bands, the pager bands, 2 ISM bands,
> microwave ovens, radars, etc, in addition to the nice weak signals you
> are looking for.

Not to mention the decrease of gain from the discone.  Another factor at
my station is my discone is lower and has more blockage.  I need to
replace the coax at my test antenna site because the N connector came
off and the coax got exposed to the rain.

I am upset because I want to test my code to make sure my later changes
did not break anything but hardware keep breaking!  So it may be
possible the code in my branch does not work as good as past code.  I
want to do some testing of various setups but I need to get everything
working again. :(

73 Eric

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