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Re: [Discuss-gnuradio] DBSRX Modification and Antenna Bias Question

From: Eric Cottrell
Subject: Re: [Discuss-gnuradio] DBSRX Modification and Antenna Bias Question
Date: Wed, 11 Jul 2007 20:10:38 -0400 (EDT)

----- Start Original Message -----
Sent: Wed, 11 Jul 2007 15:58:09 -0700
From: Matt Ettus <address@hidden>
To: Eric Cottrell <address@hidden>
Subject: Re: [Discuss-gnuradio] DBSRX Modification and Antenna Bias Question

> > In looking at the schematic of the DBSRX I noticed there is no real DC path 
> > to ground when DC Bias is not used.  Would there be any problem to either 
> > jumper the DC Bias connector when not used or to add a high value shunt 
> > resistor across the DC Bias connector to keep any DC levels down?  I assume 
> > the choke will keep most of the RF on the proper side.
> >   
> Shorting J101 puts 5 Volts (up to 500 mA I believe) on the antenna line.
> Applying voltage to J100 pin 1 allows you to send bias other than 5V.
> The DC-blocking cap to the LNA should be good to 50V, but I wouldn't
> push it.
> Shorting across J100 would give a DC short to your amplifier, which
> seems ok from the DBSRX side, but make sure your amp won't send
> significant current down that line.
> Matt

----- End Original Message -----


Thanks.  Maybe it would be safer to put a 10k or 100k resistor across there.  I 
can just picture the damage if the diode in the preamplifier ever failed.

I managed to change the DBSRX LNA chip on my second attempt.  I managed to lift 
the pad on the input side so I had to jumper from the capacitor to Pin 3.  
Actually the second attempt was quicker and easier.  So I got 8 LNA spares that 
will be lost the next time I need them.  I just need to test the board when I 
get home.  Hopefully the board will get a second life but I doubt it will get a 

After this experience changing that huge surface mount GAs MESFET in the AR5000 
will be a piece of cake.

73 Eric

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