[hpsdr] Question re: RX attenuator

[KA2WEU at aol.com]

My experience using  "large antennas" in deed is NOT to need a  high gain 
PreAmp but to connect the AGC and a pre attenuator after about 40 dB  S/(S+N) 
has been achieved.
 
The EB 500 RX is a good example how effective this can work, and yes 1 dB  
steps are fine.
 
Ulrich
 
 
 
 
 
 
In a message dated 10/28/2012 11:57:42 A.M. W. Europe Standard Time,  
phil at pharman.org writes:

*****  High Performance Software Defined Radio Discussion List *****

Hi  John,

You are quite correct that the attenuator control is simply  in/out and the 
LTC6400-20 preamp is  actually in circuit at all  times.  This was a 
necessary compromise when designing Mercury since  we found that switching 
the preamp in/out of circuit introduced an  unacceptable number  of spurs. 
After revising the Mercury PCB some  four times we were concerned for the 
sanity of Lyle, KK7P, who was doing  the PCB layout and decided on the 
current  implementation.

Theoretically there is a slight degradation in Dynamic  Range in taking 
this 
approach but given the resulting high sensitivity and  the amazing lack of 
spurs in Hermes it appears to be an acceptable  compromise.

It would indeed be possible to implement 1dB attenuation  steps over a 31dB 
range for Hermes.  If you can construct a *totally  convincing* argument as 
to why this is necessary then you are in with a  good chance of having it 
implemented in the future :)

73  Phil...VK6APH


-----Original Message----- 
From: John  Marvin
Sent: Sunday, October 28, 2012 5:42 PM
To: HPSDR
Subject:  [hpsdr] Question re: RX attenuator

***** High Performance Software  Defined Radio Discussion List *****

I've been looking at the  Mercury and Hermes schematics, along with the
protocol document.  I've  noticed that some software has a preamp
enable/disable, while others have a  -20db attenuator control.  Looking
at the schematics, it appears that  the preamp (which I believe is a
LTC6400-20) is always enabled.   Mercury has a fixed 20 db attenuator
that can be activated by a relay under  control from the FPGA. Hermes has
a 31 db step attenuator (DAT-31-SP) that  can be controlled from the FPGA.

Looking at the protocol, it appears  that there is only a preamp
enable/disable bit.  Is that actually the  control for the attenuator, or
am I missing something? I assume this is  only an on/off control due to
the fact that Mercury only has a fixed 20 db  attenuator.  Are there any
plans to add support in the protocol (and  PowerSDR, KK, cuSDR, etc.) for
the Hermes attenuator, i.e. allow the  attenuator to be set to anything
from 0 to  -31db?

Thanks,

John
AC0ZG
_______________________________________________
HPSDR  Discussion List
To post msg: hpsdr at openhpsdr.org
Subscription help:  
http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
HPSDR web  page: http://openhpsdr.org
Archives:  http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/  

_______________________________________________
HPSDR Discussion  List
To post msg: hpsdr at openhpsdr.org
Subscription help:  
http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
HPSDR web page:  http://openhpsdr.org
Archives:  http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20121028/f2860cf4/attachment-0004.htm>