[hpsdr] Question re: RX attenuator

[Phil Harman]

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
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