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<DIV>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.</DIV>
<DIV> </DIV>
<DIV>The EB 500 RX is a good example how effective this can work, and yes 1 dB
steps are fine.</DIV>
<DIV> </DIV>
<DIV>Ulrich</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
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<DIV>In a message dated 10/28/2012 11:57:42 A.M. W. Europe Standard Time,
phil@pharman.org writes:</DIV>
<BLOCKQUOTE
style="BORDER-LEFT: blue 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px"><FONT
style="BACKGROUND-COLOR: transparent" color=#000000 size=2 face=Arial>*****
High Performance Software Defined Radio Discussion List *****<BR><BR>Hi
John,<BR><BR>You are quite correct that the attenuator control is simply
in/out and the <BR>LTC6400-20 preamp is actually in circuit at all
times. This was a <BR>necessary compromise when designing Mercury since
we found that switching <BR>the preamp in/out of circuit introduced an
unacceptable number of spurs. <BR>After revising the Mercury PCB some
four times we were concerned for the <BR>sanity of Lyle, KK7P, who was doing
the PCB layout and decided on the <BR>current
implementation.<BR><BR>Theoretically there is a slight degradation in Dynamic
Range in taking this <BR>approach but given the resulting high sensitivity and
the amazing lack of <BR>spurs in Hermes it appears to be an acceptable
compromise.<BR><BR>It would indeed be possible to implement 1dB attenuation
steps over a 31dB <BR>range for Hermes. If you can construct a *totally
convincing* argument as <BR>to why this is necessary then you are in with a
good chance of having it <BR>implemented in the future :)<BR><BR>73
Phil...VK6APH<BR><BR><BR>-----Original Message----- <BR>From: John
Marvin<BR>Sent: Sunday, October 28, 2012 5:42 PM<BR>To: HPSDR<BR>Subject:
[hpsdr] Question re: RX attenuator<BR><BR>***** High Performance Software
Defined Radio Discussion List *****<BR><BR> I've been looking at the
Mercury and Hermes schematics, along with the<BR>protocol document. I've
noticed that some software has a preamp<BR>enable/disable, while others have a
-20db attenuator control. Looking<BR>at the schematics, it appears that
the preamp (which I believe is a<BR>LTC6400-20) is always enabled.
Mercury has a fixed 20 db attenuator<BR>that can be activated by a relay under
control from the FPGA. Hermes has<BR>a 31 db step attenuator (DAT-31-SP) that
can be controlled from the FPGA.<BR><BR>Looking at the protocol, it appears
that there is only a preamp<BR>enable/disable bit. Is that actually the
control for the attenuator, or<BR>am I missing something? I assume this is
only an on/off control due to<BR>the fact that Mercury only has a fixed 20 db
attenuator. Are there any<BR>plans to add support in the protocol (and
PowerSDR, KK, cuSDR, etc.) for<BR>the Hermes attenuator, i.e. allow the
attenuator to be set to anything<BR>from 0 to
-31db?<BR><BR>Thanks,<BR><BR>John<BR>AC0ZG<BR>_______________________________________________<BR>HPSDR
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