[hpsdr] Project proposal - Hermes

[Phil Harman]

Lyle,

Looks like a case of dejavue all over again!!  One implementation of Hermes 
is to use Alex for its HPF/LPF/antenna switching and 6m preamp.  In which 
case it will need the LTC6400-20 preamp unless we want to redesign Alex.

A preamp is essential before the LT2208 if you want to have usable 
sensitivity on the higher HF bands. As many are finding with Mercury, the 
inherent selectivity of your antenna system is often sufficient for 
satisfactory performance on the HF bands without any additional filtering. 
By not putting a pre-amp on Hermes you are forcing the user to use an 
external one.

Having been through this preamp - no preamp loop a few times I recommend we 
leave the LTC6400 preamp in circuit at all times and  use a switchable 
attenuator to take out the excess gain i.e. just like we do with Mercury.

Happy to debate with you - since I don't have to re-do the PCB if it doesn't 
work :)

73's Phil....VK6APH


----- Original Message ----- 
From: "Lyle Johnson" <kk7p at wavecable.com>
To: "Kevin Wheatley" <m0khz at tiscali.co.uk>
Cc: "HPSDR list" <hpsdr at lists.openhpsdr.org>
Sent: Saturday, May 23, 2009 11:18 PM
Subject: Re: [hpsdr] Project proposal - Hermes


> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello Kevin!
>
> Looking over the information for Hermes in the wiki, a few thoughts.
>
> 1) Delete the LTC6400-20 preamp.  We can get differential drive to the
> ADC using a transformer.  The 6400 reduces the dynamic range of the
> system somewhat and complicates the design as well as raising the cost.
>
> 2) Since off-board receive input filters are going to be needed anyway,
> a suitable preamp and attenuator can be incorporated on that board.  If
> Alexaires is the intended filter, perhaps it can be modified to include
> a suitable preamp for the upper HF range.  Or use of the RX1 in/out
> jacks on Alexaires can accommodate such a preamp.
>
> 3) The National LM4950TS makes a reasonable speaker amplifier. It is a
> linear device, so no switching noise.  The TPA2017 is interesting, but
> I'd be a little concerned about noise, including serious power supply
> decoupling since it shares the same analog domain as the LTC2208.
>
> ---
>
> This looks to be an exciting project!
>
> 73,
>
> Lyle KK7P
>
>
>
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