[hpsdr] Signal Paths for QSD

[Phil Harman]

Jim,

The ISD provides the correct termination of the input filter without the 
need to add preamps and attenuators. It's elegant in its simplicity.

Phil... VK6APH


----- Original Message ----- 
From: "Jim Miller" <jim at jtmiller.com>
To: "HPSDR" <hpsdr at hpsdr.org>
Sent: Sunday, October 01, 2006 6:54 AM
Subject: [hpsdr] Signal Paths for QSD


> ***** High Performance Software Defined Radio Discussion List *****
>
> I've been giving the QSD signal path some more thought. I understand that
> motivation for active integrators (with or without a twist) is the effect 
> of
> a passive integrator's impedence on input bandpass filters.
>
> I would think that solution to that would be to follow a bandpass filter
> with a suitable, e.g. pushpull Norton amp, followed by a pad. The pad 
> would
> limit impedence variations to the amp and thus to the BPF. It would also
> limit impedence variations to the QSD filter.
>
> The combination of the pad, amp and BPF would also limit radiation from 
> the
> VFO to the antenna.
>
> The output capacitors of the QSD would then be followed by suitable high
> impedence butters with the modest remaining gain necessary to match to the
> input range and impedence of the differential A/D converter. The 
> requirement
> for fancy slewrates or gainbandwidth would then be eliminated for the
> buffers making their selection considerably simpler. I suspect the 
> critical
> component count would actually be a wash or even lower for this
> configuration.
>
> Since a pushpull Norton can have a significant gain without compromising 
> IP3
> or IP2 the pad could be pretty lossy perhaps as much as 10db for a return
> loss of 20db. That could still leave a 6db net gain for the combination of
> amp and pad. Wouldn't that be enough?
>
> What have I missed? (again...)
>
> 73
>
> jim ab3cv
>
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