[hpsdr] Proposal for a 'low-end' RX/TX board for HPSDR

[Ray Anderson]

-----Original Message-----
From: CT1IZU [mailto:hands at iol.pt] 
Sent: Friday, July 21, 2006 3:13 PM
To: Lyle Johnson; Ray Anderson
Cc: hpsdr at hpsdr.org
Subject: Re: [hpsdr] Proposal for a 'low-end' RX/TX board for HPSDR

>
> 2) include the Rx "LNA" for impedance matching and with sufficient
> dynamic range to do the rest of the system justice;
>


I can see the argument for the front end LNA at 10 meters (or maybe even
15 meters), but on the bands below that I think it may actually be a
liability unless you have an extraordinarily high IP3 for the preamp.
While this can be done it comes at the expense of quite a bit of DC
current. This may not be a big deal but is something to be considered.

A QSD with a decent BPF between it and the antenna actually makes a
fairly decent receiver, though some isolation between the QSD and the
filter would be a good thing. I've seen designs where a resistive pad
was used to provide some Z matching and isolation. This obviously
degrades the noise figure, but at HF below 20 MHz or so the background
noise is such that a few dB degradation of the RX noise figure will
probably go unnoticed.

-Ray	WB6TPU




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