Xref: utzoo rec.ham-radio:13441 sci.electronics:7897
Path: utzoo!attcan!uunet!ginosko!brutus.cs.uiuc.edu!lll-winken!tekbspa!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re: New cheap 200 kHz ADC: good for digital receiver?
Summary: Digital Radio / DSP commnt
Message-ID: <2378@optilink.UUCP>
Date: 22 Sep 89 17:38:10 GMT
References: <1989Sep19.171259.1670@mentor.com> <17673@bellcore.bellcore.com>
Organization: Optilink Corporation, Petaluma, CA
Lines: 32

In article <17673@bellcore.bellcore.com>, karn@jupiter (Phil R. Karn) writes:
> >  Antenna ---> Mixer ---> LPF/gain --> ADC -----> DSP -----> DAC -----> audio
> >                 ^                                |
> >                 |                                |
> >              DDSynth                             \------> data
> >
> >A few real RF stages + cheap digital VLSI = high-performance receiver?
> 
> Actually, you'll want two mixers, low pass filters and A/D converters.
> The mixers should operate in quadrature so you can do things like SSB
> filtering in DSP software.
> 
> Phil

Correct me if I'm mistaken, but we can still do the SSB phasing demodulation
in software with the receiver as shown in the diagram (assuming blazingly fast 
processing hardware).  The only function of the mixer is to shift the input 
signal frequency down to a more comfortable range for the A-D and DSP.

Of course, if you had REALLY fast hardware and software, you could dispense
with the mixer and local oscillator altogether...  I vaguely remember a 
conversation I had with the designer of some sort of (fairly low frequency?)
(aeronautical navigation?) receiver, where they did essentially this in an 
ASIC.  It doesn't seem entirely practical to me now, so I'm probably forgetting
something critical.  I'll post again if I can come up with the details.



-- 
Paul M. Elliott      Optilink Corporation     (707) 795-9444
         {pyramid,pixar,tekbspa}!optilink!elliott
"I used to think I was indecisive, but now I'm not so sure."