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Re: [Discuss-gnuradio] why quadrature samples?

From: Dave Emery
Subject: Re: [Discuss-gnuradio] why quadrature samples?
Date: Thu, 23 Jan 2003 19:20:35 -0500
User-agent: Mutt/1.4i

On Thu, Jan 23, 2003 at 11:40:53AM -0800, Johnathan Corgan wrote:
> What needs designing is a generic RF -> IF downconverter that covers DC 
> to 3 Ghz, and has programmable bandpass filtering to restrict the 
> channel bandwidth to whatever you need before inputting to the 
> functional block above.

        For some applications one can get by with various tuners  (the
infamous cable modem tuner, and equivalent tuners for DBS and TVRO
satellite set top boxes that tune from 950-1700 or even 2100 mhz).
These, as someone on the list has reminded me, are getting better and
better each year.

        And for other more critical uses  one can simply pay the piper
and purchase existing HF/VHF/UHF high end wideband communications
receivers with wide 10.7 (or 21.4) mhz IFs.   Much as it seems retro in
a certain sense, there are lots of ICOM R-7000s, R-7100s, and R-8500s
out there both new and used and at least a few R-9000s and AOR-5000-3s
as well.    These radios have reasonably well engineered front ends with
varactor tracked preselection and decent dynamic range and respectable
LO phase noise and accuracy and acceptable noise figure.   And they all
have 5-10 mhz wide 10.7 mhz IF outputs suitable for DSP processing. And
for ham use there are transcievers that cover various ham bands with
very good front ends...

        And of course there are also PC radios like the Winradio boards
and the ICOM equivalent.

        And for those of us who like to collect antique 70's 80's and
90's coldwar era relics from the spooks, there are Watkins Johnson and
Racal and Microtel and MA/COM, and TRW, and Reggo, and Microdyne, and
ACL, and Applied Signal, and various other brands of spook receiver 
gear that  also have high spec front ends and high performance LOs
(typically with 21.4 instead of 10.7 mhz IF outputs) available on the
surplus market at hamfests and on eBay.   These can be fun to play with
if you like playing with interesting cost-was-no-object gear of the
past, though of course there is typically a rather limited supply of any
one kind of radio and so one cannot publish designs based on using them
for a wide audience.

        What all of these radios lack for modern signals is
sophisticated demodulation capability and perhaps also all the required
bandwidths.   Thus combining one of them with a gnu-radio backend as the
demodulator and final IF has a certain appeal.

        Unfortunately, engineering and producing a high perfomance tuner
for gnu-radio is quite comparable to designing and producing one of
these wide coverage VHF/UHF communications receivers, one would estimate
that perhaps half the cost of a wide coverage communications receiver is
the front end and LO synthesizers and control and power and packaging
for same.  And building such a unit at home with normal tools and
resources using modern surface mount parts and technologies is
challenging for most people whether they have a proven design to start
with or not.  Even the availablity of small quantities of the parts
involved is quite dicey these days.

        And for legal (FCC cellphone) and market reasons, there hasn't
so far been much in the way of high performance tuner only front end
devices  sold at hobbyist prices to the mass market.   Of course with
gnu-radio available and some interesting applications that might happen
(IF it is not legally blocked in the USA and other big markets).   I
think the Winradio DSP IF board is obvious the first such product...

        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18

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