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Re: [Discuss-gnuradio] why quadrature samples?

From: sam penny
Subject: Re: [Discuss-gnuradio] why quadrature samples?
Date: Thu, 23 Jan 2003 18:22:39 +0000 (GMT)

> > Thus you have sampled a BW of 2X by using 2
> > samplers of rate 2x, instead of
> > using one at a rate of 4X.
> That was probably the most succinct, easy to follow
> explanation of 
> "quadrature downconversion" I've ever seen :-) 

agreed, that should go on the FAQ without a doubt.

I understood the concept before that, but it would've
taken a lot less in terms of brain ache if I'd had
that explanation to start from! :)

> An interesting variation on this is the "Tayloe
> Detector."

I heard this was actually scarecely different from an
idea that's been around for a number of decades, but
who knows...

> The performance of this kind of detector is limited
> by the phase noise 
> of the LO and how closely matched the components are
> in each leg of the 
> circuit.

I've been thinking of maybe making a TV decoder that
uses something along these lines, but I'm not sure how
to get a switcher that will handle the ~500-900MHz
input or the switching rate (there are 8-bit, two
chanel 100M samples per second ADCs out there, it'd be
nice to be able to grab several chanels' worth at once
using such a device...) the best I've seen has a
switching rate of around 1000ns per transition
(guaranteed, more like 6ns typical), which presumably
wouldn't be enough in the worst case scenario...
(surely it has to be significantly lower than the
sampling rate you want...?)

does anyone know of any useful documentation regarding
high frequency quadrature decoding? it'd be nice, of
course to go right up to 3GHz (to take in mobile
phones, wireles LAN etc. :)

incidentally, are there any ways to make use of more
than two ADCs in super-imaginary numbers or something?
(I think I worked out that super-imaginary numbers
disolve into imaginary numbers if you look at them
hard enough, which would probably mean the answer is
no, but hey :)

> Here is a good diagram of an actual implementation:

ooh, the switch in that seems to guarantee 6.something
ns switching time, which is nice :) that should do for
a 100MHz sampling rate! would the transition time
interfere with a ~1GHz sample? (1/1GHz is a nanosecond
isn't it? I keep getting my number of zeros mixed up!)

is there any information on what kind of signals
(bandwidth, frequency) this tayloe converter can

> You can do fun and interesting (and baffling) things
> with I and Q to 
> recover just about any form of modulation on the
> original signal.  This 
> would fall under the "narrowband" version of
> software defined radio.

does it not work with wider bands, then? or is that
just a function of the sampling rate?

Cheers & God bless
    Sam "SammyTheSnake" Penny


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