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RE: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

From: Ettus, Matt
Subject: RE: [Discuss-gnuradio] Direct Conversion vs Superheterodyne
Date: Mon, 13 Jan 2003 15:57:45 -0800

> I've recently become quite interested in the Tayloe 
> Mixer/Detector/System. It's a remarkably simple circuit where you 
> moreorless connect your antenna through a fast high-speed 
> analogue bus 
> switch which is clocked at N*f Hz, connecting it to N (usually 4) 
> capacitors in sequence. 4's a particularly nice number as you end up 
> with 4 nice quadrature signals 90 degrees apart - convenient for SDR 
> work. Charge capacitors a,b,c,d,a,b,c,d, then use simple op-amp-ish 
> circuits to give you a gain boost and make Q = a - c, I = b - 
> d... and 
> the rest of it feels like a digital circuit anyway!   :-)

I've got a bone to pick with Mr. Tayloe.  He didn't invent that.  Its been
around for about 30+ years.  Its called a commutating mixer, differential
quadrature switched mixer, or multi phase sampler.  It is a nice easy
circuit, though.

> That's an ADVANTAGE of direct conversion - any undesired 
> mixer products 
> appear as DC on your outputs, whereas in superhets they 
> appear at IF+F 
> and IF-F or whatever. If you have multiple IFs in your 
> superhet,  you've 
> got all kinds of accidental products produced by all kinds of 
> products 
> of multiple input carriers and IFs interacting in all kinds 
> of odd ways. 
> 2 nearby carriers on adjacent channels can even produce all kinds of 
> products with themselves. The superhet requires far more 
> shielding and 
> filtering because otherwise you end up with all kinds of 
> messy products 
> all over the place, whereas in the Direct Conversion 
> receiver, you have 
> ONE frequency to worry about, and if it leaks, it gives you DC.

As someone who works every day on direct conversion receivers every day, I
have to say that they are not as easy as people say, unless you don't care
about the DC content of the signal.  If that is the case, then you're not
really using Direct conversion anyway, you're doing low-if.

> > 3) Some sort of automatic frequency control or phase 
> locking is needed on the
> > local oscillator to make sure it's on the same carrier 
> frequency as the desired
> > signal or else the beat note will dominate the analog to 
> digital converter
> > output, chewing up dynamic range.
> Now that's reasonably true if tuning an AM broadcast, but in 
> SSB there's 
> no such carrier to get in the way, and in FM your carrier is moving 
> around and that's exactly what you WANT to dominate your A2D 
> and eat up 
> your dynamic range   :-)

Not sure that makes sense.  But anyway, in all comm systems you essentially
need to have some form of afc.  In SSB, your ear does it, and in FM voice,
the DC-block does it.  But AFC can (and probably should be) be done
digitally, and isn't really a function of the architecture used.

> Even in the case of AM, CW, RTTY etc with a strong carrier or 
> carrier-like signal, you can tune (say) 10kHz away from it, treating 
> your "direct conversion" more like an IF to get within the frequency 
> response of your A2D, then tune down 10kHz in the software 
> (or do your 
> CW/RTTY/whatever decoding centered around 10kHz instead of DC).

Just because you have only one conversion, and do everythign else digitally
doesn't make it direct conversion.  If you convert the carrier down to a
frequency other than 0, it is a low-if receiver.

> See Fig 10, 12, 14 in "SDR for the masses part 1" at 
> http://www.arrl.org/tis/info/sdr.html - some of the rest of 
> the article 
> even explains the harmonic/alias frequency rejection better 
> than I can.

Gerald Youngblood, the author of that article, has since stopped calling it
a Tayloe detector, since it isn't a Tayloe Detector any more than it is an
Ettus Detector or a Waterman Detector.

Matt Ettus

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