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Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

From: Nick Waterman
Subject: Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne
Date: Mon, 13 Jan 2003 23:15:18 +0000
User-agent: Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.1) Gecko/20020919 Debian/1.1-1

Crusty Curmudgeon wrote:
Ok.. a direct conversion receiver is simple and cheap, but I think there are
some flies in the ointment too:

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! :-)

1) It requires very good shielding of the local oscillator and great
mixer-to-antenna isolation or else it will leak signals right on top of what
you're trying to listen to, potentially jamming the channels.

But in most true direct-conversion receivers, including the Tayloe detector, you're leaking signals at EXACTLY the frequency you're trying to listen to, producing (if anything) a simple DC offset. PC Soundcards are quite good (or arguably bad) at ignoring DC for you :-)

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.

2) It does not tolerate strong near channel signals well. That can lead to
overload of the mixer(s) or follow-on baseband amplifiers and active filters.
Very high level LO mixers can improve things a little, but that impacts the
signal leakage problem too.

The Tayloe detector was originlly conceived as a tuneable narrow bandpass filter centered at exactly (clock freq)/4. In the same way that charging a capacitor through a resistor gives you a low pass filter, charging 4 capacitors in sequence acts like a bandpass filter at a quarter of the clock rate - basically a bandpass filter at the frequency you're trying to tune. Bonus! Tweak the circuit a bit if you want narrower/wider bandpass.

3) It is sensitive to any harmonically (sp?) related signals, so good bandpass
filtering between the antenna and mixer is needed. Of course this make band
hopping a problem.

I've already said the Tayloe detector acts like quite a good bandpass filter. F/4 gives your perfect quadratures. F*2/4 effectively gets added to a&c and subtracted from b&d, so getting Q from a-c and I from b-d cancel this out. F*3/4 sort of cancels itself out by appearing 120 degrees out of phase 3 times in a row (I'm probably not describing this too well)

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 :-)

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).

4) An agc system is also needed to help handle the extreme amplitude shifts
multipath fluttering causes to the signal or else you'll need a rather expensive
analog to digital converter.

Not a disadvantage of Direct Conversion at all - exactly the same applies to superhets! :-p

In my opinion the coolest thing about the Tayloe system, and what makes it a natural partner to SDR - It feels like half-analogue, half-digital technology - you're using a component that's designed to quickly select 1 of 4 busses in a computer, and you're moreorless using it to "demultiplex" your desired frequency into 4 quadrature signals separated by 90 degrees. Now you just need a nice stable VFO, doesn't even need to be a nice pure sine wave as it's only being used as a clock - a square wave is in fact ideal, as long as it's nice and stable... Dare I say it might even be generated by digital hardware, or even software if you can get the stability?... and then you probably want a bit of AGC, and even THAT might be better controlled by software feedback rather than being all analogue. It's almost the ideal bare minimum analogue stuff, letting you do the rest in the digital and software domains.

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.

"Nosey" Nick Waterman, Senior Sysadmin.  #include <stddisclaimer>
address@hidden  http://noseynick.net/
Those who live in glass houses...shouldn't.

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