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Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

From: Mark Smith
Subject: Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne
Date: Tue, 14 Jan 2003 10:48:51 -0800
User-agent: Mutt/

On Mon, Jan 13, 2003 at 10:43:23PM -0500, David Bengtson wrote:
> undersampling and Direct Conversion are slightly different approaches.
>  In a Direct Conversion receiver, there is a Mixer stage that mixes
> the desired signal down to an IF frequency of zero. In other words,
> you would had a mixer stage at, say 10.7 MHz, and a local oscillator
> injection signal at 10.7 MHz. These two signal's mix, with one mixing
> product at DC, and the other at 2*10.7 MHz, or 21.4 MHz. This can be
> sampled out quite easily by a simple low pass filter. 

How is this different than a single stage supper-het?  Isn't that just
mixing down to base band?  Is there something I'm missing?

> In an undersampled receiver, the desired signal, at an IF frequency
> (Use 10.7 MHz) is sampled by a A/D operating at a clock rate of 500
> kHz. You then sample signal's from DC to 250 kHz, in the typical
> fashion. Signals from 250 kHz to 500 kHz are also sampled by the A/D,
> as are signals at every frequency presented to the input of the A/D. 
> In this approach, you must filter out the unwanted frequency bands,
> since any signal present will alias into the passband, and then appear
> as an in-band jammer. Notice that this also applies to DC offset's
> that appear in circuits. 

Yup.  Absolutely.  This is one of the main drawbacks to undersampling;
you had better have darn good filtering of your IF and a darn quiet 
circuit between the IF output filter and the ADC.

> Analog Devices has a decent app note by Brad Brannon that discusses
> Digital Radio Fundamentals on their web site, Titled Radio 101. I
> don't have a URL at hand, though. 

I'll Google it and take a look at it later, when I'm not at work.  
Thanks for the info!  :)

73 de KR6ZY
Mark Smith - finger address@hidden for GPG v1.0.1 public key

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