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Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

From: Lisa Bengtson
Subject: Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne
Date: Tue, 14 Jan 2003 18:19:31 -0500

On Tue, 14 Jan 2003 10:48:51 -0800, you wrote:

>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?

Nope, you are not missing a thing. It's the same as a single Stage
Super-het, with an IF of Zero. Typically, a Super-het has a final IF
that isn't at DC. 
>> 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

Super-Het's are pretty well understood at this point, people have been
hacking on them for 70 years. Direct Conversion and Undersampling have
some gotcha's that are not fully understood and publicized yet. That's
what makes them interesting. 


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