Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

[Mark Smith]

On Mon, Jan 13, 2003 at 01:33:34PM -0500, Tanner Lovelace wrote:
> > Why does everyone insist on using an if?  I seem to recall from my radio 
> > classes (admittedly quite a few years ago) that direct-to-baseband IQ 
> > conversion keeps all the modulation information present in the signal.  
> 
> Well, admittedly my radio knowledge isn't as much as it should be, but
> from what I understand, the main reason for superhet designs was that
> it was easier to process signals at a fixed frequency than over a 
> range of frequencies.  While this may be true in analog, it's entirely
> possible that direct conversion would be better for an SDR.  In
> fact, I was just looking at direct conversion circuits last night
> in the ARRL Handbook and wondering this very thing.  Perhaps someone
> with better knowledge could speak about this?

The theory you're talking about here is called undersampling.  You 
still need to tune and mix your signal to an IF (eg: 455kHz), but if
your signal is only 10kHz wide, you can sample your 455kHz IF at at least
20kHz and aliasing will effectively do the final mix-down to base band.
Aliasing is usually a Bad Thing(tm) that makes a 11kHz tone sound like
a 9kHz tone when sampled at 20kHz.  In this case, it works in our favor.

There are some limitations to this, however.  You need to make sure
that your IF signal will all be aliased into the same image; if your
sample rate is set wrong, you'll get half your signal in a positive
image and the other half in a negative image.  This is a function of 
your IF, the bandwidth of the signal you care about, and your sample rate.

To undersample, you need to look for an ADC that has an input bandwidth
at least the IF + Bandwidth/2, and can sample at at least Bandwidth * 2.
In our example above, we'd need an ADC with an input bandwidth of at least
460kHz and a sample rate of at least 20kHz.

Because of this large input bandwidth, you need to make sure that you do
_VERY_GOOD_FILTERING_ of _EVERYTHING_ that's not your signal.  Anything
that's on the input of the ADC within the bandwidth _WILL_ get aliased
down into your data, and you won't be able to tell the difference.  
(And you thought that intermod was tough to deal with.  :)

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

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