Re: [Discuss-gnuradio] Direct Conversion vs Superheterodyne

[Crusty Curmudgeon]

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

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. 

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.

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. 

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. (Note: if using IQ conversion, two mixers, two
matched sets of audio LPF filters and two channels of ADC are needed, plus extra
DSP programming is required, all to eliminate the AFC. Seems a high price, but
it may be worth it in some cases.) 

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. 16 bits gets you only 96dB or so dynamic range and
a good portion of that is needed for demodulating the signal so a 22 bit ADC
maybe in order without AGC. With AGC, a 12 bit ADC would give good performance
and cost a lot less. 

Anyway, that's my two bits worth. 

On 13 Jan 2003 13:33:34 -0500, you wrote:

>Sorry about the previous message.  Apparently the mailing list software
>doesn't like PGP signed e-mail. :-(
>
>On Mon, 2003-01-13 at 13:00, John E. Perry 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.  
>> 
>> For all voice and program broadcast signals the bandwidth is well within 
>> even low-end PC sound cards.  Phase-locked loops are not trivial, but 
>> neither are they hard, to build. Essentially all common signals except 
>> hi-fi FM and video signals could be decoded in practically any low-end 
>> PC, couldn't they?
>> 
>> What am I not understanding?
>> 
>> John Perry
>
>
>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?
>
>Cheers,
>Tanner Lovelace
>KB4TYE

---------------------------------------------------------------------------------------
"Make peace an attitude. Want it. Make peace a habit. Live it. Make peace a 
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published poet and television celebrity, 
spoke at the annual peace assembly at Community of Christ headquarters, May 
2002.