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November 5th 03, 10:14 PM

In article ,
writes:

Experimental Methods in RF Design points out that direct conversion
receivers have become highly popular in the past couple of decades... this
seems somewhat surprising; I would have guessed people back in the, e.g.,
'60s, would have gone to great lengths to avoid image reject filters and
long IF chains.

The nice thing about DC IQ receivers (apart from their zero image problem) is
that, any kind of demodulation can be solved in software, and is fully
updatable ... whereas if it's done in hardware, you'd need new hardware for
each mode
required etc.

The "first" complete receivers for HF were done over 30 years ago.
Totally in hardware.
Equally capable of CW or SSB reception without switching anything.

The lack of image problems, simplicity of hardware and fully updatable
modulation schemes is what makes DC IQ so nice -. so it's not surprising to
me at all why it's becoming so popular.

Demodulation for In-phase and Quadrature had a much wider
application than you would normally consider, and by the millions
without software of any kind: NTSC chrominance demodulation in
TV receivers. Did it at the HF level, too, 3.58 MHz. :-)

I'm not positive about it, but I believe the first "image-less" mixer
systems were for radar applications in the 1950s, specifically for
monopulse radar tracking. I first encountered monopulse around
1959.

I agree that most everything can be done in software...provided one
gets the proper A-to-D arrangement capable of operating at extremely
low noise levels. Such is NOT easy.

DC receivers in general have many good features. But, one has to
do a realistic comparison versus the superheterodyne structure.

1. To get good sensitivity one has to work with truly low-level
signals that cannot take advantage of narrow bandpass filtering
ahead of the demodulator. A DC mixer input is relatively
broadband and that increases the total noise power in the
circuit.

2. Relative broadbandedness of the input absolutely requires a
high IP3 since adjacent, normally-unheard signals can be at
a high relative level. Few DC receivers have any AGC to the
input stages.

3. The "image-less" condition (receiving only high-side or low-
side of LO frequency) is resolved in demodulated audio and
that absolutely requires a broadband phase-shifting network.
Even with high IP3 specs on the input mixer, strong input
signals adjacent in frequency can get through and fail to be
cancelled in the audio network...if the adjacent signals are
outside of the phasing network's range.

4. If the phasing network is simulated in software, then the
microprocessor or microcontroller must be adequately
shielded to avoid transitent RFI from getting into the input.
Physical proximity would be very close and the input has no
narrowband filtering to help that. Software demodulation
will depend heavily on the type of processor and a lot of
specs that have no direct relationship to software.

The end result is really a compromise of fewer parts traded for
a whole new set of potential problems. One kind is not
"superior" to another kind, just different.

Len Anderson
retired (from regular hours) electronic engineer person

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