In article , "Joel Kolstad"
writes:

Avery Fineman wrote:
Can one get separated sidebands on AM DSB with a DC receiver?
Absolutely!

That's good to know. At present I'm going to quit performing these
"intellectual experiments" and start building something, and while I'm after
C-QUAM AM stereo (rather than upper/lower sideband stereo), it's good to
know what else _could_ be received.

BTW, if anyone wants to see the block diagram of what I'm planning to do,
see he http://oregonstate.edu/~kolstadj/RadioProj.gif . Keep in mind
it's designed primarily for simplicity, not for phoenomenally good noise
performance, sensitivity, selectivity, etc.

(I'd be particularly interested in comments on how to implement the low pass
filters -- it seems one would want phase preserving filters such as Bessels
or a cascade of a Chebyshev followed by an all-pass phase restoration
filter.)

OK, I got the block diagram. If you are using even a rudimentary
R-C lowpass following the two mixers, you need the parts rather
well matched in order to preserve identical relative phases. It is
important to HOLD the relative phase error at audio to a very small
number in order to do the In-phase/Quadrature thing. DSP will
work with BOTH magnitude and phase regardless of the kind of
modulation going into the mixers. You CAN realize a lowpass
function in DSP but the TI chip inputs probably needs some sort
of hardware lowpass filtering...? A simple R-C lowpass can be
checked out independently just from equal parts values to assure
minimum relative phase error.

Here's a good hint on melding hardware with software using DSP:

"Scientist's and Engineer's Guide to Digital Signal Processing,"
by Stephen W. Smith, PhD, California Technical Publishing.

Despite the title, this is a good text on DSP from the beginner's
point of view on to the more advanced. What is special is that
ALL the chapters can be downloaded absolutely FREE! :-)
[or pay about $68 for the hardcover]

http://www.DSPguide.com

Well organized book and a good "teaching style" to the writing.

Once you have the hardware fairly well in shape, it's time to go
nuts with the programming. This book ought to help whatever it
is you are going to code.

Nooo...AM "came about" with absurdly SIMPLE components first,
not even using any vacuum tubes!

Wow... I realize now there's a large gap in my knowledge of the history of
the progression of radio inbetween "spark gap transmitter" and "diode-based
envelope detector!" I have read of coherers before in Lee's book, "Design
of CMOS Radio-Frequency Integrated Circuits" where he claims that nobody
ever really did figure out _how_ they worked -- interest wanted as better
detectors were available before they were around long enough for someone to
do so.

Actually, that's irrelevant and a historical curiosity.

The galena crystal and "cat's whisker" formed a rudimentary point-
contact diode. I had one of those in 1946, a Philmore Crystal Set my
Dad got for me (el cheapo quality, but it worked after a fashion). A
half year later a new electronics store opened up in town and they
were selling surplus WW2 radar set silicon mixer diodes, type 1N21
and 1N23. Put one of those in the Philmore and really "souped up"
the audio. :-)

Long-distance telephony was the birthplace of SSB. Frequency
multiplexing was the only practical way to cram four telephone
circuits on a single pair of wires running many miles way back when.

If you tell me people were already using IQ modulation back then as well
I'll be quite impressed...

As far as I've seen, the old telephony "carrier" equipment used ordinary
4-diode ring mixers, usually copper-oxide stacked plate types, the
small ones the size of old multimeter AC rectifiers. That was pre-1930.

I'm not sure when the In-phase/Quadrature demod/mod sub-systems
were first used other than probably just before 1940...or maybe in the
WW2 years. I know the beginning applications were there in the late
1940s.

If you want synchronous detection of AM DSB, then you concentrate
on getting a carrier reinsertion oscillator locked to the received
carrier. Primary object is to get that lock.

I'm planning to write a (software) quadrature detector, and once that works,
start worrying about obtaining phase lock so that stereo can be decoded.

Good luck on that.

Can you get a synchronous detection of AM SSB? Difficult unless
the transmitter at the other end has sloppy carrier suppression.

Without a carrier of some pilot tone (as a reference) it seems as though
it's difficult to even claim there could be such a thing as 'synchronous
detection.'

I've seen it claimed in text, but no details, that a quasi-lock could
be obtained via voice, working on the harmonics of speech tones.
I'm not going to buy that until I see a demo.

DC receivers (also called "Zero-IF") came into popularity in Europe
THREE decades ago. RSGB's Radio Communication magazines of
1973 were showing stuff in Pat Hawker's monthly column. I got
interested in the Mike Gingell polyphase R-C network by seeing it
first in there.

I took a quick look at the Gingell networks and they seem quite novel --
even made their way into a Real Commerical Product (a Maxim IC).
(Interestingly enough, Dr. Gabor Temes -- who spent a long time designing
telephone network filters before going into academia, where he is now, all
of about 500' away from me here -- says there is still some black magic
involved in making them work. :-) )

The polyphase network was the subject of Michael Gingell's PhD
thesis in the UK. Material on that was seen on the Internet. Mike
is a USA resident now (or was a couple years ago when he had a
website...has a US ham callsign, too). A Japanese ham got busy
on that polyphase network and came up with an optimum set of
component values. That was published in QEX. Hans Summers'
website has links to all those.

Gabor Temes is a familiar name to textbook thumbers. :-) There
isn't a lot of black magic associated with the Gingell network, but it is
a thorough #$%^!!!! to try and analyze with its busy interconnections.
I stole a few minutes of CPU time on the RCA corporate computer,
using their LECAP (a frequency-domain version of IBM's ECAP...the
SPICE thing hadn't been developed yet) back in the 1970s. It worked
as advertised with only 0 and 180 degree audio input, producing nice
relative quadrature phases on all four outputs. Was surprised!

Some scrounged parts, not well measured as to values, and a quick
open-air toss-together showed excellent broadband relative quadrature
with less than 1 degree error in the 'voice' bandspace. Checked that
with a time-interval function on a homebuilt frequency counter.

Thanks for all the advice Len... I'd be offering to take you to dinner by
now if you were halfway local!

Thank you but I'll just wave as my wife and I roll through Oregon
along I-5 about once a year from southern California to Puget Sound
area of Washington. :-) Want to bypass the usual clogging just
before crossing the river into WA and vice-versa.

Len Anderson
retired (from regular hours) electronic engineer person