This gets to the question of whether DC receivers can be used to copy
DSB and SSB:
By Goodman, W1DX, explained the problem in the 1965 edition of
"Single Sideband for the Radio
Amateur" (page 11): "Unfortunately, if both sidebands are received at
the detector where the carrier is
introduced, the carrier has to have exactly the correct phase
relationship with the sidebands if distortion is
to be avoided. Since exact phase relationship precludes even the
slightest frequency error, such a system
is workable only with very complicated receiving techniques. However,
if only one sideband is present at
the detector, there is no need for an exact phase relationship and
there can be some frequency error
without destroying intelligibility. " Modern SSB transcievers send
only one of the sidebands to the
detector, so this distortion problem only occurs when receiving a DSB
signal on a receiver that sends both
sidebands to the detector.

73 Bill M0HBR




"Joel Kolstad" wrote in message ...
I'm curious... with the current popularity of simple (e.g., QRP usage)
direct conversion receivers, whatever happened to the problem of having to
synchronize the cariier phases? I'm looking at Experimental Methods in RF
Design, and they just use an LC oscillator for the input to the mixer. If
input carrier is cos(f*t) and the LC oscillator is generating cos(f*t+phi),
where phi is the phase offset between them, you end up with a cos(phi) term
coming out of the mixer. If the frequencies are ever-so-slightly off, phi
essentially varies slowly and cos(phi) should slowly cause the signal to
fade in and out.

Why isn't this a problem in practice?

Thanks,
---Joel Kolstad

Bill Meara wrote:
This gets to the question of whether DC receivers can be used to copy
DSB and SSB:
By Goodman, W1DX, explained the problem in the 1965 edition of
"Single Sideband for the Radio
Amateur" (page 11): "Unfortunately, if both sidebands are received at
the detector where the carrier is
introduced, the carrier has to have exactly the correct phase
relationship with the sidebands if distortion is
to be avoided. Since exact phase relationship precludes even the
slightest frequency error, such a system
is workable only with very complicated receiving techniques.

In 1965 I can imagine that a Costas loop, two mixers, etc. was considered
'very complicated.' It doesn't seem all that horribly fancy by today's
standards, however. But of course it's not like I've actually _built_ such
a thing yet! :-)

However,
if only one sideband is present at
the detector, there is no need for an exact phase relationship and
there can be some frequency error
without destroying intelligibility. " Modern SSB transcievers send
only one of the sidebands to the
detector, so this distortion problem only occurs when receiving a DSB
signal on a receiver that sends both
sidebands to the detector.

It's ironic that DSB, which came about due to the ease of detection with
diode (envelope detectors) turns out to be somewhat challenging to recover
with a more sophisticated synchronous detection scheme.

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.

---Joel Kolstad

In article , "Joel Kolstad"
writes:

Bill Meara wrote:
This gets to the question of whether DC receivers can be used to copy
DSB and SSB:
By Goodman, W1DX, explained the problem in the 1965 edition of
"Single Sideband for the Radio
Amateur" (page 11): "Unfortunately, if both sidebands are received at
the detector where the carrier is
introduced, the carrier has to have exactly the correct phase
relationship with the sidebands if distortion is
to be avoided. Since exact phase relationship precludes even the
slightest frequency error, such a system
is workable only with very complicated receiving techniques.

In 1965 I can imagine that a Costas loop, two mixers, etc. was considered
'very complicated.' It doesn't seem all that horribly fancy by today's
standards, however. But of course it's not like I've actually _built_ such
a thing yet! :-)

Most of you guys are knocking yourself out on what is little more than
an "intellectual experiment." Get a receiver and do a practical
experiment. An old receiver with a "BFO" isn't "sophisticated" and
will receive DSB and SSB just dandy, very "workable" if the LO is
warmed up and stable and the fine-tuning ("bandspread") can zero-
beat where the carrier is (or was). Do the same thing with a newer
receiver that has a "product detector" (nothing more than a mixer,
the same as what a DC receiver front end has but at IF, not HF).
Very "workable" and done all over in everyday HF comm, both ham
and maritime radio. Been done for decades.

The only "distortion" comes from not being able to set the tuning
precisely without some AFC. With AM and a "product detector"
(or BFO on), there's the carrier beat, strong, and won't go away
unless there's a terrific lowpass audio filter in there. If using a more
modern, basically-SSB receiver, it probably has a "RIT" or Receiver
Incremental Tuning that allows making the carrier beat almost to
DC. That's a frequency distortion still and manual tweaking can't
get the low-frequency, absolutely non-phase (or rapidly changing
phase) all the way out.

Can one get separated sidebands on AM DSB with a DC receiver?
Absolutely! No problem with a manual tuning DC receiver that has
TWO audio networks out of the mixer. A stereo-like effect (amazing
to hear for the first time) with lower SB = left ear, upper SB = right
ear is possible, even if the tuning doesn't hit right on carrier zero
beat. The "phase distortion" manifests itself solely in the amount of
rejection of the unwanted side of tuning...too great a phase from ideal
results in poor unwanted side rejection...very close phase and the
the rejection of unwanted side is best.

However,
if only one sideband is present at
the detector, there is no need for an exact phase relationship and
there can be some frequency error
without destroying intelligibility. " Modern SSB transcievers send
only one of the sidebands to the
detector, so this distortion problem only occurs when receiving a DSB
signal on a receiver that sends both
sidebands to the detector.

Pfui. The old receivers with BFOs could "work" SSB. Problem is,
those old receivers were so finicky and unstable, had such wide
final bandwidths that those faults predominated. I have a nice
1948 National NC-57 gathering dust as proof of that. :-)

It's ironic that DSB, which came about due to the ease of detection with
diode (envelope detectors) turns out to be somewhat challenging to recover
with a more sophisticated synchronous detection scheme.

Nooo...AM "came about" with absurdly SIMPLE components first,
not even using any vacuum tubes! Case in point: Reginald
Fessenden's famous Christmas Eve, 1906, voice transmission from
Brant Rock, MA. Used a rotary alternator LF generator with a special
(probably carbon) microphone in series with the antenna lead. The
few who heard it along the east coast used galena crystal (the first
point-contact diode?) or "coherer" or "liquid barreter" detectors.
Technologically primitive by today's standards.

The existance of two sidebands in AM wasn't known for sure
until the first Johnny Carson (John R. Carson, AT&T) published his
modulation equations to show the presence of identical-information
sidebands. Few labs had the equivalent of spectrum analyzers and
vacuum tubes were still rare in the 1915-1922 era. "Detectors" of
that early time were still the simple "rectifying" types...a regenerative
detector still does "rectifying" (averaging of amplified signal input) to
recover the modulation 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.
Frequency multiplexing uses SSB techniques. When RF amplifiers
using tubes got going, the first SSB was four-voice-channel long-lines
"carrier" frequency multiplexed modulation with radio replacing the
wire pairs. Those applications needed AFC for unattended operation.

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. Worry about "phase
differences due to distortion" in intellectual experiments. Lock
guarantees that the synchronous detection will hold. There won't be
any noticeable recovered audio distortion EXCEPT from unusual
selective fading propagation on very long-distance radio circuits; you
can hear that over old receivers with "rectifying" detectors.

Can you get a synchronous detection of AM SSB? Difficult unless
the transmitter at the other end has sloppy carrier suppression. The
commercial HF SSB stuff uses "pilot carriers" and the like to provide
an AFC lock...deliberate steady tones at unused sideband
frequencies.

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.

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. The UK ham who was experimenting with it was Peter
Martinez, G3PLX. Hams of today will know him as the innovator of
PSK31.

To get good sensitivity with DC receivers you need ultra-low-noise
mixers and following audio stages. Since the input side selectivity
is poor (compared to IF xtal filters), those mixers need a terrifically
high intermodulation specification which precludes low-noise
operation. The Tayloe Mixer handles both superbly, the "mixer"
being a CMOS switch IC with very low conversion loss as a mixer
(all other passive mixers have at least 6 db loss). The CMOS switch
IC has very low internal noise. Absolutely the best of both worlds.

In order to achieve selectivity and unwanted frequency side rejection
the Tayloe Mixer system needs a basic LO at four times the carrier
frequency to get wideband phase quadrature using digital devices.
The In-phase and quadrature mixer output is in the audio range.

It seems to me that a modification of the Tayloe circuit would suit a
synchronous detector application. How to go about it is another
matter. Planning for THAT can start with an "intellectual experiment"
but trying to implement it requires bench experimentation. There
won't be any "distortion due to phase" once carrier lock has been
achieved. Carrier lock methods have to concentrate on the narrow
frequency region (tolerance of tuning offset) of the carrier. In practical
reception the carrier of AM DSB is relatively constant (within a 20 db
spread of amplitude if some AGC exists elsewhere).

Once the carrier is locked, the remainder of the detection process
(recovering modulation audio) is straightforward. Any "phase
distortion" is due to phasing network errors...which can be checked
and trimmed independently prior to applying them.

Go for it! :-)

Len Anderson
retired (from regular hours) electronic engineer person

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.)

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.

You have a phoenomenal memory, Len... I wish I could recall the details as
well as you have!

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...

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.

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.'

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. :-) )

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

---Joel

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

Avery Fineman wrote:
You CAN realize a lowpass
function in DSP but the TI chip inputs probably needs some sort
of hardware lowpass filtering...?

An anti-alias filter at the very least! The TI chip in question is actually
a microcontroller rather than a DSP; I chose that based on desiring a low
power design and only requiring one chip rather than two (although I'd grant
you that there are some, e.g., SO-8 package serial interface ADCs out there
that almost don't count as another chip...) and, uh, because I already have
experience with it from other projects. It can sample up to ~200ksps, but I
was shooting for a much lower rather (perhaps some 20-50 ksps) since there
isn't much number crunching 'oomph' available. (I.e., no multipy-accumulate
instruction. In fact, no hardware multiplier at all! :-) I'll build the
mind-numbingly fast DSP monster receiver that can pull a signal 10dB beneath
the noise floor and turn it into CD quality audio once I get the simple ones
working.)

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.

How very interesting -- I actually have a copy of that with from Analog
Devices (under a slightly different name); I always figured it was some
third party and not the application engineers at Analog Devices that wrote
it. Now I just have to crack the thing open...

Good luck on that.

It should be up and running within a month here.

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.

I figured that's what they might have had in mind. Seems like a lot of
effort to avoid sending a pilot tone (but then again, what else to use the
aforementioned DSP for... oh, wait... fancy digital modulation schemes...
ok...)

---Joel

Also regarding microcontrollers vs. DSPs: TI has an application note where
they use so-called wave filters (which I know next to nothing about other
than the name and that they can be designed to only require
additions/subtractions and not multiplications in their implementations and
hence execute quickly) for detecting DTMF. I thought it was pretty
clever...

---Joel

Also regarding microcontrollers vs. DSPs: TI has an application note where
they use so-called wave filters (which I know next to nothing about other
than the name and that they can be designed to only require
additions/subtractions and not multiplications in their implementations and
hence execute quickly) for detecting DTMF. I thought it was pretty
clever...

---Joel

Avery Fineman wrote:
You CAN realize a lowpass
function in DSP but the TI chip inputs probably needs some sort
of hardware lowpass filtering...?

An anti-alias filter at the very least! The TI chip in question is actually
a microcontroller rather than a DSP; I chose that based on desiring a low
power design and only requiring one chip rather than two (although I'd grant
you that there are some, e.g., SO-8 package serial interface ADCs out there
that almost don't count as another chip...) and, uh, because I already have
experience with it from other projects. It can sample up to ~200ksps, but I
was shooting for a much lower rather (perhaps some 20-50 ksps) since there
isn't much number crunching 'oomph' available. (I.e., no multipy-accumulate
instruction. In fact, no hardware multiplier at all! :-) I'll build the
mind-numbingly fast DSP monster receiver that can pull a signal 10dB beneath
the noise floor and turn it into CD quality audio once I get the simple ones
working.)

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.

How very interesting -- I actually have a copy of that with from Analog
Devices (under a slightly different name); I always figured it was some
third party and not the application engineers at Analog Devices that wrote
it. Now I just have to crack the thing open...

Good luck on that.

It should be up and running within a month here.

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.

I figured that's what they might have had in mind. Seems like a lot of
effort to avoid sending a pilot tone (but then again, what else to use the
aforementioned DSP for... oh, wait... fancy digital modulation schemes...
ok...)

---Joel

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