Of course, if you were demodulating DSB suppressed carrier and you
injected the carrier at the wrong phase, you indeed would get the two
sidebands going through constructive and destructive phases. If
you're 90 degrees out with your LO, it looks a lot like narrowband FM,
though very slightly different as I posted in the thread on SSB-FM.
If you do the quadrature detector thing with DSB-suppressed carrier,
then when one of the two is just the wrong phase (and you get no
output from that one), the other will be just the right phase, and
vice-versa. When it's in between, does it work out right to just sum
the two? I suppose so, though it's worth going through the math to
make sure. And of course, with quadrature mixers, you can combine the
outputs with audio phase shifting to select just one of the two
sidebands (or just CW signals on one side of the LO). In fact, the
mixer LO inputs don't have to be exactly in quadratu it's possible
to apply a calibration to account for a phase error (and also an
amplitude error, where the gain through one mixer path is slightly
different from the gain through the other). That's all practical to
do digitally...we do that sort of thing at 100 megasamples per second
with some custom chips.

Cheers,
Tom


"Joel Kolstad" wrote in message ...
Dan Tayloe wrote:
This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if
the signal is a cw signal, we listen to a signal that is 600 Hz or so
away from the VFO so that we hear the 600 Hz tone difference.

...or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz
over time, eh? Good enough.

With SSB, presumably you have the same 'problem' -- the entire voice signal
is shifted in pitch by the difference between the LO and the real carrier.
In fact, with SSB and direct conversion, how do you even decide you have the
correct LO frequency? Just when people sound 'most natural?'

Thanks,
---Joel Kolstad

Dan Tayloe wrote:
This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if
the signal is a cw signal, we listen to a signal that is 600 Hz or so
away from the VFO so that we hear the 600 Hz tone difference.

....or at least, say, 595-605Hz is the local oscillator tends to drift +/-5Hz
over time, eh? Good enough.

With SSB, presumably you have the same 'problem' -- the entire voice signal
is shifted in pitch by the difference between the LO and the real carrier.
In fact, with SSB and direct conversion, how do you even decide you have the
correct LO frequency? Just when people sound 'most natural?'

Thanks,
---Joel Kolstad

This is indeed what happens only if the VFO and an incoming single are
at almost the same frequency ("zero beat"). However, in practice, if
the signal is a cw signal, we listen to a signal that is 600 Hz or so
away from the VFO so that we hear the 600 Hz tone difference.

For a SSB signal, we listen to the audio content contained in the
sideband, which is 300 Hz to 3 KHz away from the VFO signal when it is
tuned in correctly.

- Dan, N7VE

Joel Kolstad wrote:

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

This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90
degrees apart, so that the phasing is no longer a problem. (the sqrt of sum
of squares of the signal out of the two channels (or "magnitude") is not
sensitive to phase.)

Cliff

"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

Cliff Curry wrote:
This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90
degrees apart, so that the phasing is no longer a problem. (the sqrt of
sum of squares of the signal out of the two channels (or "magnitude") is
not sensitive to phase.)

Hmm... I went through the math, and indeed, this is the case!

But this then begs the question: Since the quadrature detector obtains the
correct magnitude of the transmitted signal for ANY phase difference between
the carrier and the LO, and if we model the phase difference as a function
of time that slowly changes due to the fact that, in actuality, our LO isn't
_quite_ the same frequency as the carrier, will the system still work? This
almost seems too good to be true...

Thanks,
---Joel Kolstad

Cliff Curry wrote:
This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90
degrees apart, so that the phasing is no longer a problem. (the sqrt of
sum of squares of the signal out of the two channels (or "magnitude") is
not sensitive to phase.)

Hmm... I went through the math, and indeed, this is the case!

But this then begs the question: Since the quadrature detector obtains the
correct magnitude of the transmitted signal for ANY phase difference between
the carrier and the LO, and if we model the phase difference as a function
of time that slowly changes due to the fact that, in actuality, our LO isn't
_quite_ the same frequency as the carrier, will the system still work? This
almost seems too good to be true...

Thanks,
---Joel Kolstad

This is a problem in general in direct conversion schemes: and this is the
reason that "quadrature" detectors are made, with two mixing channels 90
degrees apart, so that the phasing is no longer a problem. (the sqrt of sum
of squares of the signal out of the two channels (or "magnitude") is not
sensitive to phase.)

Cliff

"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

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