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

With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

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.

I went through the math and you end up with the magnitude of the original
signal. What's unclear to me is how to recover the phase offset between
your signal and the original -- although adding a DC component (or some
other unique frequency component) to either I or Q (or placed at some
strategic angle between them) would allow you to synchronize the phases.

Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

---Joel Kolstad

With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

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.

I went through the math and you end up with the magnitude of the original
signal. What's unclear to me is how to recover the phase offset between
your signal and the original -- although adding a DC component (or some
other unique frequency component) to either I or Q (or placed at some
strategic angle between them) would allow you to synchronize the phases.

Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

---Joel Kolstad

"Joel Kolstad" wrote in message ...
With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R? All this is not my forte; I know only
enough to be dangerous with it. But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver. If you're sending symbols, presumably
there can be some symbol you transmit periodically to insure things
stay synched, a complex version of the old RS-232 start and stop bits
if you will. For analog signals, you can transmit some sort of pilot
tone, perhaps the carrier itself, which of course must be done for
compatible AM anyway.

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.

I went through the math and you end up with the magnitude of the original
signal. What's unclear to me is how to recover the phase offset between
your signal and the original -- although adding a DC component (or some
other unique frequency component) to either I or Q (or placed at some
strategic angle between them) would allow you to synchronize the phases.

Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors. The I and Q outputs are V(C1)-V(C3) and
V(C2)-v(C4) respectively. It must be driven with a "LO" at four times
the detected frequency: that is, the switch must rotate through all
four positions in one cycle of what would normally be considered the
LO frequency. If you use care in its construction, you should be able
to get very good balance. The size of the capacitors determines the
bandwidth. There are commercial quadrature active mixers, too, but
they typically cover a modest frequency range in the VHF region or
higher.

Cheers,
Tom

"Joel Kolstad" wrote in message ...
With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R? All this is not my forte; I know only
enough to be dangerous with it. But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver. If you're sending symbols, presumably
there can be some symbol you transmit periodically to insure things
stay synched, a complex version of the old RS-232 start and stop bits
if you will. For analog signals, you can transmit some sort of pilot
tone, perhaps the carrier itself, which of course must be done for
compatible AM anyway.

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.

I went through the math and you end up with the magnitude of the original
signal. What's unclear to me is how to recover the phase offset between
your signal and the original -- although adding a DC component (or some
other unique frequency component) to either I or Q (or placed at some
strategic angle between them) would allow you to synchronize the phases.

Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors. The I and Q outputs are V(C1)-V(C3) and
V(C2)-v(C4) respectively. It must be driven with a "LO" at four times
the detected frequency: that is, the switch must rotate through all
four positions in one cycle of what would normally be considered the
LO frequency. If you use care in its construction, you should be able
to get very good balance. The size of the capacitors determines the
bandwidth. There are commercial quadrature active mixers, too, but
they typically cover a modest frequency range in the VHF region or
higher.

Cheers,
Tom

Tom Bruhns wrote:
"Joel Kolstad" wrote in message
...
For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R?

They may well -- I'm not sure which 'standard' we're talking about here
(there seem to be several out there...). :-) But note that with L+R on I
and L-R on Q, you only get L+R if you manage to synchronize with the phase
(the original problem)! If you use a quadrature detector and extract the
magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R
either. The Motorola C-QUAM AM stereo system forces the magnitude of the
I-Q phasor to be L+R, which makes it compatible with both envelope detectors
and quadrature detectors. (I probably sound like a Motorola sales guy these
day... I just think it's clever and the end goal here is to build a direct
conversion receiver to decode it...)

But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver.

For AM, it would appear that the carrier itself is what people use for that
purpose.

If it very clear to me now why TV transmission need to send the colorburst
sequence these days -- otherwise they'd have no way to synchronize the
chroma decoder, which is taking in a DSB-SC modulated in both I and Q.

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors.

I was just thinking of doing something like this -- from a square wave, get
a 4 bit shift register (that always has one output active) and feed the
outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1
and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering
and -- presto chango! -- we've got I and Q.

Thanks for all the help -- this just might end up working after all.

---Joel Kolstad

Tom Bruhns wrote:
"Joel Kolstad" wrote in message
...
For standard broadcast, don't they always put L+R on I and L-R on Q,
so standard receivers get L+R?

They may well -- I'm not sure which 'standard' we're talking about here
(there seem to be several out there...). :-) But note that with L+R on I
and L-R on Q, you only get L+R if you manage to synchronize with the phase
(the original problem)! If you use a quadrature detector and extract the
magnitude, you of course get sqrt(L^2+R^2) -- good, but not exactly L+R
either. The Motorola C-QUAM AM stereo system forces the magnitude of the
I-Q phasor to be L+R, which makes it compatible with both envelope detectors
and quadrature detectors. (I probably sound like a Motorola sales guy these
day... I just think it's clever and the end goal here is to build a direct
conversion receiver to decode it...)

But I assume that in any transmission
standard, there is something transmitted that lets you recover the
right phase at the receiver.

For AM, it would appear that the carrier itself is what people use for that
purpose.

If it very clear to me now why TV transmission need to send the colorburst
sequence these days -- otherwise they'd have no way to synchronize the
chroma decoder, which is taking in a DSB-SC modulated in both I and Q.

One easy way is with a "Tayloe mixer" -- you should be able to find
info on that on the web, but it's basically just a commutating switch
that switches the signal (through its source resistance) among four
different capacitors.

I was just thinking of doing something like this -- from a square wave, get
a 4 bit shift register (that always has one output active) and feed the
outputs to a 4066 such that I sample the 'I' channel during, say, clocks #1
and #4 and the 'Q' channel during clocks #1 and #2. Add some filtering
and -- presto chango! -- we've got I and Q.

Thanks for all the help -- this just might end up working after all.

---Joel Kolstad

In article , "Joel Kolstad"
writes:

With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

You are using an example of separation of conventional AM
sidebands. Phase synchronization to the carrier can be done
separately or by using parts of the multi-mixer I-Q circuitry.
Phase synchronization is not absolutely necessary for listening
and still hearing separate sidebands.

Relative phase in mixers is NOT disturbed. (basic fact)

Single-sideband phasing systems use at least two mixers, the
LO of one in quadrature (90 degrees) phase with the other LO.
Since the LO frequency is the same, the two mixers' output will
have a relative phase difference of 90 degrees.

In addition to that, the mixer outputs are put through an audio-
frequency-range wideband phasing network. The Gingell 4-phase
network is ideal for this (it works fine with just two phase inputs).
With the Yoshida value optimization, the Gingell network can be
made with excellent constant-relative-phase-quadrature over a
broad audio frequency range without using precision tolerance parts.

The "trick" now is to linearly combine two of the four audio phases
such that the TOTAL relative phase shift is 0 degrees (or very nearly
so). With the LO having a relative phase differential of 90 degrees
the audio output of the mixers will have a differential phase of 90
degrees. Since the audio polyphase network provides additional
90 degrees relative phase difference, the total is 180 degrees...or 0
degrees if an inverting unity gain amplifier is used.

So, what happens if the LO isn't "locked" to the incoming carrier?
Actually, very little. If the 2 LOs remain in quadrature phase realation-
ship, the two mixer output relative phase relationships are STILL in
quadrature. The only thing that has changed is the slight frequency
difference in the mixer outputs relative to the original modulation
frequency. This has no effect on any broadband audio phasing network
following the mixer outputs...those maintain the additional quadrature
relative phase and linear addition and subtraction will be the same.
Unwanted sideband AND carrier suppression in demodulation will be
essentially unaffected.

In going back through messages after a short absence, I detect some
worry about a slow "beat" effect if the LO isn't synchronized. That's not
a real worry if you've gone through the full expansion of the basic AM
equation and shifted the whole series in phase by 90 degrees, then did
a linear comparison with the same series unshifted in phase, then taking
the TWO audio frequency components from the series and did a linear
addition or subtraction with an additional 90 degree relative difference.

Synchronization-to-the-carrier-frequency-and-phase is necessary only
with conventional AM and the audio circuitry being broadband all the
way down to DC. There are several ways to make the DC component
represented by the carrier mixed down to baseband either disappear or
reduce greatly in value. Hint: Using the full series expansion, use a
small phase shift error and note the comparisons of all series
components, including the carrier.

With SSB, there's no real worry since the transmitting end carriers are
already reduced reduced in amplitude. If the LO isn't quite in sync or
even not on-frequency, all that will be noticed is the slight change in
demodulated audio frequency relative to original frequency. The
amount of rejection of RF in the unwanted side of the carrier will
vary by the error of exact LO quadrature and the error of the phasing
network quadrature. On conventional AM, those errors are the same
as the isolation between sideband modulation content. If the AM has
left ear content on lower sideband and right ear content on upper, that
isolation is the same as "left-ear v. right-ear separation of stereo."


Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

"A" mixer, no. You must use at least two to get an In-phase and
Quadrature output. The Tayloe detector has several advantages.
First, the CMOS switch can be driven with 4 phases, not just 2, and
the equivalent conversion transconductance is far higher than any
passive mixer; mixer noise is also reduced relative to active mixers
due to the nature of the CMOS switch structure. With four phases
in the output, all at 90 degree multiples, it fits the Gingell-Yoshida
polyphase network just dandy such that quadrature errors from
network components are greatly diminished.

The original Gingell polyphase network as described by Peter
Martinez* in RSGB's Radio Communication magazine in 1973 had
only 0 and 180 degree audio phase differences at the input. The
network outputs were still at 90 degree multiples over a wide audio
bandwidth. That bandwidth will increase and with less error when
inputs are already at four phases of relative quadrature.

The only disadvantage of the Tayloe mixer is the need to use a 4x
frequency master LO if the four phases are derived digitally for
broad tuning range.

*G3PLX, the same that inovated PSK31 some years later.

Len Anderson
retired (from regular hours) electronic engineer person

In article , "Joel Kolstad"
writes:

With all this discussion of phasing fun... could someone answer the
following question for me?

Say I'm transmitting binaural audio, with I being L and Q being R. I
receive this signal and generate my own I' and Q' outputs. However, if the
RF carrier and my LO have a phase difference, the entire IQ (phasor) diagram
is rotated by that difference and, e.g., a 90 degree difference will result
in the left and right channels I receive being swapped.

How do IQ-binaural receivers recover a phase lock to present this?

You are using an example of separation of conventional AM
sidebands. Phase synchronization to the carrier can be done
separately or by using parts of the multi-mixer I-Q circuitry.
Phase synchronization is not absolutely necessary for listening
and still hearing separate sidebands.

Relative phase in mixers is NOT disturbed. (basic fact)

Single-sideband phasing systems use at least two mixers, the
LO of one in quadrature (90 degrees) phase with the other LO.
Since the LO frequency is the same, the two mixers' output will
have a relative phase difference of 90 degrees.

In addition to that, the mixer outputs are put through an audio-
frequency-range wideband phasing network. The Gingell 4-phase
network is ideal for this (it works fine with just two phase inputs).
With the Yoshida value optimization, the Gingell network can be
made with excellent constant-relative-phase-quadrature over a
broad audio frequency range without using precision tolerance parts.

The "trick" now is to linearly combine two of the four audio phases
such that the TOTAL relative phase shift is 0 degrees (or very nearly
so). With the LO having a relative phase differential of 90 degrees
the audio output of the mixers will have a differential phase of 90
degrees. Since the audio polyphase network provides additional
90 degrees relative phase difference, the total is 180 degrees...or 0
degrees if an inverting unity gain amplifier is used.

So, what happens if the LO isn't "locked" to the incoming carrier?
Actually, very little. If the 2 LOs remain in quadrature phase realation-
ship, the two mixer output relative phase relationships are STILL in
quadrature. The only thing that has changed is the slight frequency
difference in the mixer outputs relative to the original modulation
frequency. This has no effect on any broadband audio phasing network
following the mixer outputs...those maintain the additional quadrature
relative phase and linear addition and subtraction will be the same.
Unwanted sideband AND carrier suppression in demodulation will be
essentially unaffected.

In going back through messages after a short absence, I detect some
worry about a slow "beat" effect if the LO isn't synchronized. That's not
a real worry if you've gone through the full expansion of the basic AM
equation and shifted the whole series in phase by 90 degrees, then did
a linear comparison with the same series unshifted in phase, then taking
the TWO audio frequency components from the series and did a linear
addition or subtraction with an additional 90 degree relative difference.

Synchronization-to-the-carrier-frequency-and-phase is necessary only
with conventional AM and the audio circuitry being broadband all the
way down to DC. There are several ways to make the DC component
represented by the carrier mixed down to baseband either disappear or
reduce greatly in value. Hint: Using the full series expansion, use a
small phase shift error and note the comparisons of all series
components, including the carrier.

With SSB, there's no real worry since the transmitting end carriers are
already reduced reduced in amplitude. If the LO isn't quite in sync or
even not on-frequency, all that will be noticed is the slight change in
demodulated audio frequency relative to original frequency. The
amount of rejection of RF in the unwanted side of the carrier will
vary by the error of exact LO quadrature and the error of the phasing
network quadrature. On conventional AM, those errors are the same
as the isolation between sideband modulation content. If the AM has
left ear content on lower sideband and right ear content on upper, that
isolation is the same as "left-ear v. right-ear separation of stereo."


Have any suggestions for a nice simple mixer (ala the NE602) that retains
both the I and Q signals at the output?

"A" mixer, no. You must use at least two to get an In-phase and
Quadrature output. The Tayloe detector has several advantages.
First, the CMOS switch can be driven with 4 phases, not just 2, and
the equivalent conversion transconductance is far higher than any
passive mixer; mixer noise is also reduced relative to active mixers
due to the nature of the CMOS switch structure. With four phases
in the output, all at 90 degree multiples, it fits the Gingell-Yoshida
polyphase network just dandy such that quadrature errors from
network components are greatly diminished.

The original Gingell polyphase network as described by Peter
Martinez* in RSGB's Radio Communication magazine in 1973 had
only 0 and 180 degree audio phase differences at the input. The
network outputs were still at 90 degree multiples over a wide audio
bandwidth. That bandwidth will increase and with less error when
inputs are already at four phases of relative quadrature.

The only disadvantage of the Tayloe mixer is the need to use a 4x
frequency master LO if the four phases are derived digitally for
broad tuning range.

*G3PLX, the same that inovated PSK31 some years later.

Len Anderson
retired (from regular hours) electronic engineer person

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