On 22 Oct 2003 20:21:16 GMT, (Avery Fineman)
wrote:

Bill, I just dug out the 1977 issues of HR from storage and looked
the article over. Author Richard Slater (W3EJD) said almost the
same thing at the end of the article on page 15 under "closing
comments." The nomenclatures for different modulations were
formalized by the ITU-R since then but the FCC still doesn't have
anything covering this "single-sideband FM" modulation type for
U. S. amateur radio.

The ITU-R emission designations are quite outdated and many modern
emissions use din commercial and military systems would be designated
as XXX. In each case the X means "none above" in the corresponding
column.

Anyway, why should the amateur radio regulations contain these ITU-R
designations ? Here in Finland, ITU-R emission designations were
removed from amateur radio regulations and exam in 1997 and only band
specific power and bandwidth limits are used. I haven't heard of any
problems due to this decision.

Paul OH3LWR

Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it?

Here is a question that has plagued many for years:
If you have a plate modulated transmitter, the plate voltage will
swing down to zero and up to two times the plate voltage with 100%
modulation. At 100% negative modulation the plate voltage is cutoff
for the instant of the modulation negative peak.

How is the carrier still transmitted during the time there is zero
plate voltage?

If we lower the modulation frequency to say 1 cps or even lower, 1
cycle per minute, then wouldn't the transmitter final be completely
off for half that time and unable to produce any carrier output??

Question is, at what point does the carrier start to be effected?


73
Gary K4FMX


On Wed, 22 Oct 2003 20:39:20 -0700, Roy Lewallen
wrote:

The amplitudes of the sideband components are symmetrical (at least for
modulation by a single sine wave), but the phases aren't. The phases of
all the upper sideband components are in phase with the carrier; in the
lower sideband, the odd order components (and only the odd order ones)
are reversed in phase. With multitone modulation, things get a whole lot
more complex. Unlike AM, FM is nonlinear, so there are sideband
components from each tone, plus components from their sum, difference,
and harmonics. The inability to use superposition makes analysis of
frequency modulation with complex waveforms a great deal more difficult
than AM.

Note also that unlike AM, whatever fraction of the carrier that's left
when transmitting FM also contains part of the modulation information.
Of course, at certain modulation indices with pure sine wave modulation,
the carrier goes to zero, meaning that all the modulation information is
in the sidebands. But this happens only under specific modulation
conditions, so you'd certainly have an information-carrying carrier
component present when modulating with a voice, for example.

Roy Lewallen, W7EL

Joel Kolstad wrote:
Avery Fineman wrote:

There isn't any corresponding similarity of
FM and PM to AM for the repetition of sidebands' information when
looking at the spectral content.


Umm... last I looked the spectrum of FM and PM was symmetrical about the
carrier frequency? (Well, the lower sideband is 180 degrees out of phase
with the upper, but that's true of AM as well.) Looking at a single sine
wave input to an FM or phase modulator, this comes about from the Bessel
function expansion of the sidetones and J-n(x)=-Jn(x)?

I know you're far more experienced in this area than I am, however, so I'll
let you explain what I'm misinterpreting here!

---Joel Kolstad

Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it?

Here is a question that has plagued many for years:
If you have a plate modulated transmitter, the plate voltage will
swing down to zero and up to two times the plate voltage with 100%
modulation. At 100% negative modulation the plate voltage is cutoff
for the instant of the modulation negative peak.

How is the carrier still transmitted during the time there is zero
plate voltage?

If we lower the modulation frequency to say 1 cps or even lower, 1
cycle per minute, then wouldn't the transmitter final be completely
off for half that time and unable to produce any carrier output??

Question is, at what point does the carrier start to be effected?


73
Gary K4FMX


On Wed, 22 Oct 2003 20:39:20 -0700, Roy Lewallen
wrote:

The amplitudes of the sideband components are symmetrical (at least for
modulation by a single sine wave), but the phases aren't. The phases of
all the upper sideband components are in phase with the carrier; in the
lower sideband, the odd order components (and only the odd order ones)
are reversed in phase. With multitone modulation, things get a whole lot
more complex. Unlike AM, FM is nonlinear, so there are sideband
components from each tone, plus components from their sum, difference,
and harmonics. The inability to use superposition makes analysis of
frequency modulation with complex waveforms a great deal more difficult
than AM.

Note also that unlike AM, whatever fraction of the carrier that's left
when transmitting FM also contains part of the modulation information.
Of course, at certain modulation indices with pure sine wave modulation,
the carrier goes to zero, meaning that all the modulation information is
in the sidebands. But this happens only under specific modulation
conditions, so you'd certainly have an information-carrying carrier
component present when modulating with a voice, for example.

Roy Lewallen, W7EL

Joel Kolstad wrote:
Avery Fineman wrote:

There isn't any corresponding similarity of
FM and PM to AM for the repetition of sidebands' information when
looking at the spectral content.


Umm... last I looked the spectrum of FM and PM was symmetrical about the
carrier frequency? (Well, the lower sideband is 180 degrees out of phase
with the upper, but that's true of AM as well.) Looking at a single sine
wave input to an FM or phase modulator, this comes about from the Bessel
function expansion of the sidetones and J-n(x)=-Jn(x)?

I know you're far more experienced in this area than I am, however, so I'll
let you explain what I'm misinterpreting here!

---Joel Kolstad

You have to be careful in what you call the "carrier". As soon as you
start modulating the "carrier", you have more than one frequency
component. At that time, only the component at the frequency of the
original unmodulated signal is called the "carrier". So you have a
modulated RF signal, part of which is the "carrier", and part of which
is sidebands.

General frequency domain analysis makes the assumption that each
frequency component has existed forever and will exist forever. So under
conditions of modulation with a periodic signal, you have three
components: A "carrier", which is not modulated, but a steady, single
frequency, constant amplitude signal; and two sidebands, each of which
is a frequency shifted (and, for the LSB, reversed) replica of the
modulating waveform.

You can take each of these waveforms, add them together in the time
domain, and get the familiar modulated envelope.

So, the short answer is that the carrier, which is a frequency domain
concept, is there even if you're modulating at 0.001 Hz. But to observe
it, you've got to watch for much longer than 1000 seconds. You simply
can't do a meaningful spectrum analysis of a signal in a time that's not
a lot longer than the modulation period.

Roy Lewallen, W7EL

Gary Schafer wrote:
Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it?

Here is a question that has plagued many for years:
If you have a plate modulated transmitter, the plate voltage will
swing down to zero and up to two times the plate voltage with 100%
modulation. At 100% negative modulation the plate voltage is cutoff
for the instant of the modulation negative peak.

How is the carrier still transmitted during the time there is zero
plate voltage?

If we lower the modulation frequency to say 1 cps or even lower, 1
cycle per minute, then wouldn't the transmitter final be completely
off for half that time and unable to produce any carrier output??

Question is, at what point does the carrier start to be effected?


73
Gary K4FMX

You have to be careful in what you call the "carrier". As soon as you
start modulating the "carrier", you have more than one frequency
component. At that time, only the component at the frequency of the
original unmodulated signal is called the "carrier". So you have a
modulated RF signal, part of which is the "carrier", and part of which
is sidebands.

General frequency domain analysis makes the assumption that each
frequency component has existed forever and will exist forever. So under
conditions of modulation with a periodic signal, you have three
components: A "carrier", which is not modulated, but a steady, single
frequency, constant amplitude signal; and two sidebands, each of which
is a frequency shifted (and, for the LSB, reversed) replica of the
modulating waveform.

You can take each of these waveforms, add them together in the time
domain, and get the familiar modulated envelope.

So, the short answer is that the carrier, which is a frequency domain
concept, is there even if you're modulating at 0.001 Hz. But to observe
it, you've got to watch for much longer than 1000 seconds. You simply
can't do a meaningful spectrum analysis of a signal in a time that's not
a lot longer than the modulation period.

Roy Lewallen, W7EL

Gary Schafer wrote:
Speaking of AM modulation,, we all know that the carrier amplitude
does not change with modulation. Or does it?

Here is a question that has plagued many for years:
If you have a plate modulated transmitter, the plate voltage will
swing down to zero and up to two times the plate voltage with 100%
modulation. At 100% negative modulation the plate voltage is cutoff
for the instant of the modulation negative peak.

How is the carrier still transmitted during the time there is zero
plate voltage?

If we lower the modulation frequency to say 1 cps or even lower, 1
cycle per minute, then wouldn't the transmitter final be completely
off for half that time and unable to produce any carrier output??

Question is, at what point does the carrier start to be effected?


73
Gary K4FMX

You're probably thinking of AM vs. narrow band FM. Although the equations
look very similar on paper and the MAGNITUDE spectrum is identical, the
phase spectrum is different

Joel-

Perhaps that is what I'm remembering. Now, if you use a filter to eliminate
the other sideband, the higher frequency components and the carrier, don't you
have a nearly identical remainder?

See the message I posted earlier tonight for a discussion of whether or not
you can recover NBFM with an envelope detector

Somehow I missed that one. It seems that AOL does not post messages in the
order in which they were originated!

I think we are in agreement that you can't recover FM modulation with just an
envelope detector, but there is another approach. Again, you need a filter,
but maybe one that is not as sharp as above. If you tune the radio so the
carrier is just outside the passband, an amplitude variation will occur as the
signal slides up and down the shoulder of the filter. The result is a pseudo
AM signal that is detected by the envelope detector. I recall that this
approach is called "slope detection".

73, Fred, K4DII

You're probably thinking of AM vs. narrow band FM. Although the equations
look very similar on paper and the MAGNITUDE spectrum is identical, the
phase spectrum is different

Joel-

Perhaps that is what I'm remembering. Now, if you use a filter to eliminate
the other sideband, the higher frequency components and the carrier, don't you
have a nearly identical remainder?

See the message I posted earlier tonight for a discussion of whether or not
you can recover NBFM with an envelope detector

Somehow I missed that one. It seems that AOL does not post messages in the
order in which they were originated!

I think we are in agreement that you can't recover FM modulation with just an
envelope detector, but there is another approach. Again, you need a filter,
but maybe one that is not as sharp as above. If you tune the radio so the
carrier is just outside the passband, an amplitude variation will occur as the
signal slides up and down the shoulder of the filter. The result is a pseudo
AM signal that is detected by the envelope detector. I recall that this
approach is called "slope detection".

73, Fred, K4DII

Fred McKenzie wrote:
Perhaps that is what I'm remembering. Now, if you use a filter to
eliminate the other sideband, the higher frequency components and the
carrier, don't you have a nearly identical remainder?

At that point I don't think you could tell the difference since there's no
longer any local phase reference (i.e., the carrier) to compare with. I
suppose this is why your SSB-AM rig is able to (somewhat) receive low
frequency (and thereby presumably narrowband) FM broadcasts; this is what
you were saying in your last post, correct?

I think we are in agreement that you can't recover FM modulation with
just an envelope detector

Yes, at least you can't recover a signal that directly corresponds to what
you transmitted. It does appear that you can recover the signal's square,
however, so this approach might be useful for, e.g., remote command
transmissions. (But probably just for the novelty of having said you did
it... since it's probably not much harder to build the slope detector you
describe!)

---Joel

Fred McKenzie wrote:
Perhaps that is what I'm remembering. Now, if you use a filter to
eliminate the other sideband, the higher frequency components and the
carrier, don't you have a nearly identical remainder?

At that point I don't think you could tell the difference since there's no
longer any local phase reference (i.e., the carrier) to compare with. I
suppose this is why your SSB-AM rig is able to (somewhat) receive low
frequency (and thereby presumably narrowband) FM broadcasts; this is what
you were saying in your last post, correct?

I think we are in agreement that you can't recover FM modulation with
just an envelope detector

Yes, at least you can't recover a signal that directly corresponds to what
you transmitted. It does appear that you can recover the signal's square,
however, so this approach might be useful for, e.g., remote command
transmissions. (But probably just for the novelty of having said you did
it... since it's probably not much harder to build the slope detector you
describe!)

---Joel

Along the same line consider that the envelope of an SSB signal has no
direct relationship to the original modulation the way that an AM
signal does.

This is why you can not use RF derived ALC to control the audio stage
of an SSB transmitter the way you can with an AM transmitter.
Or audio clipping that works on AM but does not work the same on SSB.

Transmit a square wave on an AM transmitter and you see a square wave
in the AM envelope. Do the same with an SSB transmitter and you only
see sharp spikes in the envelope.

73
Gary K4FMX


On Thu, 23 Oct 2003 12:08:31 -0700, "Joel Kolstad"
wrote:

Fred McKenzie wrote:
Perhaps that is what I'm remembering. Now, if you use a filter to
eliminate the other sideband, the higher frequency components and the
carrier, don't you have a nearly identical remainder?

At that point I don't think you could tell the difference since there's no
longer any local phase reference (i.e., the carrier) to compare with. I
suppose this is why your SSB-AM rig is able to (somewhat) receive low
frequency (and thereby presumably narrowband) FM broadcasts; this is what
you were saying in your last post, correct?

I think we are in agreement that you can't recover FM modulation with
just an envelope detector

Yes, at least you can't recover a signal that directly corresponds to what
you transmitted. It does appear that you can recover the signal's square,
however, so this approach might be useful for, e.g., remote command
transmissions. (But probably just for the novelty of having said you did
it... since it's probably not much harder to build the slope detector you
describe!)

---Joel