[IC-7000] IC-7000 AM Modulation Problem


Dean Gibson AE7Q <yahoo1@...>
 

I just tried it at full power on 20m, and using the built-in bar meter for all observations:

* Carrier-only level was 40%.
* Normal voice peaks were below 50% with a slightly loud voice and the mic. gain at normal (50%).
* Normal voice peaks were about 60% with a slightly loud voice and the mic. gain at 100%.
* With both mic. gain settings, a low-volume whistle drove the meter to 100% power.

If my math is correct, an AM transmitter that has a 40w carrier with modulation peaks at 100w, then the modulation level is 60%.

-- Dean

On 2006-04-04 21:09, Larry Harrison wrote:
...

I recently purchased an IC-7000. A fine radio in all respects except my
radio will only modulate 30% or less with VOICE.
It will modulate 100% with a 1000 Hz tone.

...

The 7000 is rated for 100 Watts PEP therefore, 25 Watts of carrier output
would be the correct resting carrier output level. Not 40 Watts as stated in
the manual. With a 25 Watt carrier at 100% modulation the TX output would be
100 Watts PEP.

73
Larry K3JRR


RL Luns
 

Your math is right but your theory may be wrong. Most ham SSB transceivers, when used for AM, appear to use a certain amount of "negative" AM modulation. This allows a stronger carrier level than conventional AM theory predicts.

Conventional AM theory says that AM modulation is obtained by starting with a reference carrier level then modulating the RF envelope voltage symmetrically higher and lower about the reference carrier voltage. Full 100% modulation is obtained when the modulation positive peaks double the voltage and negative peaks tangientially go to zero voltage. Because power is proportional to voltage squared, this arrangement results in a 100% modulated PEP four times higher than the carrier power. The 100% modulation also results in a 50% increase in the overall average power transmitted.

I have never seen the theory of "negative" AM modulation written up, but I believe it is based on the following. Again start with an unmodulated reference RF carrier voltage. Now consider the following different type of AM modulation. When modulated by a sine wave, the envelope of the RF is only negatively AM modulated such that the peak of the RF envelope is at the reference level at the positive peak of the modulating sine wave and proportially lower throughout the remainder of the sine wave. Full 100% modulation with this scheme results in the RF envelope voltage varying from the reference level to zero. In this approach the PEP and the carrier power are always the same regardless of the percentage of modulation. The 100% modulation results in a 50% DEcrease in the overall average power transmitted. This can sometimes be noticed as a downward deflection of the S-meter in time with voice peaks, hence the term "negative" modulation.

It is my impression that most ham SSB transceivers when operated in the AM mode use a hybrid AM modulation that is somewhere in between the two limiting theoretical cases described above. If this is the case, then the percentage of modulation can not be accurately calculated using the formulas from just the conventional AM theory alone. An oscilloscope and listening to the AM qualtity received are the best guide to the modulation quality.

I have held off buying an IC-7000 because I am not convinced that it has a credible AM xmit capability. I would be be very happy to learn otherwise.

< Grey

Dean Gibson AE7Q <yahoo1@ae7q.net> wrote:
I just tried it at full power on 20m, and using the built-in bar meter
for all observations:

* Carrier-only level was 40%.
* Normal voice peaks were below 50% with a slightly loud voice and the
mic. gain at normal (50%).
* Normal voice peaks were about 60% with a slightly loud voice and the
mic. gain at 100%.
* With both mic. gain settings, a low-volume whistle drove the meter to
100% power.

If my math is correct, an AM transmitter that has a 40w carrier with
modulation peaks at 100w, then the modulation level is 60%.

-- Dean

On 2006-04-04 21:09, Larry Harrison wrote:
...

I recently purchased an IC-7000. A fine radio in all respects except my
radio will only modulate 30% or less with VOICE.
It will modulate 100% with a 1000 Hz tone.

...

The 7000 is rated for 100 Watts PEP therefore, 25 Watts of carrier output
would be the correct resting carrier output level. Not 40 Watts as
stated in
the manual. With a 25 Watt carrier at 100% modulation the TX output
would be
100 Watts PEP.

73
Larry K3JRR



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Dean Gibson AE7Q <yahoo1@...>
 

I've never heard of that, but that doesn't mean much.

However, note that a certain amount of carrier is required (absent one inserted by the receiving radio) in order to properly demodulate the AM signal. Too little, and you get distortion with a conventional AM detector. In the extreme it becomes double sideband, no carrier. It's OK to have too much carrier (from a a demodulating point of view), but not too little.

Now, my recollection (which is fuzzy) is that you needed all of the carrier (again, absent one inserted by the receiving radio) to demodulate. You can obviously reduce the carrier by 1/2 if you get rid of one of the sidebands, and an AM radio will demodulate that without a problem.

Now, if you are saying that the "no modulation" carrier is 40% of max RF power, but that under 100% modulation, the carrier drops to 25% of the peak power, I can understand that. What the benefits/consequences of that are, I don't know.

However, your question prompted me to do a little experiment: Given the same voice level of speech, the power meter on the IC-7000 showed the same peaks for a strong voice, whether SSB or AM. This was not a scientific test, so "your results may vary".

Any references/links that anyone can provide would be illuminating, to say the least.

-- Dean

On 2006-04-08 15:07, RL Luns wrote:
Your math is right but your theory may be wrong. Most ham SSB transceivers, when used for AM, appear to use a certain amount of "negative" AM modulation. This allows a stronger carrier level than conventional AM theory predicts.
Conventional AM theory says that AM modulation is obtained by starting with a reference carrier level then modulating the RF envelope voltage symmetrically higher and lower about the reference carrier voltage. Full 100% modulation is obtained when the modulation positive peaks double the voltage and negative peaks tangientially go to zero voltage. Because power is proportional to voltage squared, this arrangement results in a 100% modulated PEP four times higher than the carrier power. The 100% modulation also results in a 50% increase in the overall average power transmitted. I have never seen the theory of "negative" AM modulation written up, but I believe it is based on the following. Again start with an unmodulated reference RF carrier voltage. Now consider the following different type of AM modulation. When modulated by a sine wave, the envelope of the RF is only negatively AM modulated such that the peak of the RF envelope is at the reference level at the positive peak of the modulating sine wave and proportially lower throughout the remainder of the sine wave. Full 100% modulation with this scheme results in the RF envelope voltage varying from the reference level to zero. In this approach the PEP and the carrier power are always the same regardless of the percentage of modulation. The 100% modulation results in a 50% DEcrease in the overall average power transmitted. This can sometimes be noticed as a downward deflection of the S-meter in time with voice peaks, hence the term "negative" modulation.
It is my impression that most ham SSB transceivers when operated in the AM mode use a hybrid AM modulation that is somewhere in between the two limiting theoretical cases described above. If this is the case, then the percentage of modulation can not be accurately calculated using the formulas from just the conventional AM theory alone. An oscilloscope and listening to the AM qualtity received are the best guide to the modulation quality.
I have held off buying an IC-7000 because I am not convinced that it has a credible AM xmit capability. I would be be very happy to learn otherwise.
< Grey

Dean Gibson AE7Q <yahoo1@ae7q.net> wrote:
I just tried it at full power on 20m, and using the built-in bar meter
for all observations:

* Carrier-only level was 40%.
* Normal voice peaks were below 50% with a slightly loud voice and the mic. gain at normal (50%).
* Normal voice peaks were about 60% with a slightly loud voice and the mic. gain at 100%.
* With both mic. gain settings, a low-volume whistle drove the meter to 100% power.

If my math is correct, an AM transmitter that has a 40w carrier with modulation peaks at 100w, then the modulation level is 60%.

-- Dean

On 2006-04-04 21:09, Larry Harrison wrote:
...

I recently purchased an IC-7000. A fine radio in all respects
except my radio will only modulate 30% or less with VOICE.
It will modulate 100% with a 1000 Hz tone.

...

The 7000 is rated for 100 Watts PEP therefore, 25 Watts of carrier
output would be the correct resting carrier output level. Not 40 Watts as stated in the manual. With a 25 Watt carrier at 100% modulation the TX output would be 100 Watts PEP.

73
Larry K3JRR


RL Luns
 

Hi Dean,

Your 4th paragraph is what I was referring to: "no modulation" carrier is 40% of max RF power, but that under 100% modulation, the carrier drops to 25% of the
peak power.

The main advantage of such an approach might be that the manufacturer can advertise a higher AM transmitter power. It would also result in less receiver noise when transmitting a dead carrier as happens during pauses between speach.

The results of your experiment sound like what I would expect from most any SSB transceiver operating in the AM mode. Guess I need to hear an IC-7000 transmitting AM on the air.

73,

Grey

Dean Gibson AE7Q <yahoo1@ae7q.net> wrote:
I've never heard of that, but that doesn't mean much.

However, note that a certain amount of carrier is required (absent one
inserted by the receiving radio) in order to properly demodulate the AM
signal. Too little, and you get distortion with a conventional AM
detector. In the extreme it becomes double sideband, no carrier. It's
OK to have too much carrier (from a a demodulating point of view), but
not too little.

Now, my recollection (which is fuzzy) is that you needed all of the
carrier (again, absent one inserted by the receiving radio) to
demodulate. You can obviously reduce the carrier by 1/2 if you get rid
of one of the sidebands, and an AM radio will demodulate that without a
problem.

Now, if you are saying that the "no modulation" carrier is 40% of max RF
power, but that under 100% modulation, the carrier drops to 25% of the
peak power, I can understand that. What the benefits/consequences of
that are, I don't know.

However, your question prompted me to do a little experiment: Given the
same voice level of speech, the power meter on the IC-7000 showed the
same peaks for a strong voice, whether SSB or AM. This was not a
scientific test, so "your results may vary".

Any references/links that anyone can provide would be illuminating, to
say the least.

-- Dean

On 2006-04-08 15:07, RL Luns wrote:
Your math is right but your theory may be wrong. Most ham SSB
transceivers, when used for AM, appear to use a certain amount of
"negative" AM modulation. This allows a stronger carrier level than
conventional AM theory predicts.

Conventional AM theory says that AM modulation is obtained by
starting with a reference carrier level then modulating the RF
envelope voltage symmetrically higher and lower about the reference
carrier voltage. Full 100% modulation is obtained when the modulation
positive peaks double the voltage and negative peaks tangientially go
to zero voltage. Because power is proportional to voltage squared,
this arrangement results in a 100% modulated PEP four times higher
than the carrier power. The 100% modulation also results in a 50%
increase in the overall average power transmitted.

I have never seen the theory of "negative" AM modulation written up,
but I believe it is based on the following. Again start with an
unmodulated reference RF carrier voltage. Now consider the following
different type of AM modulation. When modulated by a sine wave, the
envelope of the RF is only negatively AM modulated such that the peak
of the RF envelope is at the reference level at the positive peak of
the modulating sine wave and proportially lower throughout the
remainder of the sine wave. Full 100% modulation with this scheme
results in the RF envelope voltage varying from the reference level to
zero. In this approach the PEP and the carrier power are always the
same regardless of the percentage of modulation. The 100% modulation
results in a 50% DEcrease in the overall average power transmitted.
This can sometimes be noticed as a downward deflection of the S-meter
in time with voice peaks, hence the term "negative" modulation.

It is my impression that most ham SSB transceivers when operated in
the AM mode use a hybrid AM modulation that is somewhere in between
the two limiting theoretical cases described above. If this is the
case, then the percentage of modulation can not be accurately
calculated using the formulas from just the conventional AM theory
alone. An oscilloscope and listening to the AM qualtity received are
the best guide to the modulation quality.

I have held off buying an IC-7000 because I am not convinced that it
has a credible AM xmit capability. I would be be very happy to learn
otherwise.

< Grey

Dean Gibson AE7Q <yahoo1@ae7q.net> wrote:
I just tried it at full power on 20m, and using the built-in bar meter
for all observations:

* Carrier-only level was 40%.
* Normal voice peaks were below 50% with a slightly loud voice and the
mic. gain at normal (50%).
* Normal voice peaks were about 60% with a slightly loud voice and the
mic. gain at 100%.
* With both mic. gain settings, a low-volume whistle drove the meter
to 100% power.

If my math is correct, an AM transmitter that has a 40w carrier with
modulation peaks at 100w, then the modulation level is 60%.

-- Dean

On 2006-04-04 21:09, Larry Harrison wrote:
...

I recently purchased an IC-7000. A fine radio in all respects
except my radio will only modulate 30% or less with VOICE.
It will modulate 100% with a 1000 Hz tone.

...

The 7000 is rated for 100 Watts PEP therefore, 25 Watts of carrier
output would be the correct resting carrier output level. Not 40 Watts
as stated in the manual. With a 25 Watt carrier at 100% modulation the
TX output would be 100 Watts PEP.

73
Larry K3JRR


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subject to deletion and banning."
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Adam Farson
 

Grey, Dean,

Although I cannot check this on the IC-7000 (I do not have one), I am
reasonably sure that the DSP algorithm in the IC-7000 is reasonably similar
to that in the 756Pro series. The drop in carrier level as we approach 100%
modulation is not a deliberate design feature; it is carrier starvation
caused by ALC action to maintain the 100W PEP power budget of the PA.

I performed some measurements in AM mode on an IC-756Pro2, using a spectrum
analyser. Details are here:

http://www.ab4oj.com/icom/pro_notes.html#AM

Summarising:

At 25W: Carrier drops less than 0.5 dB.
At 40W: Carrier drops 3 dB.

Best 73,
Adam, VA7OJ/AB4OJ
Owner, Yahoo! ic7000 Group



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