Tried that, still the sideband sweet spot, Most power is still down considerably?? Not so with the SSB.  That is the CW level of peak carrier and the SSB are about right.  Just confusing.



Mel, K6KBE

One other thing Grant is that when a BC station transmits a single tone, the sidebands at averaging or open should be 6 dB down.  It never is?  As a side note, my HP 8920 shows a proper power distribution in the sidebands?   Is what I am observing be some artifact of the DSP in the K3/P3 algorithm?

Mel, K6KBE

Final comment after some bench testing.  I put the signal generator with 1 KHz tone at 100% modulation into the 8920.  Sidebands were 6 dB down.  I then put the signal generator on 7.2 MHz at +20 dBm into a long wire antenna and went inside and tuned the K3/P3 to 7.2 MHz and the display was about 8dB down on the sidebands.  Not sure of the difference, but I am convinced the averaging algorithm is not following the voice patterns on AM modulation like it does on SSB. Probably a timing issue but nothing to worry about.  Glad to see that. 

Mel, K6KBE 

In general, the power level of a voice signal displayed on the P3 will be less than the average power. The reason is that each display point on the P3 is only showing a portion of the total power.

You would see a similar effect on the K3 or other receiver if you monitored an SSB signal with a narrow CW filter. The S meter would read lower since only a portion of the signal is within the passband.

If an AM signal is modulated with a single-tone sine wave at 100% modulation, then each sideband will be -6 dB from the carrier, and it will show that way on the P3. But with voice modulation it will be much lower, both because of the effect I just mentioned and because the average modulation is less than the peak.

Alan N1AL

Yes, Alan, I understand all of that.

Here is the question.  Why is the sideband power on AM on the same band, 75 M, less than the displayed signal on SSB. 

An example.  Watching the 100 watt signal on 3963 KHz SSB signal on the P3, the envelop of the energy fills the IF KHz window and is smooth in the display, if the audio is flat, to the top of the same RF level as CW. With audio frequency variances allowed.  While on the same settings on a AM signal at 3870 KHz at 1KW carrier the sideband are 20 to 30 dB lower and ragged in display.!!!!!!!!!!!  All P3 settings the same...... 

If my math is correct, 100 w SSB and 1KW AM should have within ~3 dB the same displayed sideband power level?????  Where is my reasoning wrong???

Mel, K6KBE

Hi Mel,

I'm trying to understand the test conditions. Are the 100W SSB signal and the 1 kW AM signal transmitted from the same station on the same antenna? If not, then no direct comparison can be made.

I can easily believe the sidebands of the AM signal are 20 to 30 dB below the carrier. Say the audio has a 10 dB peak-to-average ratio. Right away that puts the average power of each sideband at 6 + 10 = 16 dB below the carrier, assuming 100% peak modulation.

Furthermore, if the P3 span is 20 kHz, then each display point is roughly 40 Hz wide, perhaps 1/100 of the total sideband, assuming 4 kHz audio. That's another 20 dB.

Here's something to try: Turn on peak hold on the P3 and watch the AM signal for awhile as the program content varies between music, voice announcements, ads, etc. You'll probably notice the sidebands are much flatter and higher in amplitude on the peak trace than on the real-time trace.

Alan N1AL

OK, Alan, Thanks for the input.

Here goes.  I am monitoring a friend near by, 15 miles.  I took care to have him set up the 100 W SSB station and his 1KW AM station like this.

In CW mode, send a 100 W measured carrier.  I recorded the level on the P3.  Then had him transmit normal voice levels and the peak SSB audio components reached the same level as the CW signal on the P3. 

Then I had him set up his RCA AM transmitter to monitored 1 KW carrier on the bird.  I recorded the level on the P3, ~1-2 dB of 10 dB more than the CW signal.  Same antenna on both.

I had him transmit a 1 KHz tone 100 % modulated and yes, the 1 KHz was 6 db below the carrier at 1 KW on the P3.

Now I had him talk to the 100% modulation point on the AM transmitter.

With the Peak averaging set to max, the side band AM signals were 20-25 dB down from the carrier.

If the premise is that a 100% modulated AM signal has 50% of the power in the sidebands and 25% on each side, that is 500 watts total audio and 250 watts on each side.  So the level should be only 6 dB down from the carrier.

The only difference is the complex audio components vs the signal tone.  So why is the SSB signal peaking at the 100 watt level and the AM signal 10 to 15 dB lower?

Comments appreciated....

Mel, K6KBE

… I’d love to hear the answers from somebody who really knows… by my speculation is that they need to drop the power output on AM because it emits a steady carrier, while in SSB mode those peaks are ms duration peaks, and can be easily handled by the instantaneous power available from capacitors and with endurable dissipation in the final transistors. But for those power levels under steady carrier conditions it would be intolerable to the finals and the power supply is insufficient.

Added note.

In my lab, I set up the following:  Advert 7200A signal generator and HP 8920A analyzer.

All tests prove that a AM 100 % modulation carrier the sidebands are ALWAYS 6 dB down from the carrier.....

This is regardless of the absolute level power level mW to MW. 

How come the 8920A on some local BC stations shows this to be true to the above and the P3 does not.............

Not a problem, just a interesting issue about the algorithms used I guess.

Mel, K6KBE

Then I had him set up his RCA AM transmitter to monitored 1 KW
carrier on the bird.

Which is, of course, illegal as soon as he modulates the transmitter.
A 100% modulated 1 KW AM signal is 4KW PEP ... the legal limit (in the
US) for AM is now 375 W carrier => 1500 W PEP just like every other
mode.

With the Peak averaging set to max, the side band AM signals were
20-25 dB down from the carrier.

You are not measuring peak audio components - you are measuring the
average audio level. Instead of average, select MENU:PEAK to turn
on "spectrum peak hold" and set averaging to the minimum.

73,

... Joe, W4TV


On 2015-07-03 1:29 PM, farrerfolks@... [Elecraft_K3] wrote:

OK, Alan, Thanks for the input.

Here goes. I am monitoring a friend near by, 15 miles. I took care
to have him set up the 100 W SSB station and his 1KW AM station like
this.

In CW mode, send a 100 W measured carrier. I recorded the level on
the P3. Then had him transmit normal voice levels and the peak SSB
audio components reached the same level as the CW signal on the P3.

Then I had him set up his RCA AM transmitter to monitored 1 KW
carrier on the bird. I recorded the level on the P3, ~1-2 dB of 10
dB more than the CW signal. Same antenna on both.

I had him transmit a 1 KHz tone 100 % modulated and yes, the 1 KHz
was 6 db below the carrier at 1 KW on the P3.

Now I had him talk to the 100% modulation point on the AM
transmitter.

With the Peak averaging set to max, the side band AM signals were
20-25 dB down from the carrier.

If the premise is that a 100% modulated AM signal has 50% of the
power in the sidebands and 25% on each side, that is 500 watts total
audio and 250 watts on each side. So the level should be only 6 dB
down from the carrier.

The only difference is the complex audio components vs the signal
tone. So why is the SSB signal peaking at the 100 watt level and the
AM signal 10 to 15 dB lower?

Comments appreciated....

Mel, K6KBE

But the difference is the display is the question. WHY?

Are you using the peak hold mode for voice modulation as Al ask
and I told you how to enable? There is a significant difference
between a static display of 1 KHz modulation and the dynamic
display of voice modulation. When you enable averaging, the
sideband level will appear lower based on the length of each
dynamic element as a percentage to the total averaging time.

*USE PEAK HOLD* and see if the peaks in the sidebands aren't
close to -6dB compared to the carrier.

73,

... Joe, W4TV


On 2015-07-03 2:53 PM, Mel Farrer farrerfolks@... [Elecraft_K3] wrote:

Yes, Joe,
But the difference is the display is the question. WHY? My 8920 show proper power distribution on the sidebands, the P 3does not!!!!!!!


Mel, K6KBE
From: "'Joe Subich, W4TV' lists@... [Elecraft_K3]" <Elecraft_K3@...>
To: Elecraft_K3@...
Sent: Friday, July 3, 2015 11:35 AM
Subject: Re: [Elecraft_K3] K3/P3 video display on AM vs SSB


> Then I had him set up his RCA AM transmitter to monitored 1 KW
> carrier on the bird.


Which is, of course, illegal as soon as he modulates the transmitter.
A 100% modulated 1 KW AM signal is 4KW PEP ... the legal limit (in the
US) for AM is now 375 W carrier => 1500 W PEP just like every other
mode.

With the Peak averaging set to max, the side band AM signals were

> 20-25 dB down from the carrier.


You are not measuring peak audio components - you are measuring the
average audio level. Instead of average, select MENU:PEAK to turn
on "spectrum peak hold" and set averaging to the minimum.


73,


... Joe, W4TV


On 2015-07-03 1:29 PM, farrerfolks@... [Elecraft_K3] wrote:

OK, Alan, Thanks for the input.

Here goes. I am monitoring a friend near by, 15 miles. I took care
to have him set up the 100 W SSB station and his 1KW AM station like
this.

In CW mode, send a 100 W measured carrier. I recorded the level on
the P3. Then had him transmit normal voice levels and the peak SSB
audio components reached the same level as the CW signal on the P3.

Then I had him set up his RCA AM transmitter to monitored 1 KW
carrier on the bird. I recorded the level on the P3, ~1-2 dB of 10
dB more than the CW signal. Same antenna on both.

I had him transmit a 1 KHz tone 100 % modulated and yes, the 1 KHz
was 6 db below the carrier at 1 KW on the P3.

Now I had him talk to the 100% modulation point on the AM
transmitter.

With the Peak averaging set to max, the side band AM signals were

20-25 dB down from the carrier.

If the premise is that a 100% modulated AM signal has 50% of the
power in the sidebands and 25% on each side, that is 500 watts total
audio and 250 watts on each side. So the level should be only 6 dB
down from the carrier.

The only difference is the complex audio components vs the signal
tone. So why is the SSB signal peaking at the 100 watt level and the
AM signal 10 to 15 dB lower?

Comments appreciated....

Mel, K6KBE

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OK, as stated, I ran the peak hold which actually the average with a setting of 20.  No difference, sorry.

You have to perform the test to see it your self.  No more on this thread unless P3 tech support joins in..  Tired of repeating myself..

Mel  K6KBE

OK, as stated, I ran the peak hold which actually the average with a
setting of 20.

Peak Hold and Average are completely different settings. If you are
setting average to 20, you are not using peak hold.

You have to perform the test to see it your self.

I have run peak-hold while watching several of the local AM stations.
The "hold" line reliably settles to -6dB relative to carrier within
one or two dB over several minutes of time while a 20 period average
is often -15 or -20 dB compared to carrier.

73,

... Joe, W4TV


On 2015-07-03 6:54 PM, farrerfolks@... [Elecraft_K3] wrote:

OK, as stated, I ran the peak hold which actually the average with a setting of 20. No difference, sorry.

You have to perform the test to see it your self. No more on this thread unless P3 tech support joins in.. Tired of repeating myself..

Mel K6KBE

Thanks Joe, found the peak hold.  Still on my P3 the blue modulation line is a slow arc but unless you are looking at the really low audio tones, -500 Hz, which are about -12 dB, the rest of the audio spectrum is -20 dB on my BC stations?  Still confused...  I took a picture and put it in my call sign photo.

Mel, K6KBE