AM Carrier Adjustments (Transmit Tab)

Ken
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AM Carrier Adjustments (Transmit Tab)

Postby Ken » Wed Jan 25, 2023 3:40 pm

Hello,

How does the AM Carrier adjustment under the Transmit Tab actually work in the Anan Software?

Typically as I understand it, if you have a 100 watt transceiver that transmits 100 watts on SSB, the output power for AM needs
to be set for 25 watts. (AM modulation = 4x the carrier at 100% ) (4x 25 watts with 100% modulation = 100 Watts)

However lowering the carrier to less then 25 watts will allow more audio head room, perhaps 110%

Is this the case with the Anan?
I have experimented by lowering the carrier achieving more audio head room.
But is this what is actually going on in the Anan, or is there more?

Thank you.
Ken
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W1AEX
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Re: AM Carrier Adjustments (Transmit Tab)

Postby W1AEX » Thu Jan 26, 2023 5:30 pm

Hi Ken,

Thetis does a remarkable job with the AM mode and is capable of producing a broadcast quality signal using the CFC processing if that is what is desired. My simplest understanding of what's going on is that Thetis produces a double-sideband signal and adds the carrier in at the end. So, unlike plate modulation, you are not actually modulating the carrier.

If you are using a PA power profile that produces a 100 watt carrier in Tune mode when the TX drive slider is set to the maximum setting, when you run AM with the ANAN set for maximum output the default AM Carrier Level setting of 100 will produce around 25 watts of carrier and your positive peaks will be allowed to reach just before the point where the DAC would clip, which will be right around 100 watts or slightly more. The PA in the ANAN has enough headroom to handle that signal and will be happy to do it all day along. It is worth noting that the algorithms that are in place in Thetis will absolutely not allow you to exceed 100% modulation in the negative direction with an AM carrier level setting of 100. It behaves exactly like a perfect "soft" negative peak limiter.

If you reduce the AM carrier level below a setting of 100 the carrier output will drop, but you are not affecting the sideband power that is produced, so the AM signal you are generating will not be any louder to stations you are talking with. Also, the further you drop the AM carrier level setting, the less that perfect negative peak limiting algorithm will exert itself to control negative peaks.

At some point, with an AM carrier level setting of less than 100, depending on how much audio you are using your signal will begin to exceed 100% in the negative direction. You will not splatter, but when observed with a scope you will see little wavelets passing through the baseline between your positive peaks. They will not be heard by someone receiving with a synchronous detector (SAM in Thetis) but if a diode detector is used, you will begin to sound distorted. If you go to extremes with a low AM carrier level setting, you will begin to sound like a quacking sideband signal to someone using a diode detector.

To directly answer your question, you won't go wrong with the default AM carrier level setting of 100, and if your signal is observed with a scope, you will easily reach 100% positive peaks with 99% negative peaks that do not hit the baseline. If you use the soft-limiting processing in the CFC tools you can attain incredible audio density and loudness that most plate modulated rigs will not be able to produce. If you want to generate a signal that "looks" like it has asymmetry I have found that an AM carrier level setting of 90 will make it look like I am modulating around 120% positive and there is enough resistance in the negative direction that I am not exceeding 100% in the negative direction. It looks like a broadcast station on a scope, but it's actually "fake" asymmetry because you are not gaining any additional power in the sidebands.

Note that if you run pure signal while running the AM mode you will find that it interprets asymmetry as distortion and it will remove it completely to hold your signal to 100% positive and 99% negative peaks. There is an added advantage to running pure signal on AM due to the noticeable improvement in THD. A major disadvantage to using pure signal with AM is that if you use the MON function in Thetis to hear your own audio you will only hear yourself in one side of your headphones and what you hear is the pre-distorted signal which is not what is heard by stations receiving you.

Hope that helps!

73, Rob W1AEX

Note: This YouTube video was done with PowerSDR mRX PS v3.4.1 but what is shown is exactly what you will see in Thetis and it illustrates the effect that the AM carrier level has on the signal that is generated.

"One thing I am certain of is that there is too much certainty in the world."
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Re: AM Carrier Adjustments (Transmit Tab)

Postby w-u-2-o » Thu Jan 26, 2023 5:50 pm

Rob--great write up! Any thoughts on the phase rotator for AM in Thetis?
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Re: AM Carrier Adjustments (Transmit Tab)

Postby W1AEX » Thu Jan 26, 2023 6:07 pm

Hi Scott,

I was very curious about how the phase rotator in the CFC tools might affect the AM mode in Thetis having seen what guys were doing to gain asymmetry with plate modulated transmitters. My experiments with it did not produce any true positive peak asymmetry on AM. It was very easy to remove asymmetry, but I could not find a solution that would exaggerate any asymmetry in my voice. I would love to see if anyone has any success producing positive asymmetry with the Thetis phase rotator in the AM mode. My guess is that it's impossible because we are not actually modulating the carrier with the ANAN, but I'll bet you could figure it out with your broadcast quality announcer's voice!

73, Rob W1AEX

The video below shows what my AM signal looks like on a panadapter with Pure Signal toggled on and off. The audio in the video is the actual ANAN signal as it was received by my RSP2Pro SDR Play using the SDRuno software. I don't think there is any other ham radio equipment that can "right out of the box" produce a better AM signal. The CFC audio tools in Thetis are very impressive!

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Re: AM Carrier Adjustments (Transmit Tab)

Postby w-u-2-o » Thu Jan 26, 2023 7:27 pm

Good points on the phase rotator, Rob. You definitely know a lot more about AM processing in Thetis than I do!
Ken
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Re: AM Carrier Adjustments (Transmit Tab)

Postby Ken » Fri Jan 27, 2023 12:12 am

W1AEX wrote:Hi Ken,

Thetis does a remarkable job with the AM mode and is capable of producing a broadcast quality signal using the CFC processing if that is what is desired. My simplest understanding of what's going on is that Thetis produces a double-sideband signal and adds the carrier in at the end. So, unlike plate modulation, you are not actually modulating the carrier.

If you are using a PA power profile that produces a 100 watt carrier in Tune mode when the TX drive slider is set to the maximum setting, when you run AM with the ANAN set for maximum output the default AM Carrier Level setting of 100 will produce around 25 watts of carrier and your positive peaks will be allowed to reach just before the point where the DAC would clip, which will be right around 100 watts or slightly more. The PA in the ANAN has enough headroom to handle that signal and will be happy to do it all day along. It is worth noting that the algorithms that are in place in Thetis will absolutely not allow you to exceed 100% modulation in the negative direction with an AM carrier level setting of 100. It behaves exactly like a perfect "soft" negative peak limiter.

If you reduce the AM carrier level below a setting of 100 the carrier output will drop, but you are not affecting the sideband power that is produced, so the AM signal you are generating will not be any louder to stations you are talking with. Also, the further you drop the AM carrier level setting, the less that perfect negative peak limiting algorithm will exert itself to control negative peaks.

At some point, with an AM carrier level setting of less than 100, depending on how much audio you are using your signal will begin to exceed 100% in the negative direction. You will not splatter, but when observed with a scope you will see little wavelets passing through the baseline between your positive peaks. They will not be heard by someone receiving with a synchronous detector (SAM in Thetis) but if a diode detector is used, you will begin to sound distorted. If you go to extremes with a low AM carrier level setting, you will begin to sound like a quacking sideband signal to someone using a diode detector.

To directly answer your question, you won't go wrong with the default AM carrier level setting of 100, and if your signal is observed with a scope, you will easily reach 100% positive peaks with 99% negative peaks that do not hit the baseline. If you use the soft-limiting processing in the CFC tools you can attain incredible audio density and loudness that most plate modulated rigs will not be able to produce. If you want to generate a signal that "looks" like it has asymmetry I have found that an AM carrier level setting of 90 will make it look like I am modulating around 120% positive and there is enough resistance in the negative direction that I am not exceeding 100% in the negative direction. It looks like a broadcast station on a scope, but it's actually "fake" asymmetry because you are not gaining any additional power in the sidebands.

Note that if you run pure signal while running the AM mode you will find that it interprets asymmetry as distortion and it will remove it completely to hold your signal to 100% positive and 99% negative peaks. There is an added advantage to running pure signal on AM due to the noticeable improvement in THD. A major disadvantage to using pure signal with AM is that if you use the MON function in Thetis to hear your own audio you will only hear yourself in one side of your headphones and what you hear is the pre-distorted signal which is not what is heard by stations receiving you.

Hope that helps!

73, Rob W1AEX

Note: This YouTube video was done with PowerSDR mRX PS v3.4.1 but what is shown is exactly what you will see in Thetis and it illustrates the effect that the AM carrier level has on the signal that is generated.



Excellent Demonstration!

I mostly leave it at 100 watts anyway but now I will leave it at a 100 with the giving information.
Very well done.

Rob, any chance of sharing any of your screen shot of your "CFC processing" ?
Thank you Rob,
Ken
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Re: AM Carrier Adjustments (Transmit Tab)

Postby w-u-2-o » Fri Jan 27, 2023 12:51 am

Ken wrote:Rob, any chance of sharing any of your screen shot of your "CFC processing"

https://community.apache-labs.com/viewtopic.php?f=6&t=2550
Ken
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Joined: Thu Feb 13, 2020 11:43 pm

Re: AM Carrier Adjustments (Transmit Tab)

Postby Ken » Sat Jan 28, 2023 11:47 pm

w-u-2-o wrote:
Ken wrote:Rob, any chance of sharing any of your screen shot of your "CFC processing"

https://community.apache-labs.com/viewtopic.php?f=6&t=2550


You are awesome Rob !

Thank you,
Ken

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