GeographyFlyer
Topic Author
Posts: 6
Joined: Tue Dec 19, 2017 6:59 pm

Amplitude Modulation (B777)

Sun Feb 10, 2019 4:16 pm

Hello everyone,

When the AM (amplitude modulation) is used, what effect does it have and what is the difference between USB and AM? (everything related to 777)


Every answer is appreciated, GeographyFlyer.
 
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Francoflier
Posts: 4634
Joined: Wed Oct 31, 2001 12:27 pm

Re: Amplitude Modulation (B777)

Sun Feb 10, 2019 5:02 pm

https://www.pprune.org/archive/index.php/t-10746.html

These HF radio settings have nothing to do with the 777. All airplanes use the same system.
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
GalaxyFlyer
Posts: 2246
Joined: Fri Jan 01, 2016 4:44 am

Re: Amplitude Modulation (B777)

Sun Feb 10, 2019 8:15 pm

If you select AM, reception of a USB transmission is unreadable and vice versa. It’ll sound vaguely recognizable but unreadable.


GF
 
Caryjack
Posts: 395
Joined: Tue May 08, 2007 9:45 am

Re: Amplitude Modulation (B777)

Tue Feb 12, 2019 11:14 am

Francoflier wrote:
https://www.pprune.org/archive/index.php/t-10746.html


This from the link:
Cat1234
30th Dec 2001, 14:02
"In normal Amplitude Modulation (AM) signal generation a carrier and two side bands are produced. The side bands are designated Upper Side Band (USB) and Lower Side Band (LSB). It is actually not necessary to transmit all these signals to provide the receiver with enough information to reconstruct the original modulation, audio in this case. The carrier may be removed or attenuated, and so can one of the two sidebands. The resulting signals will require less transmitted power and will occupy less bandwidth.
USB is the norm for aeronautical HF communications, AM is used when a carrier is required such as while being checked for maintenance and as far as I am aware LSB is unused by acft."

A 1 Kw AM carrier requires 500 watts of audio to produce 100% modulation. The transmitted signal is made of the carrier plus the USB and the LSB which are spaced from the carrier by the audio frequency. Both side bands contain the same information. Commercial equipment (maritime, airline, etc.) save much energy by transmitting just 1 side-band. Receivers select a BFO frequency (beat frequency oscillator) as a reference carrier for the side-band.

The HF band is too low for Frequency Modulation (FM). An FM carrier contains all the audio (or digital) information and requires no extra modulator power for 100% modulation, plus it has other advantages.
 
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akoma
Posts: 15
Joined: Sun Jun 18, 2017 12:51 pm

Re: Amplitude Modulation (B777)

Thu Feb 14, 2019 12:25 pm

Caryjack wrote:
Francoflier wrote:
https://www.pprune.org/archive/index.php/t-10746.html


This from the link:
Cat1234
30th Dec 2001, 14:02

The HF band is too low for Frequency Modulation (FM). An FM carrier contains all the audio (or digital) information and requires no extra modulator power for 100% modulation, plus it has other advantages.


Not strictly true, as FM can be used on any frequency, including HF & lower. There are other reasons why it is not used for aviation HF (and "normal" aviation VHF) though.
 
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akoma
Posts: 15
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Re: Amplitude Modulation (B777)

Thu Feb 14, 2019 12:32 pm

GalaxyFlyer wrote:
If you select AM, reception of a USB transmission is unreadable and vice versa. It’ll sound vaguely recognizable but unreadable.


GF


If you select AM on your receiver, a USB transmission will be almost unreadable. If you select USB and there is a AM transmission on frequency, the AM transmission will be perfectly readable.
 
Caryjack
Posts: 395
Joined: Tue May 08, 2007 9:45 am

Re: Amplitude Modulation (B777)

Sat Feb 16, 2019 4:06 am

akoma wrote:
Caryjack wrote:
Francoflier wrote:
https://www.pprune.org/archive/index.php/t-10746.html


This from the link:
Cat1234
30th Dec 2001, 14:02

The HF band is too low for Frequency Modulation (FM). An FM carrier contains all the audio (or digital) information and requires no extra modulator power for 100% modulation, plus it has other advantages.


Not strictly true, as FM can be used on any frequency, including HF & lower. There are other reasons why it is not used for aviation HF (and "normal" aviation VHF) though.

FM is not used on HF and below, otherwise known as shortwave (3 to 30 Mhz). Shifting the carrier in that band (modulation) sufficiently to add usable information requires the carrier to exceed the resonance of the transmitter, receiver and the antenna.

What is "normal" aviation VHF? Is it AM, FM, PM, SSB or some combination? What are the other reasons FM is not used for aviation?

I'd like to know the make and model of an FM HF transceiver. Do you have one?
Thanks,
Cary
 
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akoma
Posts: 15
Joined: Sun Jun 18, 2017 12:51 pm

Re: Amplitude Modulation (B777)

Mon Feb 18, 2019 6:35 am

Caryjack wrote:

"FM is not used on HF and below, otherwise known as shortwave (3 to 30 Mhz). Shifting the carrier in that band (modulation) sufficiently to add usable information requires the carrier to exceed the resonance of the transmitter, receiver and the antenna"

FM does not have to be used only above 30 MHz. There are services which use FM transmissions in the HF band, like transmissions using FSK (which is a form of FM), SSTV, Wefax - to name a few.

Some countries have 27 MHz CB Citizens Band operations using FM.

I have been in FM parties on the quieter parts of the 20m band (14 MHz) where communications were carried out over distances of thousands of km.

FM for communications purposes with a modulation index below 1.0 can be made to swing less than 3 kHz on the fundamental frequency - less bandwidth than a typical AM broadcast with 2 sidebands. Such a small frequency swing does not affect the resonance of a transmitter's tank circuit - if it does, AM transmissions would not be possible, as AM sidebands exceed the FM frequency swing.

Antenna resonance is generally pretty broad - even the highly tuned and matched HF antenna on a commercial aircraft, for example, will have a broadly flat resonance curve of tens of kHz either side of the tuned frequency (not saying they use FM though).

Resonance in the tuned circuits of any receiver will be designed such that it can pass or reject whatever signals are required or not required.

"What is "normal" aviation VHF? Is it AM, FM, PM, SSB or some combination? What are the other reasons FM is not used for aviation?"

"Normal" aviation VHF (I should also add in UHF, but I did not want to be too confusing) is purely AM mode.

"I'd like to know the make and model of an FM HF transceiver. Do you have one?"

I have had access to several HF transceivers which had FM as one of the modes, besides having LSB, USB and AM. A couple of the older basic models which I remember fondly were the Icom 725 and Yaesu 747.

I have also modified old CW transmitters by adding a simple low-level audio amplifier fed to a varicap/varactor attached to the VFO - an instant HF FM transmitter.

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