Question of the Month Answers, April 5, 2006

Richard Chong from Asia Satellite Telecommunications Co. Ltd. has submitted the March question:

Dear Harlan,
I recently came across the term "modulation loss" in the receiver portion shown in the (satellite) link budget. When I check my resources, however, I cannot find a proper definition. Here are my questions:

1. What is the definition of "modulation loss"?
2. Is there a theoretical number associated with each type of modulation scheme (or just a receiver type dependent number)?
3. Where can I locate a table summarizing the "theoretical" modulation loss versus types of modulations, if any?

From: Randolph Kallas, Kallas Consulting

1. Yes, there is a precise definition: "The amount of available signal power for a particular channel which is passed by the INPUT SYSTEM RESPONSE, relative to TOTAL RECEIVED POWER, is called the MODULATION LOSS for that particular channel (MLch)."

2. There are theoretical expressions available that enable one to calculate MLch for the channel and are dependent upon the modulation process (for example, FM or PM) that is being used. Since FM and PM are both forms of what is generally referred to as ANGLE MODULATION, the MLch, in this case, is represented by products involving J0 and J1, where: J0, J1 = Bessel functions, first kind; zero and first order, respectively. See Schwartz, Bennett and Stein, Communication Systems and Techniques, McGraw-Hill, 1966, Ch. 5, for a discussion of angle modulation (FM, PM) in general and transmission through a linear network (for example, channel) in particular.

3. One may use the Bessel function representations of #2 above, but while working on the Shuttle Program in the '70s, I acquired a copy of the Apollo Comm Link Models which contained the appropriate expression, which I still have occasion to use every now and then.

From: Amir Mohammad Kherzri, IUST

Modulation loss is the amount of decreasing in power level after modulating a wave than before it.

From: Xie Maoxu, ZTEIT

The "modulation loss" is the ratio between the output signal power of a certain modulation system and the input signal power of this system. I also hope to know the answers to parts two and three of the question.

From: David Cacciaglia, Space Ground System Solutions

Modulation loss on a satellite downlink is caused by multiple services present on the downlink. The total received signal power is apportioned among the services. My experience is with the Air Force Satellite Control Network (AFSCN) SGLS configuration. In the basic case there would be Telemetry on a sub-carrier and ranging both PM modulated on the carrier. The signal power is divided between carrier, telemetry and ranging. The modulation loss is computed for each and then subtrated from the total power to give the actual power for the service being budgeted for. The best reference is document SIS-000502 by AFSCN.

From: Gopalan Sampath, DoD Australia

1. Modulation loss could be the conversion loss in the mixer process. Since the term appears in the satellite link budget it could also be the front-end converter loss if the budget includes the term IF-signal to noise.

2. As such there is no loss in any modulation process except in wideband FM systems as is used in satellite links.

3.The FM detection is characterized by an "FM advantage" factor used to arrive at the baseband signal to noise.

From: Tharaka Perera, Millennium Information Technologies

1. The reason for the modulation loss is the ever-existing imperfections of the modulators that we use. As we all know, we cannot guarantee that our local oscillator will provide a perfect pure tone all of the time. Certainly not in real world applications. Hence, apart from the desired local oscillator signal there can be other harmonics present with that. When these are multiplied with the incoming message signal, it will result in unwanted harmonics in the output containing finite amounts of power when compared with the desired output power. This unwanted power distribution variation is what we call the modulation loss.

2. Yes. Theoretically, there are infinite amounts of unwanted harmonics that can be present in the output but only the third harmonic contains more power. Then the fifth harmonic, seventh harmonic and so on.

3. I have not come across such a table.