AM Power Measurement in Software

Making ANSI/AIM Code 128 in Software AM Power Measurement

The power of an AM signal can be measured as the peak envelope power (PEP), which is utilized to gauge the average peak power, with 100% modulation applied, of the transmitted signal, or: PEP = V2RMS/R or VRMS IRMS or I2RMS R

The carrier power can also be calculated with these same formulas, but with zero transmitter modulation applied

by increasing its power is not linear To extend a transmitter s distance by two, multiply the transmitter s power, in watts, by 22 (or 4 power) To increase the distance by three, multiply the power by 32, (or 9 power)

The disadvantages of AM are many: The bandwidth of an AM signal is twice what is required for the reception of the intelligence being sent, since only one sideband is absolutely necessary to convey the baseb and information; a significant amount of power is in the carrier, which is not even required to furnish the intelligence; the phase relationship between the carrier and the sidebands must be precise, or severe fading will result within the demodulated signal (which is quite difficult to maintain under most atmospheric and multipath conditions)

Frequency modulation was originally invented as an answer to the many deficiencies inherent in AM, which is fundamentally that of excessive noise sensitivity Since noise is normally produced by undesired amplitude variations in a signal, this can easily be removed in frequency-modulated receivers by amplitude limiters Two techniques can be employed to generate an FM signal The first, which directly alters the frequency of the carrier in step with the baseband s amplitude variations, is called direct FM; and the second method, indirect FM, changes the phase of the carrier, which creates phase modulation However, both of these techniques produce the end effect of frequency modulation of the RF carrier Both methods are classified under the designation of angle modulation

Modulation is the method we use to insert baseband information on an RF carrier wave The baseband information can be voice, digital data, analog video, and so on Demodulation is the procedure of extracting this baseband information, which is then

sent to a speaker to reproduce the original voice and music, or on to digital circuits for processing or storage FM accomplishes this modulation process by altering the carrier s frequency in step with the baseband signal s changes in amplitude When this frequency-modulated RF carrier arrives at the receiver, the frequency variations as created by the original baseband modulations are changed back into amplitude variations This baseband is then amplified and inserted into an appropriate transducer As stated, in FM the baseband s amplitude alters the frequency of the RF carrier, and not the amplitude as it does with AM, while the amount of the actual frequency deviation of the FM carrier is dependant on the increase or decrease in this baseband s amplitude Frequency deviation is considered to be the amount the RF carrier deviates from its center frequency in one direction during modulation Without any baseband modulation present, however, the frequency of the RF carrier will stay at the transmitter s predetermined center frequency, which is the frequency of the master oscillator after any multiplication Thus, as the baseband modulation occurs, the carrier will increase and decrease in frequency; as the baseband swings positive in amplitude, the carrier will increase in frequency, but as the baseband modulation swings negative in amplitude, the frequency of the carrier will fall below its rest frequency (Fig 211) The frequency of the baseband signal will change the rate that the frequencymodulated RF carrier intersects its own rest frequency, and will vary at this same baseband rate As an example, if a baseband audio tone is inserted at 2 kHz, the FM carrier will actually swing past its own rest frequency 2000 times in 1 second Unlike AM modulation, the percent of modulation for FM is directed by government rules and regulations, and not by any natural limitations For instance, narrowband voice communications is considered to be 5-kHz deviation for 100% frequency modulation, while with wideband FM broadcast, a maximum allowed deviation is 75 kHz But if the baseband signal s amplitude should induce the FM deviation to go above this 100% limit, then more frequency sidebands will be created, which broadens the bandwidth conceivably causing interference to any adjacent channels