Anritsu S113C Guide de l'utilisateur télécharger pdf (Page 76)

The markers read the frequency difference between the two signals, which is equal to the

modulating frequency. The marker also reads the difference in amplitude. This difference

in amplitude between the two signals can be used to determine the percentage of modula

tion. Use the following formula to calculate the percentage of modulation:

dB E

() () log-=20

Where E

(dB) is the energy in the sidebands in dB, E

(dB) is the energy in the carrier in

dB, and m is the degree of modulation (multiply by 100 to get the percentage).

NOTE: Unequal amplitudes of the lower and upper sidebands indicate inciden-

tal FM on the input signal. Incidental FM can reduce the accuracy of percent-

age-of-modulation measurements.

Frequency Modulation

This section contains general information about frequency modulation, and an example of

using the Site Master to display a FM signal.

Frequency modulations are generated when a modulating signal, fmod, causes an instanta-

neous frequency deviation of the modulated carrier. The peak frequency deviation, Dfpeak,

is proportional to the instantaneous amplitude of fmod, and the rate of deviation is propor-

tional to the frequency of the fmod.

The FM index,

, is defined as:

D fpeak

f mod

In general, the spectrum analyzer is a very useful tool for measuring Dfpeak and

adjust-

ments of FM transmitters. FM is composed of an infinite number of sidebands. In practice,

the spectrum of a FM signal is not infinite. The side band amplitudes become negligibly

small beyond a certain frequency offset for the carrier, depending on the value of

To demonstrate the properties of an FM signal, we will use a carrier frequency of 100 MHz

and test for FM deviation accuracy at 25 kHz. With a

value of 2, we need a 50 kHz modu-

lating signal.

Step 1. Connect the FM signal source to the input of the Site Master.

Step 2. Set the reference level to 0 dBm.

Step 3. Set the attenuation coupling to AUTO.

Step 4. On the signal source, set the carrier frequency to 100 MHz, modulating fre-

quency to 50 kHz, carrier power to –10 dBm, and deviation to 25 kHz.

Step 5. Set the center frequency of the Site Master to 100 MHz, span to 500 kHz, reso-

lution bandwidth to 10 kHz, and video bandwidth to 1 kHz.

You will see a carrier signal at 100 MHz and 3 sidebands on each side (Figure 5-2). The

side bands are separated by 50 kHz (shown by the value of DM2), which is the fmod.

5-4

Chapter 5 Spectrum Analyzer Measurements

SgLabs

www.sglabs.it

email: [email protected]

tel. +39 0755149360

SgLabs

www.sglabs.it

email: [email protected]

tel. +39 0755149360

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Anritsu S113C Guide de l'utilisateur Page 76

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