HP 8901B Operation And Calibration Manual page 26

General Information Model 8901B
The frequency of the audio signal at the MODULATION OUTPUT/AUDIO INPUT connector,
whether internal or external, is measured by a reciprocal-type Audio Counter. In the Audio Counter,
the input signal is used to gate the 10 MHz Time Base Reference into the main Counter. (This gating
function is also used by the Voltage-to-Time Converter.) The number of time base pulses received
during the count is read by the Controller which computes and displays the signal frequency.
The AM and FM Calibrators provide a nominal 10.1 MHz signal with a precisely known amount of
AM or FM. When this signal is applied to the instrument's RF INPUT connector (either directly
or via the Sensor Module), the modulation is measured and the calibration factor of the AM or FM
Demodulator is computed and displayed. Related front-panel functions are automatically set for proper
demodulation of the calibrator signal.
P
(a) BASEBAND SIGNAL
1.5
1
.o
0.5
0
(b)
AMPLITUDE MODULATED CARRIER
I
Figure 1-3. A Baseband Signal and the Corresponding Amplitude Modulated Carrier
1-1 1. MODULATION BASICS
The Modulation Analyzer can demodulate and measure three types of modulation: amplitude
modulation (AM), frequency modulation (FM), and phase modulation (@M). In general, modulation is
that
characteristic of a signal which conveys the information.
A signal without modulation is said to be
a
continuous-wave (CW) signal. CW signals contain two information-carrying parameters: amplitude and
frequency. These two parameters, however, are static (time invariant). Consequently, the information
conveyed by them is scant-you know only that a signal is present at a certain frequency. When one
or both of these parameters is altered as a function of time, the signal is said to be modulated.
The RF signal which is modulated is called the carrier. The modulating signal is referred to as the
baseband signal and can be of any arbitrary form (for example, voice, tone, noise). Demodulation is
the process of recovering the baseband signal from the modulated carrier. The Modulation Analyzer
can measure the modulation on carriers in the range of 150 kHz to 1300 MHz. Measurement accuracy
is specified
for modulation rates generally between 20 Hz and 100 kHz. The demodulated signal is
present at the MODULATION OUTPUT connector.
Amplitude Modulation
As the name implies,
a
carrier is amplitude modulated when its amplitude is varied as a function of
time. Figure 1-3 shows a carrier with amplitude modulation and, for reference, also shows the baseband
signal. As you can see, the tips of the carrier trace out a waveform that resembles the baseband signal.
This trace is called the envelope. The envelope rises to a maximum called the peak and drops to a
minimum called the trough. A quantity which describes the amount of AM or the AM depth is the
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