T 7.2.1.3 Amplitude Modulation

TPS 7.2.1.3
Measuring instruments
Fig. 2.1-4:The time law of electrical telecommunications engineering
(A): Input signals: pulses with equal amplitude A and period T
(B): Lowpasses with critical frequencies f c1
and f c2
; f c1
< f c2
(C): Output signals: pulse of varying period and amplitude
If, for example, a spectrum analysis has to be performed
over a wide frequency domain, and, in
addition, a very short sawtooth period is selected,
then the result of this is a very large change in frequency
per unit time. The mixer output signal
passes through the mid-frequencies of the BPF
with corresponding speed. According to the time
law the selected bandwidth of the BPF now has to
be “sufficiently” large if the BPF is to attain the
input amplitude. However, at greater bandwidth
of the bandpass filter the analyzer's spectral resolution
capacity drops. For that reason, work with
the spectrum analyzer always involves the compromise
between spectral RESOLUTION and
fault-free reproduction of the amplitude. For the
relationship between the SCAN TIME T, bandwidth
b and frequency window SPAN the following
approximately applies:
b = 20 ( f − f )
T
max min (2.1)
Where:
f max : maximum frequency
f min : minimum frequency
b : bandwidth of the filter
T : sawtooth period SCAN TIME.
The difference f max – f min is called frequency
window = SPAN.
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