PRBS
A Pseudo-Random Bit Sequence (PRBS) has two levels (high and low), and it switches between them in a manner
that is difficult to predict without knowing the sequence generation algorithm. A PRBS is generated by a linear-
feedback shift register (LFSR), shown below.
An LFSR is specified by the number of stages it contains and which stages ("taps") feed the exclusive-or (XOR) gates
in its feedback network. The PRBS output is taken from the last stage. With properly chosen taps, an L-stage LFSR
produces a repetitive PRBS of length 2
L
- 1. The clocking frequency of the LFSR determines the "bit rate" of the
PRBS.
You can set L to 7, 9, 11, 15, 20, or 23, resulting in sequences from 127 to 8,388,607 bits in length.
The default value for L is 7, resulting in a sequence of 127 bits in length.
Modulation
Amplitude Modulation (AM)
The instrument implements two forms of AM:
– Double-sideband full-carrier (DSB-FC), which has an ITU designation of A3E and is used in AM broadcasting.
The equation for DSB-FC is
y(t)= [(½)+(½)•d•m(t)]•A
c
•sin(ω
c
t)
where
m(t) is the modulating signal
A
c
is the carrier amplitude
ω
c
is the carrier frequency of the carrier
d is the "modulation depth," or fraction of the amplitude range is used by the modulation
For example, a depth setting of 80% varies the amplitude from 10% to 90% of the amplitude setting (90% - 10% =
80%) with either an internal or a full-scale (±5 V) external modulating signal. You may set depth as high as 120%, as
long as you do not exceed the instrument’s maximum output voltage of (±5 V into 50 Ω, ±10 V into high impedance).
The top trace below represents the modulating signal; the bottom trace represents the modulated carrier.
156
Keysight EDU33210 Series User's Guide
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