33
Subject to change without notice
Spectrum Analyzer Requirements
1350.85 MHz (150 kHz + 1350.7 MHZ) and for an input signal of
1050 MHz the 1st LO must oscillate at 2400.7 MHz (1050 MHz
+ 1350.7 MHz).
b) The complete input spectra as present at the analyzer input.
After having passed through the attenuator, this is also pre-
sent at the mixer output.
c) The mixing product sum of the 1st LO and the complete in-
put spectra. For 150 kHz the 1st LO frequency is 1350.85 MHz
which results in a sum of 1351 MHz. In case of 1050 MHz input
frequency the 1st LO frequency is 2400.7 MHz and the sum
is 3450.7 MHz.
d) The mixing product difference of the 1st LO and the complete
input spectra. At 150 kHz the 1st LO frequency is 1350.85 MHz
so that the difference (1350.85 MHz – 150 kHz) is 1350.7 MHz.
Tuned to 1050 MHz the 1st LO frequency is 2400.7 MHz and
the difference is 1350.7 MHz (2400.7 MHz – 1050 MHz).
After the mixing stage these signals enter a band pass fi lter
(IF fi lter) with a center frequency of 1350.7 MHz. Except for one
special condition, only the mixing product difference can pass
the fi lter and is displayed after further processing. The excep-
tion is the 1
st
LO signal which is 1350.7 MHz if the analyzer is
tuned to 0 kHz.
Note:
This 1st LO signal at „0 kHz“ is named Zero Peak, or local oscil-
lator feedthrough and is unavoidable. It can be seen at the left
of the display. Its presence can be disturbing on frequencies
between 150 kHz and approx. 2.5 MHz if e.g. 1 MHz resolution
bandwidth (RBW) is selected. To avoid such problems a lower
resolution bandwidth should be selected.
Depending on whether measurements are made with or without
SPAN, the following conditions occur.
In ZERO SPAN mode the 1
st
LO generates a frequency that
must be 1350.7 MHz higher than the selected input frequency.
The analyzer then displays only the input frequency and those
frequency fractions that can pass the IF fi lter, depending on the
actual resolution bandwidth (RBW) setting.
In nor mal frequency span conditions (ZERO SPAN not selected),
a frequency range is displayed dependent on the SPAN setting.
In the condition that the center frequency is 500 MHz and a span
of 1000 MHz (full span) is chosen, the measurement starts with
0 kHz at the left side of the display and ends with 1000 MHz at
the right side. This means that the 1
st
LO frequency is increased
repeatedly from 1350.7 MHz to 2400.7 MHz. After each sweep is
performed, a new one starts.
There is a relationship between the frequency range to be ana-
lyzed (SPAN setting dependent) and the resolution bandwidth
that can cause the display of erroneous (too low) signal levels.
Such errors occur if the measuring time does not meet the re-
quirements of the IF and/or Video Filter settling time, which is
the case if the measuring time is too short. A warning of this
state is indicated by the readout displaying „uncal“.
Spectrum Analyzer Requirements
To accurately display the frequency and amplitude of a signal
on a spectrum analyzer, the instrument itself must be properly
adjusted. A spectrum analyzer properly designed for accurate
frequency and amplitude measurements has to satisfy many
requirements:
a) Wide tuning range
b) Wide frequency display range
c) Stability
d) Resolution
e) Flat frequency response
f) High sensitivity
g) Low internal distortion
Frequency Measurements
A Spectrum Analyzer allows frequency measurement whether
SPAN mode is present or not (ZERO-SPAN).
In „full span“ (1000MHz) mode, the complete frequency range
is displayed and a signal frequency can roughly be determined.
This frequency then can be input as center frequency and dis-
played with less SPAN. The measurement display and MAR-
KER accuracy increases with less SPAN and smaller resolution
bandwidth (RBW). In combination with „ZERO SPAN“, a signal
which is not modulated is displayed as a straight horizontal line.
To determine the signal frequency, the center frequency should
be adjusted so that the signal line moves up the screen to the
maximum top position (maximum level). Then the frequency can
be read from the readout. In the zero scan mode, the analyzer
acts as a fi xed tuned receiver with selectable bandwidths.
Relative frequency measurements can be made by measuring
the relative separation of two signals on the display. It is impor-
tant that the spectrum analyzer be more stable than the signals
being measured. The stability of the analyzer depends on the
frequency stability of its local oscillators. Stability is usually
characterized as either short term or long term. Residual FM
is a measure of the short term stability that is usually specifi ed
in Hz peak-to-peak. Short term stability is also characterized
by noise sidebands which are a measure of the analyzers spec-
tral purity.
Noise sidebands are specifi ed in terms of dB down and Hz away
from a carrier in a specifi c bandwidth. The frequency drift of the
analyzer’s Local Oscillators characterizes long term stability.
Frequency drift is a measure of how much the frequency chan-
ges during a specifi ed time (i.e., Hz/min. or Hz/hr).
Resolution
Before the frequency of a signal can be measured on a spectrum
analyzer it must fi rst be resolved. Resolving a signal means di-
stinguishing it from its nearest neighbours. The resolution of
a spectrum analyzer is determined by its IF bandwidth. The IF
bandwidth is usually the 3 dB bandwidth of the IF fi lter. The ra-
tio of the 60 dB bandwidth (in Hz) to the 3 dB bandwidth (in Hz) is
known as the shape factor of the fi lter. The smaller the shape
factor, the greater the analyzer’s capability to resolve closely
spaced signals of unequal amplitude. If the shape factor of a fi lter
is 15:1, then two signals whose amplitudes differ by 60dB must
differ in frequency by 7.5 times the IF bandwidth before they can
be distinguished separately. Otherwise, they will appear as one
signal on the spectrum analyzer display.
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