Spectrum Analyzer Requirements; Frequency Measurements; Resolution; Sensitivity - Hameg hm5012-2 Handbuch

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
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 filter, depending on the
actual resolution bandwidth (RBW) setting.
In normal 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
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
analyzed (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
requirements 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
displayed with less SPAN. The measurement display and MARKER
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 fixed tuned
receiver with selectable bandwidths.
Relative frequency measurements can be made by measuring
the relative separation of two signals on the display.
Subject to change without notice
st LO generates a frequency that must
st LO frequency is increased
It is important 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 specified in Hz
peak-to-peak. Short term stability is also characterized by noise
sidebands which are a measure of the analyzers spectral purity.
Noise sidebands are specified in terms of dB down and Hz away
from a carrier in a specific bandwidth. The frequency drift of the
analyzer's Local Oscillators characterizes long term stability.
Frequency drift is a measure of how much the frequency changes
during a specified time (i.e., Hz/min. or Hz/hr).

Resolution

Before the frequency of a signal can be measured on a spectrum
analyzer it must first be resolved. Resolving a signal means
distinguishing 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 filter. The ratio
of the 60 dB bandwidth (in Hz) to the 3 dB bandwidth (in Hz) is
known as the shape factor of the filter. 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 filter
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.
The ability of a spectrum analyzer to resolve closely spaced signals
of unequal amplitude is not a function of the IF filter shape factor
only. Noise sidebands can also reduce the resolution. They appear
above the skirt of the IF filter and reduce the off band rejection of
the filter. This limits the resolution when measuring signals of
unequal amplitude.
The resolution of the spectrum analyzer is limited by its narrowest
IF bandwidth. For example, if the narrowest bandwidth is 9kHz
then the nearest any two signals can be and still be resolved is
9kHz. This is because the analyzer traces out its own IF band
pass shape as it sweeps through a CW signal. Since the resolution
of the analyzer is limited by bandwidth, it seems that by reducing
the IF bandwidth indefinitely, infinite resolution will be achieved.
The fallacy here is that the usable IF bandwidth is limited by the
stability (residual FM) of the analyzer. If the internal frequency
deviation of the analyzer is 9kHz, then the narrowest bandwidth
that can be used to distinguish a single input signal is 10kHz.
Any narrower IF-filter will result in more than one response or an
intermittent response for a single input frequency. A practical
limitation exists on the IF bandwidth as well, since narrow filters
have long time constants and would require excessive scan time.

Sensitivity

Sensitivity is a measure of the analyzer's ability to detect small
signals. The maximum sensitivity of an analyzer is limited by its
internally generated noise. This noise is basically of two types:
Thermal (or Johnson) and non thermal noise. Thermal noise power
can be expressed as: PN = k x T x B
where: PN = Noise power in watts
k = Boltzmanns Constant (1.38 x ?10-23 Joule/K)
T = absolute temperature, K
B = bandwidth of system in Hertz
Introduction to Spectrum Analysis
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