Spectrum Analyzer Requirements; Frequency Measurements; Resolution - Hameg Instruments Hm5012-2 Handbuch

Introduction to Spectrum Analysis
2. The complete input spectra as present at the analyzer
input.
After having passed through the attenuator, this is also
present at the mixer output.
3. The mixing product sum of the 1st LO and the complete
input spectra.
For 150kHz the 1st LO frequency is 1350.85MHz which
results in a sum of 1351MHz. In case of 1050MHz input
frequency the 1st LO frequency is 2400.7MHz and the
sum is 3450.7MHz.
4. The mixing product difference of the 1st LO and the
complete input spectra.
At 150kHz the 1st LO frequency is 1350.85MHz so that
the difference (1350.85 MHz – 150kHz) is 1350.7MHz.
Tuned to 1050MHz the 1st LO frequency is 2400.7MHz
and the difference is 1350.7MHz (2400.7MHz – 1050MHz).
After the mixing stage these signals enter a band pass filter
(IF filter) with a center frequency of 1350.7MHz. Except for
one special condition, only the mixing product difference can
pass the filter and is displayed after further processing. The
st
exception is the 1 LO signal which is 1350.7MHz if the
analyzer is tuned to 0kHz.
Note: This 1st LO signal at „0kHz" is named Zero Peak, or
local oscillator feedthrough and is unavoidable. It can be
seen at the left of the display. Its presence can be disturbing
on frequencies between 150kHz and approx. 2.5MHz if e.g.
1MHz 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
must be 1350.7MHz 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
500MHz and a span of 1000MHz (full span) is chosen, the
measurement starts with 0kHz at the left side of the display
and ends with 1000MHz at the right side. This means that
the 1 st LO frequency is increased repeatedly from
1350.7MHz to 2400.7MHz. 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:
40
st LO generates a frequency that
1. Wide tuning range
2. Wide frequency display range
3. Stability

4. Resolution

5. Flat frequency response
6. High sensitivity
7. 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.
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 3dB bandwidth of
the IF filter. The ratio of the 60dB bandwidth (in Hz) to the 3dB
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
Subject to change without notice