Measurement Reading; Indroductions To Spectrum Analysis; Types Of Spectrum Analyzers - Hameg Instruments Hm5012-2 Handbuch

Measuring in full-span mode serves mostly as a quick
overview. To analyze the detected signals more closely, the
span has to be decreased. Before decreasing the span, make
sure that the center frequency is set so the signal is at exact
center of screen. Then span can be reduced.
Then the resolution bandwidth can be decreased, and the
video filter used if necessary. Note that if the warning „uncal"
is displayed in the readout, measurement results are incorrect.

Measurement reading:

For a numerical value of a measurement result the easiest
way is by the use of the marker. The marker frequency, and
hence the marker symbol position, can be set by the TUNING
knob (on condition the MARKER LED is lit) on a spectrum line.
Then the frequency and the level can be read from the readout.
For the level value the reference level (REF.-LEVEL) and the
input attenuator setting (ATTN) are automatically considered.
If a value is to be measured without using the marker, then
measure the difference of the reference line to the signal.
Note that the scale may be either 5 dB/Div. or 10 dB/Div. In the
reference level value, the setting of the input attenuator is
already included; it is not necessary to make a correction
afterwards. The level of the 48MHz test signal (shown on
the page „Test Signal Display") is approx. 2.2 div below the
reference level graticule line of –10dBm. In combination with
a scaling of 10dB/div, 2.2div equals 22dB and consequently
the signal level is –10dBm – (22dB) = -32dBm.
Introduction to Spectrum Analysis
The analysis of electrical signals is a fundamental problem for
many engineers and scientists. Even if the immediate problem
is not electrical, the basic parameters of interest are often
changed into electrical signals by means of transducers. The
rewards for transforming physical parameters to electrical
signals are great, as many instruments are available for the
analysis of electrical signals in the time and frequency domains.
The traditional way of observing electrical signals is to view
them in the time domain using an oscilloscope. The time
domain is used to recover relative timing and phase
information that is needed to characterize electric circuit
behavior. However, not all circuits can be uniquely
characterized from just time domain information. Circuit
elements such as amplifiers, oscillators, mixers, modulators,
detectors and filters are best characterized by their frequency
response information. This frequency information is best
obtained by viewing electrical signals in the frequency domain.
To display the frequency domain requires a device that can
discriminate between frequencies while measuring the power
level at each. One instrument which displays the frequency
domain is the spectrum analyzer.
It graphically displays voltage or power as a function of
frequency on a CRT (cathode ray tube). In the time domain, all
frequency components of a signal are seen summed together.
In the frequency domain, complex signals (i.e. signals
composed of more than one frequency) are separated into
their frequency components, and the power level at each
frequency is displayed. The frequency domain is a graphical
representation of signal amplitude as a function of frequency.
The frequency domain contains information not found in the
time domain and therefore, the spectrum analyzer has certain
advantages compared with an oscilloscope.
The analyzer is more sensitive to low level distortion than a
scope. Sine waves may look good in the time domain, but in
Subject to change without notice
Introduction to Spectrum Analysis
the frequency domain, harmonic distortion can be seen. The
sensitivity and wide dynamic range of the spectrum analyzer
is useful for measuring low-level modulation. It can be used
to measure AM, FM and pulsed RF. The analyzer can be used
to measure carrier frequency, modulation frequency,
modulation level, and modulation distortion. Frequency
conversion devices can be easily characterized. Such
parameters as conversion loss, isolation, and distortion are
readily determined from the display.
The spectrum analyzer can be used to measure long and short
term stability. Parameters such as noise sidebands on an
oscillator, residual FM of a source and frequency drift during
warm-up can be measured using the spectrum analyzer's
calibrated scans. The swept frequency responses of a filter or
amplifier are examples of swept frequency measurements
possible with a spectrum analyzer. These measurements are
simplified by using a tracking generator.

Types of Spectrum Analyzers

There are two basic types of spectrum analyzers, swept-tuned
and real time analyzers. The swept-tuned analyzers are tuned by
electrically sweeping them over their frequency range. Therefore,
the frequency components of a spectrum are sampled
sequentially in time. This enables periodic and random signals
to be displayed, but makes it impossible to display transient
responses. Real time analyzers, on the other hand, simultaneously
display the amplitude of all signals in the frequency range of the
analyzer; hence the name real-time. This preserves the time
dependency between signals which permit phase information
to be displayed. Real time analyzers are capable of displaying
transient responses as well as periodic and random signals.
The swept tuned analyzers are usually of the trf (tuned radio
frequency) or super heterodyne type. A trf analyzer consists
of a band pass filter whose center frequency is tunable over a
desired frequency range, a detector to produce vertical
deflection on a CRT, and a horizontal scan generator used to
synchronize the tuned frequency to the CRT horizontal
deflection. It is a simple, inexpensive analyzer with wide
frequency coverage, but lacks resolution and sensitivity.
Because trf analyzers have a swept filter they are limited in
sweep width depending on the frequency range (usually one
decade or less). The resolution is determined by the filter
bandwidth, and since tunable filters do not usually have
constant bandwidth, it is dependent on frequency.
The most common type of spectrum analyzer differs from the
trf spectrum analyzers in that the spectrum is swept through
a fixed band pass filter instead of sweeping the filter through
the spectrum.
The analyzer is basically a narrowband receiver which is
electronically tuned in frequency by a local oscillator (1
The LO signal is the first of two inputs applied to the first
mixer. The complete input spectra (the analyzer input) is the
second signal for the first mixer. A front panel controllable
attenuator (adjacent to the input socket) can be used to reduce
the input signal level in 10dB steps. At the first mixer output,
the following four signals appear:
1. The signal of the first local oscillator (1st LO).
This is always 1350.7MHz higher then the input signal
frequency. For an input frequency of 0kHz the 1st LO is
set to 1350.7MHZ (0kHz + 1350.7MHz). At 150kHz it is
1350.85MHz (150kHz + 1350.7MHZ) and for an input
signal of 1050MHz the 1st LO must oscillate at 2400.7MHz
(1050MHz + 1350.7MHz).
st LO).
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