Indroductions To Spectrum Analysis; Types Of Spectrum Analyzers - Hameg HM5012-2 Manual

Introduction to Spectrum Analysis
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 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
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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
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:
a) The signal of the first local oscillator (1st LO).
This is always 1350.7 MHz higher then the input signal
frequency. For an input frequency of 0kHz the 1st LO is set
to 1350.7 MHZ (0 kHz + 1350.7 MHz). At 150 kHz it is
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 present
at the mixer output.
c) The mixing product sum of the 1st LO and the complete input
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 filter (IF filter)
with a center frequency of 1350.7 MHz. Except for one special
condition, only the mixing product difference can pass the filter and
is displayed after further processing. The exception is the 1
signal which is 1350.7 MHz if the analyzer is tuned to 0 kHz.
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
st
LO). The LO
st
LO
Subject to change without notice