Indroductions To Spectrum Analysis; Types Of Spectrum Analyzers - Hameg HM5014-2 Manual

I n t r o d u c t i o n t o S p e c t r u m A n a l y s i s
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
fi lter used if necessary. Note that if the warning „uncal" is dis-
played in the readout, measurement results are incorrect.
Measurement reading: For a numerical value of a measure-
ment 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 au-
tomatically 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 refe-
rence level value, the setting of the input attenuator is already
included; it is not necessary to make a correction afterwards.
The level of the 48 MHz test signal (shown on the page „Test Si-
gnal Display") is approx. 2.2 div below the reference level grati-
cule line of –10 dBm. In combination with a scaling of 10 dB/div,
2.2div equals 22 dB and consequently the signal level is –10 dBm
– (22 dB) = –32 dBm.
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 elec-
trical 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 amplifi ers, oscillators,
mixers, modulators, detectors and fi lters are best characte-
rized 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 measu-
ring 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 fre-
quency domain, complex signals (i.e. signals composed of more
than one frequency) are separated into their frequency compo-
nents, 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 ti-
me domain and therefore, the spectrum analyzer has certain
advantages compared with an oscilloscope.
32
Subject to change without notice
The analyzer is more sensitive to low level distortion than a sco-
pe. Sine waves may look good in the time domain, but in the fre-
quency 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 modu-
lation 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 oscil-
lator, 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 fi lter or amplifi er
are examples of swept frequency measurements possible with
a spectrum analyzer. These measurements are simplifi ed 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. There-
fore, 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 re-
sponses. 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 fi lter whose center frequency is tunable over a desi-
red frequency range, a detector to produce vertical defl ection on
a CRT, and a horizontal scan generator used to synchronize the
tuned frequency to the CRT horizontal defl ection. It is a simple,
inexpensive analyzer with wide frequency coverage, but lacks
resolution and sensitivity. Because trf analyzers have a swept
fi lter they are limited in sweep width depending on the frequency
range (usually one decade or less). The resolution is determined
by the fi lter bandwidth, and since tunable fi lters 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
fi xed band pass fi lter instead of sweeping the fi lter 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 fi rst of two inputs applied to the fi rst mixer.
The complete input spectra (the analyzer input) is the second
signal for the fi rst 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 fi rst mixer output, the following
four signals appear:
a) The signal of the fi rst local oscillator (1st LO).
This is always 1350.7 MHz higher then the input signal fre-
quency. 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
st
LO).