Features Of Spectrum Analyzers; Frequency Measurement; Stability; Resolution - Hameg HM5510 Handbuch

F e a t u r e s o f S p e c t r u m A n a l y z e r s
Zero span operation
If the sweep is switched off the LO will stay at a frequency
which is 1369,3 MHz above the input frequency, it functions
like a radio and displays only this one frequency and such
neighbouring frequencies which fall into the bandwidth of the
if filter.
Normal operation
In normal operation the sweep sawtooth sweeps the LO
through the selected span range. If a span of e.g. 1000 MHz
was chosen and the center frequency was 500 MHz, the display
would start on the left hand side of the display at 0 Hz and
sweep up to 1000 MHz at the right hand side. The center would
correspond to 500 MHz.
As the response time of a filter depends on its bandwidth and
shape the sweep must not be too fast, otherwise too low
amplitudes and distorted spectral lines may result. If
unsuitable combinations of span, resolution bandwidth are
chosen and UNCAL will be displayed.

Features of Spectrum Analyzers

The main applications of spectrum analyzers start where the
limited analysis performance of scopes end. As mentioned
spectrum analyzers excel especially by their enormous
dynamic range which, together with logarithmic amplitude
display allow to show several orders of magnitude on the same
display.

Frequency measurement

As the frequency scale of modern spectrum analyzers is
derived from a highly accurate and stable crystal oscillator
very precise frequency measurements are possible. First a
coarse display with large span will show the frequency to be
measured, this can then be shifted to the display center while
the span is reduced and the smallest RBW selected at the
same time, increasing the accuracy. It is also possible to select
zero span and minimum RBW and then turn the center
frequency control knob until the maximum amplitude is
reached: the frequency can then be read from the center
frequency display.

Stability

The frequency stability of a spectrum analyzer should be much
better than that of the input signal. The 1
determine the quality. Most important is the short term stability
including noise, residual FM and spectral purity.

Resolution

The smallest bandwidth and the filter slopes of the if band-
pass filter determine the available resolution of a spectrum
analyzer. The definition of bandwidth is the frequency span
between the – 3 dB points. The relationship between the – 60
dB bandwidth and the – 3 dB bandwidth is called form factor.
32
Subject to change without notice
STOP
TiPP
In addition to the form factor residual FM and spectral purity
of all oscillators will also affect the capability of a spectrum
analyzer to separate neighbouring frequencies. The noise side
bands created by residual FM and insufficient spectral purity
will deteriorate the stop band attenuation of the filters.
With the smallest RBW of 20 kHz 2 frequencies must be more
than 20 kHz apart if they should be recognized as separate.
The spectrum analyzer displays its own IF filter curve if there
is any signal. It appears that infinite resolution should be
possible with an infinitely small RBW. In practice this does
not happen. The stability of the oscillators sets one limit, if
the signal moves too much with frequency it will move back
and forth with a very narrow bandwidth filter, no usable display
would result, only jitter. Residual FM of the oscillators would
cause the display of several spectral lines instead of one. The
second practical limit is given by the relationship of filter
bandwidth and response time, the narrower the filter the
slower must the frequency be swept across, otherwise the
filter will yield a decreased amplitude and a distorted display.

Noise

The maximum sensitivity of a spectrum analyzer is determined
by the noise level, to be differentiated between thermal noise
and non-thermal noise.
Thermal noise is given by:
K = Boltzmann's constant
T = absolute temperature
B = bandwidth
Noise is hence directly proportional to bandwidth, thus if the
filter bandwidth is reduced by a factor of ten the noise will
decrease by 10 dB. The sensitivity increases by the same factor.
All other noise sources in a spectrum analyzer are regarded
as non-thermal. Sources of such non-thermal noise are e.g.:
distortions caused by nonlinear behaviour, mismatches, hf
leakage. The quality = noise figure of a system is given by the
noise figure of the non-thermal sources plus the thermal
noise. This visible noise limits the sensitivity of the instrument.
When comparing spectrum analyzers it is important to
compare identical instrument settings, i.e. the bandwidths
must be identical. Although a spectrum analyzer covers a very
broad frequency range the noise depends mainly on the IF
filter bandwidth, the detector following the IF sees only the
noise passed by it.

Video filter

st
LO' s properties
The measurement of small signals close to the noise level
becomes difficult. In order to separate the signal more from
the noise a video filter may be inserted following the detector.
This filter typically has a bandwidth of a few kHz and averages
the noise. Here it also applies that small bandwidth filters
respond slowly, hence it is advisable to switch this filter off if
the IF bandwidth becomes small compared to the scan
selected which means that the sweep speed becomes too
high, otherwise the amplitudes will be displayed too low. An
UNCAL light will indicate any unfavourable combinations of
settings.
The smaller the form factor the better can adjacent
frequencies be separated. E.g.: if the form factor is
15:1 2 frequencies which differ in amplitude by 60 dB
must differ in frequency by at least the factor of 7.5, if
they should still be discernible as separate,
otherwise they will melt into one signal.
P = K x T x B
noise