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Idiomas disponibles
Idiomas disponibles
Spektrum Analyzers
Spectrum analyzers display the amplitudes of the signal com-
ponents vs. frequency. They excel by their high sensitivity and
their large dynamic range which allow them to unveil signal
detail not visible on a scope.
Typical examples are: the distortions of a sine wave, low
amplitude modulation, measurements of AM, FM signals e.g.
carrier frequency, modulation depth, modulation frequency,
frequency displacement.
Spectrum analyzers which feature a socalled tracking
generator allow measurements on two-ports, e.g. fi lters,
amplifi ers.
Real time spectrum analyzers
They consist of a bank of narrow tuned fi lters in parallel. Ob-
viously, only as many frequencies can be detected as there are
fi lters provided. Such analyzers are rare and expensive.
Superheterodyne spectrum analyzers
Nearly all modern spectrum analyzers use the super-hetero-
dyne principle known from radio sets. In the simplest case a
spectrum analyzer is nothing else but a radio receiver where
the local oscillator does not stay tuned to one frequency (i.e.
radio station), but where it is swept by a sawtooth over the
whole frequency band to be observed. The output of the
Input
attenuator
Low pass filter
Mixer
IF filter
Local
oscillator
Sawtooth
generator
IF amplifier
Logarithmic
amplifier
Detector
Video
amplifier
Display
S p e c t r u m A n a l y z e r s
IF amplifi er is rectifi ed and used to drive the vertical defl ection
plates of a scope, the sawtooth drives the horizontal plates.
In fact simple spectrum analyzers indeed used radio tuners
and a simple scope the sawtooth of which was used for X
defl ection and sweep.
One of the advantages of this system is the fact that the pro-
perties of the IF bandpass fi lter determine the quality and
resolution of the instrument; fi lter parameters can be changed
without any change to other parts of the instrument.
As in any superheterodyne receiver this equation holds:
f
(t) = f
(t) ± f
input
LO
IF
f
(t)
= Frequency input signal
input
f
(t)
= Frequency localoszillator (LO)
LO
f
= Intermediate frequency
IF
The hf input circuit consists of an input attenuator, a mixer,
and a local oscillator.
Input fi lter
This fi lter is necessary in order to suppress signals close to
the if and outside the desired frequency range, it also prevents
the local oscillator signal from reaching the input.
Mixer, LO
The mixer mixes the input signal and that from the LO and
generates the sum and difference which is then fed to the
if stage. The mixer is a critical component as it determines
mainly the sensitivity and the dynamic range.
At the mixer output the following signals are present (ex-
ample):
1. f
= 1369.3 MHz which shall be above the input signal.
LO
For a desired input signal at 0 kHz the f
For a desired input signal at 150 kHz f
For a desired input signal of 1050 MHz f
2. Input signal spectrum, attenuated and shaped by the input
fi lter, here 150 kHz to 1050 MHz.
3. Sum of all product terms of the input frequencies and the
LO. E.g.: for an input signal of 150 kHz f
the sum will be 1369.60 MHz. for an input signal of 1050
MHz f
= 2419.3 MHz, the sum will be 3469.3 MHz.
LO
4. Difference of all product terms of the input frequencies and
the LO. E.g.: for an input signal of 150 MHz f
MHz. The difference will be 1369.3 MHz. For an input signal
of 1050 MHz f
= 2419.3 MHz the difference will be 1369.3
LO
MHz .
Summing up:
As the center frequency of the IF fi lter is 1369.3 MHz only such
mixing products will be passed which amount to 1369.3 MHz
(plus minus ½ bandwidth of the fi lter, of course). But also 0 Hz
input will yield 1369.3 MHz and thus also pass, so there will be
always a "0 Hz" spectral line in the display.
This "0 Hz" signal is hence unavoidable and may
disturb in the lower frequency range if a wide
bandwidth (500 kHz) was chosen. Selecting the
lower bandwidth (20 kHz) will diminish this
HINT
problem.
= 1369.3 MHz
LO
= 1369.45 MHz
LO
= 2419.3 MHz
LO
= 1369.45 MHz,
LO
= 1369.45
LO
Subject to change without notice
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