Spectrum Analyzers; Real Time Spectrum Analyzers; Superheterodyne Spectrum Analyzers - Hameg HM5510 Mode D'emploi

Spektrum Analyzers
Spectrum analyzers display the amplitudes of the signal
components 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. filters,
amplifiers.

Real time spectrum analyzers

They consist of a bank of narrow tuned filters in parallel.
Obviously, only as many frequencies can be detected as there
are filters provided. Such analyzers are rare and expensive.

Superheterodyne spectrum analyzers

Nearly all modern spectrum analyzers use the super-
heterodyne 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
IF amplifier is rectified and used to drive the vertical deflection
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 deflection
and sweep.
One of the advantages of this system is the fact that the
properties of the IF bandpass filter determine the quality and
resolution of the instrument; filter parameters can be changed
without any change to other parts of the instrument.
As in any superheterodyne receiver this equation holds:
f f f f f (t) = f (t) = f (t) = f (t) = f (t) = f (t) ± f (t) ± f (t) ± f (t) ± f (t) ± f
input input input LO LO LO IF IF IF IF IF
input input LO LO
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 filter
This filter 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
(example):
1. fLO = 1369.3 MHz which shall be above the input signal.
For a desired input signal at 0 kHz the fLO = 1369.3 MHz
For a desired input signal at 150 kHz fLO = 1369.45 MHz
For a desired input signal of 1050 MHz fLO = 2419.3 MHz
2. Input signal spectrum, attenuated and shaped by the input
filter, 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 fLO = 1369.45 MHz,
the sum will be 1369.60 MHz. for an input signal of 1050
MHz fLO = 2419.3 MHz, the sum will be 3469.3 MHz.
4. Difference of all product terms of the input frequencies
and the LO. E.g.: for an input signal of 150 MHz fLO =
1369.45 MHz. The difference will be 1369.3 MHz. For an
input signal of 1050 MHz fLO = 2419.3 MHz the difference
will be 1369.3 MHz .
Summing up:
As the center frequency of the IF filter is 1369.3 MHz only such
mixing products will be passed which amount to 1369.3 MHz
(plus minus ½ bandwidth of the filter, 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.
STOP
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
problem.
TiPP
A n a l y z e r s
Subject to change without notice
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