Functional Principle / Operating Instructions - Hameg HM5012-2 Manual

Functional principle
The spectrum analyzer permits the detection of spectrum
components of electrical signals in the frequency range of 0.15
to 1050MHz. The detected signal and its content have to be
repetitive. In contrast to an oscilloscope operated in Yt mode,
where the amplitude is displayed on the time domain, the
spectrum analyzer displays amplitude on the frequency domain
(Yf). The individual spectrum components of a "signal" become
visible on a spectrum analyzer. The oscilloscope would display
the same signal as one resulting waveform.
The spectrum analyser works according to the double superhet
receiver principle. The signal to be measured (fin = 0.15MHz to
1050MHz) is applied to the 1st mixer where it is mixed with the
signal of a variable voltage controlled oscillator (fLO 1350,7MHz
– 2400,7MHz). This oscillator is called the 1st LO (local oscillator).
The difference between the oscillator and the input frequency
(fLO - fin = 1st IF) is the first intermediate frequency, which passes
through a waveband filter tuned to a center frequency of
1350,7MHz. It then enters an amplifier, a second mixing stage,
oscillator and the second IF amplifier (10.7MHz). In the latter, the
signal can be selectively transferred through a filter with 1000kHz,
120kHz or 9kHz bandwidth before arriving at an AM demodulator.
The logarithmic output (video signal) is transferred directly, or via
a low pass filter to an A/D converter and the signal data are stored
in a RAM. The lowest frequency of a span is stored at the lowest
address and the highest frequency at the highest address. Then
the next span starts the same procedure once again. This means
that the signal data are continuously updated.
In addition the signal data are read and converted by a D/A
converter into an analogue signal. The latter controls the Y amplifier
and the Y deflection plates of the CRT. With increasing signal
level (amplitude) the beam is deflected from the bottom (noise)
to the top of the screen.
During the continuous read process the RAM becomes addressed
from the lowest to the highest address. The addresses become
D/A converted and consequently generate a saw tooth signal
which controls the X deflection. The sweep starts with the lowest
frequency (address) at the trace start (left) and ends with the
highest frequency (address) at the trace end (right).
The stored spectrum data can be transferred to a PC via the built
in serial interface.
Note: In Zero Span mode the measuring frequency does not
change. Then the X deflection is a time function.
The HM5014-2 also includes a tracking generator. This generator
provides sine wave voltages within the frequency range of 0.15
to 1050MHz. The tracking generator frequency is determined by
the first oscillator (1st LO) of the spectrum analyzer section.
Spectrum analyzer and tracking generator are frequency
synchronized.
Operating Instructions
It is very important to read the instructions including the paragraph
„Safety" prior to operating the HM5012-2/HM5014-2. The
straightforward front panel layout and the limitation to basic
functions, guarantee efficient operation immediately. To ensure
Subject to change without notice
optimum operation of the instrument, some basic instructions
need to be followed.
Prior to examining unidentified signals, the presence of
unacceptable high voltages has to be checked. It is also
recommended to start measurements with the highest possible
attenuation and a maximum frequency range (Span 1000MHz).
The user should also consider the possibility of excessively high
signal amplitudes outside the covered frequency range, although
not displayed (e.g. 1200MHz). The frequency range of 0Hz to
150kHz is not specified for the HM5012-2/HM5014-2 spectrum
analyser. Spectral lines within this range would be displayed with
incorrect amplitude.
High intensity settings should be avoided. The way signals are
displayed on the spectrum analyser typically allows for any signal
to be recognized easily, even with low intensity. Due to the
frequency conversion principle, a spectral line is visible at 0Hz. It
is called IF feedthrough. The line appears when the 1
frequency passes the IF amplifiers and filters. The level of this
spectral line is different in each instrument. A deviation from the
full screen does not indicate a malfunctioning instrument.
Functional principle
st LO
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