Functional Principle; First Measurements - Hameg HM5014-2 Manual

Functional principle

The spectrum analyzer permits the detection of spectrum com-
ponents of electrical signals in the frequency range of 0.15 to
1050MHz. The detected signal and its content have to be repeti-
tive. 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 super-
het receiver principle. The signal to be measured (fi n = 0.15 MHz
to 1050 MHz) is applied to the 1st mixer where it is mixed with
the signal of a variable voltage controlled oscillator (fLO 1350,7
MHz – 2400,7 MHz). This oscillator is called the 1st LO (local
oscillator). The difference between the oscillator and the input
frequency (fLO - fi n = 1st IF) is the fi rst intermediate frequen-
cy, which passes through a waveband fi lter tuned to a center
frequency of 1350,7 MHz. It then enters an amplifi er, a second
mixing stage, oscillator and the second IF amplifi er (10.7 MHz).
In the latter, the signal can be selectively transferred through a
fi lter with 1000 kHz, 120 kHz or 9 kHz bandwidth before arriving
at an AM demodulator. The logarithmic output (video signal) is
transferred directly, or via a low pass fi lter 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 fre-
quency 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 con-
verter into an analogue signal. The latter controls the Y amplifi er
and the Y defl ection plates of the CRT. With increasing signal
level (amplitude) the beam is defl ected 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 addres-
ses become D/A converted and consequently generate a saw
tooth signal which controls the X defl ection. 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: While Zero-Span-Mode the measuring fre-
quency does not change and the X-defl ection is a
time depending function.
The HM5014-2 also includes a tracking generator. It provides si-
STOP
ne wave voltages within the frequency range of 0.15 to 1050 MHz.
The tracking generator frequency is determined by the fi rst
oscillator (1st LO) of the spectrum analyzer section.
Spectrum analyzer and tracking generator are frequency syn-
chronized.
Operating Instructions
It is very important to read the instructions including the chapter
„Safety" prior to operate the HM5012-2/HM5014-2. The straight-
forward front panel layout and the limitation to basic functions,
guarantee effi cient operation immediately. To ensure optimum
F i r s t m e a s u r e m e n t s
operation of the instrument, some basic instructions need to
be followed.
Prior to examining unidentifi ed signals, the presence of unac-
ceptable high voltages has to be checked. It is also recommen-
ded to start measurements with the highest possible attenuati-
on and a maximum frequency range (Span 1000M Hz). The user
should also consider the possibility of excessively high signal
amplitudes outside the covered frequency range, although
not displayed (e.g. 1200 MHz). The frequency range of 0 Hz to
150 kHz is not specifi ed for the HM5014-2 spectrum analyser.
Spectral lines within this range would be displayed with incor-
rect amplitude.
High intensity settings should be avoided. The way signals are
displayed on the spectrum analyser typically allows for any si-
gnal to be recognized easily, even with low intensity. Due to the
frequency conversion principle, a spectral line is visible at 0 Hz.
It is called IF feedthrough. The line appears when the 1
frequency passes the IF amplifi ers and fi lters. The level of this
spectral line is different in each instrument. A deviation from the
full screen does not indicate a malfunctioning instrument.

First measurements

Settings:
Before an unknown signal is applied to the input of the instru-
ment, it should be verifi ed that the DC component is smaller
than ±25V and the signal level below +10 dBm.
ATTN:
As a protective measure the attenuation should initially be set
to 40 dB.
Frequency setting:
Set CENTER FREQ. to 500 MHz (C500MHz) and choose a span
of 1000 MHz (S1000 MHz).
Vertical scaling:
For maximum display range choose 10dB/div scaling.
RBW (resolution bandwidth):
At the start of a measurement it is recommended to select
1000 kHz (IF) bandwidth and to switch the video fi lter (VBW)
off.
If under these conditions only the noise band (frequency base
line) is visible the input attenuation can be reduced to enab-
le the measurement and display of lower signal levels. Bear
in mind that at full span, very narrow, high level signals may
low intensity and thus diffi cult to see, and should be careful-
ly sought before reducing attenuation. If the frequency base
line shifts to the top, this may be caused by a high level spec-
tra outside the measuring range. In any case the attenuator
setting must correspond to the biggest input signal (not Zero-
peak). The correct signal level is achieved if the biggest signal
(„0 Hz" - 1000 MHz) just touches the reference line. If the signal
surpasses the reference line, the attenuation must be increased,
or an external attenuator (of suitable power rating and attenua-
tion) must be used.
Measuring in full-span mode serves mostly as a quick overview.
To analyze the detected signals more closely, the span has to
st
LO
31
Subject to change without notice