Hameg HM8028 Bedienungsanleitung Seite 25

The analyzer always measures signal plus noise. There-
fore, when the input signal is equal to the internal noise
level, the signal will appear 3dB above the noise. When
the signal power is added to the average noise power,
the power level on the CRT is doubled (increased by
3dB) because the signal power = average noise power.
The maximum input level to the spectrum analyzer is
the damage level or burn-out level of the input circuit.
This is (for the HM8028) +13dBm for the input mixer
and +26dBm for the input attenuator. Before reaching
the damage level of the analyzer, the analyzer will begin
to gain compress the input signal. This gain compres-
sion is not considered serious until it reaches 1dB. The
maximum input signal level which will always result in
less than 1 dB gain compression is called the linear input
level.
Above 1 dB gain compression the analyzer is considered
to be operating nonlinearly because the signal
amplitude displayed on the CRT is not an accurate
measure of the input signal level.
Whenever a signal is applied to the input of the analyzer,
distortion products are produced within the analyzer it-
self. These distortion products are usually produced by
the non-linear behavior of the input mixer. They are typ-
ically 70dB below the input signal level for signal levels
not exceeding ~27dBm at the input of the first mixer.
To accommodate larger input signal levels, an attenuator
is placed in the input circuit before the first mixer. The
largest input signal that can be applied, at each setting of
the input attenuator, while maintaining the internally
generated distortion products below a certain level, is
called the optimum input level of the analyzer. The sig-
nal ts attenuated before the first mixer because the
input to the mixer must not exeed —27dBm, or the
analyzer distortion products may exceed the specified
70dB range. This 70dB distortion-free range is called
the spurious-free dynamic range of the analyzer. The
display dynamic range is defined as the ratio of the
largest signal to the smallest signal that can be dis-
played simultaneously with no analyzer distortion pro-
ducts present. :
Dynamic range requires several things, then. The dis-
play range must be adequate, no spurious or uniden-
tified response can occur, and the sensitivity must be
sufficient to eliminate noise from the displayed
amplitude range.
The maximum dynamic range for a spectrum analyzer
can be easily determined from its specifications. First
check the distortion spec. For example, this might be
"all spurious products down 70dB for —27dBm at the
input mixer". Then, determine that adequate sensitivity
exists. For example, 70dB down from —27dBm is
—97 dB. This is the level we must be able to detect, and
the bandwidth required for this sensitivity must not be
too narrow or it will be useless. Last, the display range
must be adequate.
Notice that the spurious-free measurement range can
be extended by reducing the level at the input mixer.
The only limitation, then, is sensitivity.
To ensure a maximum dynamic range on the CRT display,
check to see that the following requirements are satisfied.
M12 — 8028
1. The largest input signal does not exceed the op-
timum input level of the analyzer (typically —27 dBm
with OdB input attenuation).
2. The peak of the largest input signal rests at the top of
the CRT display (reference level).
Frequency Response
The frequency response of an analyzer is the amplitude
linearity of the analyzer over its frequency range. If a
spectrum analyzer is to display equal amplitudes for
input signals of equal amplitude, independent of fre-
quency, then the conversion (power} loss of the input
mixer must not depend on frequency. If the voltage
from the LO is too large compared to the input signal
voltage then the conversion loss of the input mixer is
frequency dependent and the frequency response of
the system is nonlinear. For accurate amplitude meas-
urements, a spectrum analyzer should be as flat as pos-
sible over its frequency range.
Flatness is usually the limiting factor in amplitude accu-
racy since it's extremely difficult to calibrate out. And,
since the primary function of the spectrum analyzer is to
compare signal levels at different frequencies, a lack of
flatness can seriously limit its usefulness.
TRACKING GENERATORS
The tracking generator is a special signal source whose
RF output frequency tracks (follows) some other signal
beyond the tracking generator itself. In conjunction with
the spectrum analyzer, the tracking generator produces
a signal whose frequency precisely tracks the spectrum
analayzer tuning. Because of this feature, the two in-
struments combine to make a powerful and versatile
measurement system.
The tracking generator frequency precisely tracks the
spectrum analyzer tuning since both are effectively
tuned by the same VTO. This precision tracking exists in
all analyzer scan modes. Thus, in full scan, the tracking
generator output is a start-stop sweep, in per division
scan the output is simply a CW signal.
The tracking generator signal is generated by synthesiz-
ing and mixing two oscillators. One oscillator is part of
the tracking generator itself, the other oscillator is
brought via an interface cable from the spectrum
analyzer.
The spectrum analyzer/tracking generator system is
used In two configurations: open-loop and closed-loop.
In the open-loop configuration, unknown external sig-
nals are connected to the spectrum analyzer input and
the tracking generator output is connected to a counter.
This configuration is used for making selective and sen-
sitve precise measurement of frequency.
In the closed-loop configuration, the tracking generator
signal is fed into the device under test and the output of
the device under test is connected to the analyzer input.
Subject to change without notice