Sensitivity; Video Filtering; Spectrum Analyzer Sensitivity - Hameg HM5014-2 Manual

S p e c t r u m A n a l y z e r R e q u i r e m e n t s
The ability of a spectrum analyzer to resolve closely spaced
signals of unequal amplitude is not a function of the IF fi lter
shape factor only. Noise sidebands can also reduce the resolu-
tion. They appear above the skirt of the IF fi lter and reduce the
off band rejection of the fi lter. This limits the resolution when
measuring signals of unequal amplitude.
The resolution of the spectrum analyzer is limited by its nar-
rowest IF bandwidth. For example, if the narrowest bandwidth
is 9 kHz then the nearest any two signals can be and still be re-
solved is 9 kHz. This is because the analyzer traces out its own
IF band pass shape as it sweeps through a CW signal. Since the
resolution of the analyzer is limited by bandwidth, it seems that
by reducing the IF bandwidth indefi nitely, infi nite resolution will
be achieved.
The fallacy here is that the usable IF bandwidth is limited by the
stability (residual FM) of the analyzer. If the internal frequency
deviation of the analyzer is 9 kHz, then the narrowest bandwidth
that can be used to distinguish a single input signal is 10 kHz.
Any narrower IF-fi lter will result in more than one response or
an intermittent response for a single input frequency. A practi-
cal limitation exists on the IF bandwidth as well, since narrow
fi lters have long time constants and would require excessive
scan time.

Sensitivity

Sensitivity is a measure of the analyzer's ability to detect small
signals. The maximum sensitivity of an analyzer is limited by its
internally generated noise. This noise is basically of two types:
Thermal (or Johnson) and non thermal noise. Thermal noise
power can be expressed as: PN = k x T x B
where: PN = Noise power in watts
k = Boltzmanns Constant (1.38 x 10
T = absolute temperature, K
B = bandwidth of system in Hertz
As seen from this equation, the noise level is directly proportio-
nal to bandwidth. Therefore, a decade decrease in bandwidth
results in a 10dB decrease in noise level and consequently 10dB
better sensitivity. All noise produced within the analyzer that is
not temperature dependent is known as non thermal noise. Spu-
rious emissions due to non linearities of active elements, impe-
dance mismatch, etc. are sources of non thermal noise. A fi gure
of merit, or noise fi gure, is usually assigned to this non thermal
noise which when added to the thermal noise gives the total noi-
se of the analyzer system. This system noise which is measured
on the CRT, determines the maximum sensitivity of the spec-
trum analyzer. Because noise level changes with bandwidth, it
is important when comparing the sensitivity of two analyzers,
to compare sensitivity speci-fi cations for equal bandwidths. A
spectrum analyzer sweeps over a wide frequency range, but is
really a narrow band instrument. All of the signals that appear
in the frequency range of the analyzer are converted to a single
IF frequency which must pass through an IF fi lter; the detector
sees only this noise at any time. Therefore, the noise displayed
on the analyzer is only that which is contained in the IF pass
band. When measuring discrete signals, maximum sensitivity
is obtained by using the narrowest IF bandwidth.

Video Filtering

Measuring small signals can be diffi cult when they are approxi-
mately the same amplitude as the average internal noise level
of the analyzer. To facilitate the measurement, it is best to use
video fi ltering. A video fi lter is a post-detection low pass fi lter
which averages the internal noise of the analyzer. When the noi-
34
Subject to change without notice
se is averaged, the input signal may be seen. If the resolution
bandwidth is very narrow for the span, the video fi lter should
not be selected, as this will not allow the amplitude of the ana-
lyzed signals to reach full amplitude due to its video bandwidth
limiting property.

Spectrum Analyzer Sensitivity

Specifying sensitivity on a spectrum analyzer is somewhat arbi-
trary. One way of specifying sensitivity is to defi ne it as the signal
level when signal power = average noise power.
The analyzer always measures signal plus noise. Therefore,
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 da-
mage level or burn-out level of the input circuit. This is (for the
HM5014-2) +10dBm for the input mixer and +20dBm for the input
attenuator. Before reaching the damage level of the analyzer, the
analyzer will begin to gain compress the input signal. This gain
compression is not considered serious until it reaches 1dB. The
maximum input signal level that will always result in less than
1dB gain compression is called the linear input level. Above 1dB
gain compression, the analyzer is considered to be operating non
linearly 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, distor-
tions are produced within the analyzer itself. Most of these are
caused by the non linear behavior of the input mixer. For the
HM5014-2 these distortions are typically >75 dB below the input
-23
Joule/K) signal level for signal levels not exceeding –30dBm at the input
of the fi rst mixer. To accommodate larger input signal levels, an
attenuator is placed in the input circuit before the fi rst mixer. The
largest input signal that can be applied, at each setting of the input
attenuator, while maintaining the internally generated distortions
below a certain level, is called the optimum input level of the ana-
lyzer. The signal is attenuated before the fi rst mixer because the
input to the mixer must not exceed –30 dBm, or the analyzer dis-
tortion products may exceed the specifi ed 75 dB range. This 75dB
distortion free range is called the spurious free dynamic range
of the analyzer. The display dynamic range is defi ned as the ratio
of the largest signal to the smallest signal that can be displayed
simultaneously with no analyzer distortions present. Dynamic
range requires several things then. The display range must be
adequate, no spurious or unidentifi ed response must occur, and
the sensitivity must be suffi cient to eliminate noise from the dis-
played amplitude range.
The maximum dynamic range for a spectrum analyzer can be
easily determined from its specifi cations. First check the dis-
tortion spec. For example, this might be „all spurious products
>75dB down for –30 dBm at the input mixer". Then, determine
that adequate sensitivity exists. For example, 75 dB down from
–30 dBm is –105 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 reducing the level at the input mixer can extend the
spurious free measurement range. The only limitation then, is
sensitivity. To ensure a maximum dynamic range on the CRT
display, check to see that the following requirements are sa-
tisfi ed.