Rohde & Schwarz Hameg HM5530 Скачать руководство пользователя страница 29

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F u n c t i o n a l p r i n c i p l e o f t h e H M 5 5 3 0

First measurements

Settings:

Prior to connecting an unknown signal to the instrument it

should be tested that its level is below +10 dBm and any DC content
below ±25 V.

ATTN (input attenuation):

As a protective measure, the attenuator should be set to its highest
position 50 dB (AT 50 dB).

Frequency settings:

Set the center frequency (CENTER) to 500 MHz (CF 500 MHz) and the
SPAN to 3000 MHz (SF 3000 MHz).

Vertical scaling:

Set the scaling to 10 dB/div (AT 50 dB 10 dB/) in order to have the
maximum dynamic range of 80 dB.

RBW (resolution bandwidth):

For a start, 1 MHz RBW should be selected (RB 1 MHz). The video ﬁ lter
should be switched off (VB 50 kHz).

If no signal is visible but the base noise band, the attenuation may be
carefully reduced in order to increase the sensitivity. If the base noise
band should shift upwards, this may be an indication of excessive signal
levels outside this instrument’s frequency range!

The attenuator must be set with respect to the highest input signal,
deﬁ nitely not with respect to Zero Peak! The dynamic range is used
best if the highest peak just reaches the top of the graticule (reference
level), but does not reach beyond. If the top of the graticule is exceeded,
external attenuation has to be added; the external attenuator must be
speciﬁ ed for the frequency range and the signal level (dissipation).

Please note that at full span (SF 3000 MHz) narrow peaks may be hardly
visible, hence, before increasing the sensitivity, one should search
for peaks. Full span is only good for a ﬁ rst overview, any meaningful
measurement requires a reduction of the SPAN. The correct procedure
is to shift the spectral line of interest to the screen center by adjus-
ting the CENTER frequency accordingly, then to reduce the SPAN. If
necessary, the resolution bandwidth (RBW) may be reduced to 120 or
9 kHz (RB ...), also the video ﬁ lter may be inserted (VB 4 kHz). The amp-
litude measurement results are valid as long as the message „uncal“
does not appear in place of the sweep time readout (SW ...).

Reading of measurements:

The easiest way to numerical results is the proper use of the mar-
kers. A short depression of the MARKER pushbutton will call the ﬁ rst
marker forward (symbol: cross), the tuning knob is used to position
the marker to the point of the signal to be measured. The level is
then indicated in the marker level readout (ML ...), the frequency at
this point in the marker frequency readout (MF ...). The marker level
readings automatically include the reference level (REF.LEVEL) and
attenuator (ATT) settings. With the 2

marker (symbol: rhombus,

readout (DL, DF) the difference in levels and frequency between both
markers may be determined. Please refer to the elaborate description
in the section „Functional controls and readout“ for more information
about the features of the markers.

If numerical values are to be obtained without the use of the markers,
it should ﬁ rst be noted that all measurements are referred to the re-
ference level (RL ... dBm), this is the top of the graticule. Readings are
thus taken from the top downward to the point on the spectrum to be
measured! This is contrary to oscilloscopes! The scaling may be 10 or
5 dB/div. At 10 dB/div, the screen encompasses a dynamic range of 80
dB; the bottom graticule line is equivalent to –80 dB if the reference
level is, e.g., 0 dB (RL 0 dB).

Functional principle of the HM5530

The HM5530 is a spectrum analyzer for the frequency range of 100 kHz to
3 GHz. The spectral components of signals in this range can be detected
and measured from –110 to +20 dBm.

The signal to be analyzed ﬁ rst passes through an attenuator which can
be switched from 0 to 50 dB in 10 dB steps. A preselection input ﬁ lter
follows which serves several purposes: to some degree, it prevents
multiple signal reception, it prevents the reception of signals at the
1st if (if feedthrough), and it suppresses any oscillator feedback to the
input. The purpose of the input mixer and the 1st oscillator (1st LO) is
the conversion of the input frequencies within the analyzer’s range; it
determines the frequency dependent amplitude characteristic and the
dynamic properties of the instrument.

The analyzer is designed as a triple superheterodyne receiver, it is an
electronically tuned selective ampliﬁ er. Frequency tuning is performed
with the aid of the 1

LO which can be tuned through the range of 3537.3

to 6537.3 MHz. Its output signal and the full-range input signal are fed to
the ﬁ rst mixer (input mixer). At the mixer output there are the following
frequency components present:

1. Signal of the 1

LO, the frequency of which must be 3537.3 MHz

above the frequency of the desired input signal. The frequency of
the 1

LO will thus be 3537.3 MHz if the input signal is 0 kHz (0 kHz

+ 3537.3 MHz). For an input frequency of 100 kHz the LO frequency
will be 3537.4 MHz (0,1 MHz + 3537.3 MHz). For an input frequency
of 1000 MHz the LO frequency will be 4537.3 MHz (1000 MHz +
3537.3 MHz). Hence the tuning range of the 1

LO is 3537.3 to 6537.3

MHz.

2. Input signal spectrum (f

inp

) after passing through the

attenuator and the input ﬁ lter (speciﬁ ed signal range: 0.1 to 3000
MHz).

3. Sum of the LO frequency (f

) and the whole input spectrum (f

inp

For a desired signal of 100 kHz the LO frequency will be 3537.4
MHz, the sum 3537.5 MHz. For 1000 MHz the LO frequency will be
4537.3 MHz, the sum 4437.3 MHz.

4. Difference of the LO frequency (f

) and the whole input spectrum

inp

). For an input of 100 kHz the LO frequency will be 3537.4 MHz,

the difference 3537.3 MHz (3537.4 – 0.1 MHz). For an input of 1000
MHz the LO frequency will be 4537.3 MHz, the difference 3537.3
MHz (4537.3 MHz – 1000 MHz).

All the signals from the 1

mixer mentioned above are applied to the

input of the 1

if (bandpass) ﬁ lter which is tuned to 3537.3 MHz, hence

only the mixer output difference frequency and the signal of the 1

(if tuned to 0 kHz) can pass.

Note:

The socalled „0 kHz signal“ from the 1

LO is unavoidable and

may disturb measurements with a resolution bandwidth RBW = 1 MHz
in the range from 0.1 to several MHz. By selection of a lower RBW this
problem can be solved.

The next stage in the signal path is the 2

mixer with the 2

LO (3200

MHz), the 2

if = 337.3 MHz, followed by the 3

mixer and the 3

(348 MHz), the 3

if = 10.7 MHz.

The last if stage contains a bandpass filter with a manually
or automatically selectable bandwidth of 1 MHz, 120 kHz, or
9 kHz. The signal is then fed to an AM detector, from now on it is called
the video signal. This signal is ampliﬁ ed by a logarithmic ampliﬁ er and
passes through a 50 kHz ﬁ lter which can be switched to 4 kHz (video