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(6) Special considerations
Electrical performance: The SDU5500 is designed to operate in conjunction with a companion radio
as a complete system unlike an all-in-one spectrum analyser. You are reminded of the following points:
Displayed signal level
The signal level which the SDU5500 provides is always at the RF input of the SDU5500, not the signal
level at the companion radio aerial input.
AGC
The signal level of the SDU5500 may fluctuate when the companion radio is tuned or the received signal
may appear to change in level. This is because the AGC circuit of the companion radio reacts to varying
incoming signals. When the companion radio is tuned to a very weak signal or to a frequency with no
activity at all, the radio’s RF gain is set to a maximum. Under such circumstances, when a strong signal
is received the AGC reacts to reduce the input level to the SDU5500. Some radios have an AGC OFF
position which can prevent such fluctuations, however this will inevitably increase the distortion / noise in
recovered audio when a strong signal is encountered.
Frequency characteristics
While multiple active signals are simultaneously displayed, you may notice that the received signals
change positions in signal level (i.e. a strong signal is displayed weaker and a weaker signal is displayed
stronger in comparison), particularly when the companion radio is tuned around multiple strong signals.
This is caused by a disparity in the companion radio’s RF filters, IF filters, etc which result in non-linear
amplification in the wider frequency range. This is particularly noticeable when a wide span is selected
(such as 10MHz) the extreme edge of the screen may show a reduced signal level by as much as 10dB
(when AR3000A is used), this is not a fault. This has to be taken into account when a comparison of
signal level is required.
Frequency Resolution Band Width (RBW)
The displayed signal level may be affected depending on the choice of narrow or wide filter as a result of
different losses in each filter applied. In addition the signals which use a wider bandwidth (i.e. WFM,
television / high speed digital signals) may also affect the displayed signal level. When a signal of wide
bandwidth passes through a narrow filter, only partial energy will be measured, resulting in reduced
signal level on the display when compared to the same signal measured after passing through a wide
filter. Similar phenomena may be noticed when the sweeping rate is too fast for the narrow filter path.
Image reception
You may find stray signals on the screen, like meteors or ghosts, moving in the opposite direction to
tuning or moving faster / slower randomly compared to genuine active signals. Such ghost signals are
the result of images or cross-modulation / inter-modulation caused by design characteristics of the
companion radio.
Overload from the companion radio
While monitoring a crowded band with lots of strong signals (particularly with a long wire aerial con-
nected) the noise floor of the SDU5500 may rise due to the AGC action of the companion radio.
Overloading the radio may cause a distortion within the receiver’s amplifier circuits which then affects
the SDU5500 noise floor. Turn the radio attenuator ON if such phenomena are observed.
Section 6
